“Primates with smaller genitalia compensate by being flashy and aggressive,” reads a recent headline at the New York Daily News. The articles covers new research, out in the journal Proceedings of the Royal Society B, showing an inverse correlation between testis size and sexual ornaments across species. Ornamentation here refers to traits that have no function other than to be flashy, attractive, or intimidating, e.g., the mandrill’s facial coloration or the large noses of proboscis monkeys. Basically, some primate species have large testes and less ornamentation, whereas others have smaller testes and more ornamentation.
These trends most likely reflect the different evolutionary pressures faced by males in different species with different social structures. In species where females mate with multiple males, males are more likely to evolve large testes, which can produce more and faster sperm. This is useful in sperm competition: different males’ sperms literally compete with each other within one female’s reproductive tract to be the one to fertilize her egg.
In polygynous species (where a single male monopolizes a group of females and prevents other males from mating with them), there is little to no sperm competition happening. As a result, males of these species tend to have smaller testes. They also tend to have more ornamentation. This makes sense for two reasons. First, a male primate only has so much energy to devote to developing different body parts; not having to devote that energy to testes frees it up to go toward ornamentation. Second, in these species where one male ends up with a group of females to mate with and another males gets nothing, the competitive stakes are more intense. It’s not surprising that there would be stronger pressure to evolve ornaments that are attractive to females and/or intimidating to other males.
Putting aside the obvious problems in these headlines, I want to focus on the word “compensate.” Like many words in biology, it has two meanings: an evolutionary (or ultimate) meaning, and a psychological (or proximate) one. For more on this dichotomy, I recommend checking out my recent post about cats, where I go deep into the proximate-ultimate distinction, even calling it the most important concept in biology. Here, too, the proximate-ultimate distinction can help clarify the discussion.
In evolutionary terms, “compensating” can be taken to mean “making up for a trait that is not ideal from a fitness standpoint by evolving another trait to increase fitness.” Even this isn’t exactly what’s going on behind cross-species correlations between testes and other traits. Still, this evolutionary definition of compensation is very different from the psychological one, which involves an understanding of one’s inadequacy, insecurity and effects on self-image, and a desire to mask or make up for the inadequacy. This is arguably the more colloquial meaning, and the one most people will think of as they read these headlines.
But no scientist believes that monkeys are compensating for small genitalia in the psychological sense. The evolutionary tradeoffs are happening in the monkeys’ genes, far outside their awareness. We don’t even know if monkeys form a self-image at all, let alone one based on genitalia.
Furthermore, remember that these studies looked at correlations across species. Even if primates did form a self-image around their genitalia, they probably wouldn’t be basing this on a comparison to other species! I mean, chimpanzees have testes the size of apricots, but you don’t see human men forming a complex about it.
I’m not here to be a buzzkill or say we shouldn’t joke around about science. Anything that gets people talking about science is a good thing in my book. I just want to be sure we all understand what the science is saying beforehand. Now that we’re all on the same page, go ahead and post that Trump joke.
The following adage, which I attribute to my dog-person mom (but I’m sure she got from somewhere else), aptly sums up these anti-feline sentiments:
A dog looks at its human and says, “wow, you shelter me, feed me, care for me… you must be God.” A cat looks at its human and says, “wow, you shelter me, feed me, care for me… I must be God.”
This view of cats is so pervasive that even cat-lovers
embrace it, half-joking
that their cats actually own them rather than the other way around. But how
much of our culture’s cynicism toward cats stems from cats themselves, vs. our
What our cynicism toward
cats says about us
A recent Atlantic
article (smartly titled, Why We Think Cats Are Psychopaths: It’s Just “Resting
Cat Face”) explains that most of us simply don’t understand cats’ behaviors and
emotional expressions—at least not as well as we do dogs’.
Some of this probably has to do with cultural factors. For
example, we are constantly exposed to dog behavior in the media; even other
animals are often portrayed as dog-like:
Additionally, dogs are naturally in
sync with humans’ style of emotional communication. Cats naturally communicate
in ways that are very different from dogs, and there may be somewhat of a
mismatch between cats’ communication style and that of humans. Of course this
can be overcome through familiarity, but it might account for why many people
who have never had cats find them so aloof and hard to read.
In other words, our anthropomorphic tendencies might be
leading us to underestimate cats’ affection for us. Our cynicism toward cats
might say more about our own biases than it does about cats themselves.
What Does the Science
Fortunately, we don’t have to rely solely on our flawed
intuition about cats. Science is designed to take such biases out of the picture,
to arrive at empirical truths. So, what does scientific research have to say
about cats’ attitudes towards humans?
Many things, if you believe the pop science reporting. A cursory Google search for “do cats love us” turns up tons of contradictory headlines, each claiming the science is on their side:
I’ll cut to the chase: I think the weight of evidence
suggests cats do feel affection for humans. In one
study, cats were given the choice between food, toys, a pleasurable scent,
or human interaction, and consistently chose human interaction. The same preference
has been called
evidence of love in dogs, so why not cats? The strongest contrary evidence comes
study where cats failed to show “secure attachment” to their humans,
meaning cats left alone with a stranger didn’t act particularly distressed
relative to cats left alone with their human. But the scientists behind that
study didn’t claim it says anything about love or affection. All it says is that
cats are generally autonomous and don’t necessarily depend on others to provide
a sense of security and safety. It’s not like I start panicking when my partner,
friends, or family leave the room, yet you don’t see WaPo writing headlines
about how I don’t love or care about those people.
But what about the ideas from the Cracked.com article I mentioned
earlier—the ostensibly “evil explanations” for common cat behaviors? For example,
when a cat rubs against you, is it true that they’re just scenting you in order
to claim you as their property? Is it true that a cat gifting you a dead animal
is actually insulting your capabilities as a provider?
Here’s the thing: some of these ideas floating around about cats’ selfish motives do actually reflect scientific kernels of truth. But, probably due to our biases against cats, these facts are being misinterpreted. There is a critical nuance missing from this discussion, and that’s what I’m going to talk about for the rest of this post.
Proximate and Ultimate
Let’s focus first on the rubbing behavior as an illustrative
example. It is true that the evolutionary function of a cat rubbing up against an
object (or person) is to mark it with their scent. In that sense it is accurate to say that they are
claiming you as their property when they rub against you. But selfish as that
may seem, it doesn’t actually say anything about the cat’s motivations, which
could be all about love and affection. To explain what I mean by this, I need
to turn to a concept that goes back decades: the proximate-ultimate
The proximate-ultimate distinction was initially put forth in 1961 by the biologist Ernst Mayr, and subsequently adapted for animal behavior research by one of the founders of ethology, Niko Tinbergen, in 1963. These scientists wanted to reframe how we talk about “why” questions in biology. They argued that scientific answers to those questions can be either “proximate”—pertaining to what’s going on within the organism, such as genes, hormones, neural activity, and psychology—or “ultimate”—pertaining to the evolutionary forces that have shaped the organism and its behavior over generations. Their main argument was that biologists should aim to discover and integrate both types of answers.
To illustrate the proximate-ultimate distinction, take as an
example the question, “why do humans have sex?” The ultimate-level answer is
simple: sex is the key to reproduction, and reproduction is how organisms pass
on their genes to the next generation. This has resulted in a strong evolutionary
pressure favoring those who have sex, and as a result, sex is nearly ubiquitous
in our species (though not entirely—I see you, asexual people). The
proximate-level answer in this case could involve things like an individual’s
desires, circumstances, sex hormones, and experiences.
The critical thing about proximate and ultimate answers is
that they are separate from one
another. It is true that we have sex because, on average, it has increased past
humans’ fitness by generating offspring (ultimate), but it’s not as if everyone
having sex is motivated (proximate)
by a desire for offspring. The existence of contraception is proof of this. Rather,
evolution has responded to the selection pressure by giving most humans a desire (proximate) for sex. This desire
for sex is not the same as a desire
for offspring, although many humans hold that motivation as well.
As this example demonstrates, proximate and ultimate are not
only separate from one another, they actually complement each other—in order to
get the full picture of anything biological, you need to understand both the
proximate and ultimate perspectives.
If by now you’re thinking, “I came here to learn about cats,
not think about humans boning,” here’s how it translates. For the question of
why cats rub up on people, an ultimate-level answer is that this behavior
disperses the cat’s scent which has, over evolutionary time, increased cats’
evolutionary fitness (probably by securing resources, mates, etc. from
competitors). But that ultimate function doesn’t equal the cat’s proximate
motivations. A cat rubbing against a leg isn’t necessarily thinking about
spreading their scent.
In fact, we have no reason to believe cats understand that
evolutionary function at all. From evolution’s point of view, any proximate way
of motivating them to rub would suffice—they don’t need to understand the evolutionary
function in order to reap the benefits, which are what matter from an
evolutionary perspective. Rather, it could just be that the cat experiences feelings
of affection (or even just an itchy head), and expresses this by rubbing
against you. That lovey-dovey proximate answer is totally compatible with the
ultimate answer involving scents and property.
We can also take this approach to the behavior where a cat
drops dead animal on the ground near their human. It is true that cats in the
wild do this for their infants, and that it serves the evolutionary function of
helping their infant learn how to hunt. Based on this, some have claimed that
dropping a dead animal at your feet is a way of insulting your competence. I acknowledge
that seems somewhat tongue-in-cheek, but it’s still a misinterpretation, as
becomes clear using the proximate-ultimate distinction.
The function of teaching is an ultimate answer. The cat isn’t
necessarily thinking about teaching
or doing so intentionally. In fact, because intentional teaching involves
theory of mind (to intentionally teach, I have to understand that my student
knows less than I do—this is very cognitively demanding), scientists believe that
it is extremely rare in the animal kingdom. A more likely proximate explanation
is that cats simply have a tendency to drop dead prey animals near individuals
toward whom they feel affection, whether the recipient be a human or the cat’s
own offspring. This mechanism would work just fine from an evolutionary
Now, we don’t know for sure the proximate reason for cats’
behavior, and we can’t categorically rule out the possibility that cats are insulting us and selfishly thinking
about claiming property and other evolutionary benefits. But, again, that assumes
a lot in terms of cognition; it’s pretty rare in nature for animals to
understand the ultimate functions of their behaviors. I would argue that based
on the available scientific evidence mentioned earlier—cats prefer human
interaction over food and toys—it’s more likely that feelings of affection are
involved in the behaviors discussed here, and others.
I present the proximate-ultimate distinction to my students as one of the most important concepts in all of biology, because it forces us to consider separately the evolutionary functions and proximate mechanisms behind an animal’s behavior (or any other trait), and eventually come to a greater scientific understanding.
Can “the bird with four sexes” tell us anything about gender?
Photo by Jennifer R. Merritt: White-striped (left) and tan-striped (right) birds
I recently published a piece at SAPIENS.org exploring the question of whether gender is unique to humans. A major point I tried to make in the piece was that plenty of animal species show sex differences in behavior, but that’s not really the same as gender. Human gender is much more complex than most animal sex roles; for example, it includes socially determined sex roles and an internal sense of gender identity—aspects that are missing in most if not all other species. The tl;dr of the piece was that the evidence on the topic is far from conclusive, but suggests it’s not implausible that chimpanzees and bonobos might have something comparable to human gender.
Something I was thinking about as I was working on the piece was how, in humans, gender is extricable from sex, meaning that a person’s sex doesn’t entirely determine their gender. Transgender, agender, and non-binary people demonstrate this. I wanted to explore whether we see any examples of species where the “traditional” sex roles we see in many animals—where males are aggressive and flamboyant whereas females are nurturing—are dissociated from the sexes, as human gender is sometimes dissociated from sex. This led me to consider the white-throated sparrow.
In this species, like many others, some individuals focus their energy on infant care while others focus on aggression and protecting their territory. Also, as in many other species, white-throated sparrows form mating pairs consisting of one aggressive individual and one parental individual. Where white-throated sparrows differ from other species is that these two roles aren’t tied to the two sexes. Instead, a chromosomal inversion (a mutation event where a piece of chromosome—in this case the ZAL2 chromosome—breaks and is reinserted backwards) has resulted in two “morphs” or types of bird, with both males and females of each morph represented: the white-stripe morph is aggressive, whether male or female (although male white-stripes are more so), while the tan-stripe morph is parental (but males are less so). Mating pairs almost always either consist of a white-stripe male and tan-stripe female, or a white-stripe female and tan-stripe male.
For these reasons, some have referred to the white-throated sparrow as the bird with “four sexes”—white-stripe male, tan-stripe male, white-stripe female, tan-stripe female. But just how like sexes (or genders) are these two morphs, really?
I talked to Emory University Psychology Ph.D. Candidate Jenny Merritt, who studies these birds. She explained how a bird’s morph is determined: tan-stripe birds have two normal ZAL2 chromosomes, while white-stripe birds have one normal copy of ZAL2 and one inverted copy, referred to as ZAL2m. In this way, a white-throated sparrow’s morph is determined very similarly to the system of mammalian sex determination we are all familiar with: just as the Y chromosome contains the DNA that codes for male features, the ZAL2m chromosome contains the DNA that codes for the white-stripe appearance and behavior. The critical distinction, again, is that these birds also have two sexes—morph is overlaid on, and somewhat independent of, sex. Perhaps like gender is, in humans?
Merritt is far from convinced. “I don’t think that the white-throated sparrow mating system is analogous to human gender. … I am unaware of any evidence that the sparrows have internal senses of their own morph, or whether that sense could be subject to the same kinds of factors that shape human gender.”
Furthermore, human gender roles are at least partly socially learned, and Merritt says scientists don’t know if there is a learning component to the development of behavioral differences between the two morphs. There is a strong relationship between behavior and genetic morph, and some of Merritt’s own research points to specific way that ZAL2m might influence the brain to increase aggression in white-stripe birds; these observations make it unlikely that morph roles are socially learned like gender roles are in humans.
That said, while human gender is at least partly socially determined, it probably isn’t completely free of genetic influences (although we don’t have a good sense of what those might be). Research on white-throated sparrows by Merritt and others improves our understanding of how genes generally lead to sex differences in behavior, and how sex differences evolve, with potential implications for human sex and gender.
To summarize, although there are some links to be drawn, the ways humans think about gender are almost certainly very different from how sparrows – and most (though perhaps not all) other animals – think about their behavioral roles. The best way to understand the latter might be to approach animal research with the goal of learning more about about animals themselves, absent the bias that often comes with an anthropocentric “animal model” mindset. This is a point that all of us who study nonhuman species would do well to keep in mind: while “is X unique to humans?”-type questions can be alluring, they can also lead us to miss the beauty that comes with understanding nature on its own terms.
This piece was originally published on SAPIENS. Read it here.
Is Gender Unique to Humans?
By Jay Schwartz
This summer, in the introductory course I teach on the evolution and biology of human and animal behavior, I showed my students a website that demonstrates how to identify frog “genders.” I explained that this was a misuse of the term “gender”; what the author meant was how to identify frog sexes. Gender, I told the students, goes far beyond mere sex differences in appearance or behavior. It refers to something complex and abstract that may well be unique to Homo sapiens. This idea is nothing new; scholars have been saying for decades that only humans have gender. But later that day I began to wonder: Is it really true that gender identity is totally absent among nonhuman species—even our closest evolutionary relatives, chimpanzees and bonobos?
Before tackling this question, it is necessary to define “sex” and “gender.” Sex refers to biological traits associated with male and female bodies. Sex isn’t a perfect binary, but it is relatively simple compared to gender.
Gender is multifaceted, complex, and a little abstract, and not everyone agrees on exactly what it means. That said, there are a couple of aspects of gender that most experts say are essential. The first is the existence of socially determined roles. Gender roles refer to the range of behaviors that society deems normal or appropriate for people of a particular gender based on their designated sex—the norms that (at least in many Western cultures) cause us to expect men to be assertive and brave, and women to be caring and accommodating, for instance.
It’s common for people to believe that gender roles are natural or innate, ranging from religious claims that they are God-given to the argument made by evolutionary psychologists that they are the biological result of natural selection. On the contrary, while some aspects of gender-correlated behaviors are probably largely genetic in origin (researchers don’t have a great sense of which are and aren’t), most experts agree that many gender-related expectations, such as that girls play princess and boys pretend to be soldiers, are socially determined—that is, we learn them from our culture, often without even being aware of it. This socially learned aspect is as fundamental to gender as the roles themselves.
Another fundamental aspect of gender is an internal sense of gender identity. Most people don’t just act in accordance with the roles associated with their gender identity, they also feel something inside of themselves that tells them what their gender is. For many, this sense of identity aligns with their biological sex (cisgender), but that’s not true for everyone. Plenty of people are biologically male, but they identify as women, or vice versa (transgender). Some individuals have a gender identity that is somewhere in between masculine and feminine, or it’s a mix of both or neither (androgyny). Still others are intersex, having both male and female biological traits; just like those who fit on either side of the sex spectrum, intersex people fall across a range of gender identities.
So, two criteria substantiate gender: socially determined roles and an internal sense of identity. Neither of these by itself is enough to fully encompass what gender is, but most experts appear to agree that each is a necessary aspect of gender. Therefore, to assess the common claim that gender is unique to humans, we need to look at how other species fare with respect to these two criteria.
This is a tough endeavor—most of what we know about human gender originated from talking to people, and we usually don’t have the ability to ask other species what they think. Nonetheless (as I’ve written about before on the topic of primate vocal communication), we do have some access to animals’ minds through observing their social behavior. The evidence accrued from numerous studies, while not decisive, shows that gender might, in fact, exist in other species.
First, let’s look at the question of socially determined roles. Plenty of nonhuman species show sex-based differences in behavior. From beetles to gorillas, males of many species are more aggressive than females, and they fight with one another for access to resources and mating opportunities. Males are also often the more flamboyant sex, using showy body parts and behaviors to attract females—for example, take the peacock’s tail, the mockingbird’s elaborate song, or the colorful face of the mandrill (think Rafiki from TheLion King). Females, on the other hand, are in many cases more nurturing of offspring than males; after all, by the time an infant is born the female will have already devoted significant time and energy toward forming, laying, and subsequently protecting and incubating her eggs—or, in the case of us mammals, she has gone through an intense process of gestation. The costly nature of reproduction for females limits the number of infants they can have; that’s why it generally behooves females to be conservative, expending their time and energy on mating with only the highest-quality males. Being choosy in this way has, over evolutionary time, generally yielded fitter offspring. As a result, females of many species have evolved to be the choosier sex, and their mate choices can direct the course of evolution (an idea that scandalized Victorian England when first proposed by Charles Darwin).
There are exceptions to every rule, of course. Male seahorses get pregnant. Female spotted hyenas dominate males and sport a pseudo-penis (enlarged clitoris) that is capable of erection and can be as much as 90 percent the size of a male’s penis. As matriarchal as spotted hyena society is, it doesn’t quite reach the level of the northern jacana, a wading bird species whose common territory ranges from Panama to Mexico. Female northern jacanas patrol a territory full of males and fight off intruding females; the smaller males engage in less territorial behavior than females, instead spending that time caring for a nest full of the resident female’s eggs.
Turning to our closest relatives, chimpanzees and bonobos, we see additional illustrative examples of the natural variation that exists in sex-correlated behavior. Although the two species are 99.6 percent genetically identical(and equidistant from humans), they are quite different. In general, adult male chimpanzees, like males of many species, are aggressive, domineering, and status-seeking. Much of their time is spent either patrolling territorial boundaries to deter or even kill members of other communities, or vying for social power within their own group. Adult females are generally less political and less violent—they have other priorities, like caring for offspring—but they can still influence the state of social affairs by breaking up male fights or leading rival males to reconcile. After all, as is the case in many species, much of what males stand to gain from high status is access to mating opportunities with females.
It’s been said that if chimpanzees are from Mars, then bonobos are from Venus. Bonobo society is generally female-dominated. Unlike female chimpanzees who mostly, though not always, keep their noses out of politics, female bonobos reign by forming male-dominating coalitions. They bond partly through genito-genital rubbing (it is what it sounds like), forming stronger relationships than female chimps typically have with one another. As for male bonobos, they are much less violent on average than male chimps. Unlike with chimpanzees, lethal aggression has never formally been observed in bonobos (though there has been one suspected instance); bonobos are more likely to share food (and maybe sex)with a stranger than to fight.
Some scholars look at the sex differences in behavior described in the above paragraphs as clear examples of nonhuman gender. But none of the evidence I have covered so far proves that behavioral differences between male and female chimpanzees, bonobos, or other nonhuman species are socially determined. Again, gender necessarily entails socially determined roles. Do we have any evidence that chimp and bonobo behaviors are determined socially rather than biologically?
That is the question Michelle Rodrigues, a postdoctoral researcher at the University of Illinois, and Emily Boeving, a doctoral candidate in psychology at Florida International University, set out to answer. They found that there is flexibility in some of the sex roles previously observed in chimpanzees and bonobos—specifically, in grooming. In both chimpanzees and bonobos (as well as in many other primates), grooming serves as a way of strengthening social bonds. In the wild, most of the grooming in both species is male-on-female or vice versa. Where the species differ is that among wild chimpanzees, male-male grooming is generally more common than female-female grooming—an imbalance not seen in bonobos.
Rodrigues and Boeving wondered whether chimps and bonobos living at zoos would show the same grooming patterns. To investigate this, they observed chimpanzees and bonobos at the North Carolina Zoo and Columbus Zoo, respectively, paying special attention to grooming networks. In contrast to data from the wild, zoo-living apes’ grooming seemed to be more related to individuals’ histories and personalities than their sex: Neither species showed the sex-typical grooming patterns displayed by their wild counterparts.
This is solid evidence that certain sex roles are at least partly environmentally determined in these species. But is environmental determination the same as social or cultural determination? Not exactly. Social learning could be responsible for the flexibility we see in chimpanzee and bonobo sex roles. In this hypothetical scenario, wild female chimpanzees groom less than males because growing up, they receive less grooming from other females, and they witness little, if any, female-female grooming. They are socialized in these ways not to spend as much time grooming. In the zoo, then, the “culture” around grooming is atypical, and females are socialized differently. However, an equally plausible (but not mutually exclusive) possibility is that sex-based behavioral differences in the wild are simply the result of individuals finding ways of coping with their environment: Females in the wild have the responsibility of infant care. As a result, they are too busy foraging to spend much time socializing. At the zoo, with humans providing food, females groom more simply because they have the extra time—no social learning of sex roles is required.
Again, these two explanations are not mutually exclusive. Both could play a part. I spoke with Rodrigues about what evidence would be necessary to conclude that chimpanzee or bonobo sex roles were socially determined.
“We would need to see evidence that adults are actively treating male and female infants and juveniles differently, and actively [socializing] them differently,” she said. Rodrigues pointed out that some chimpanzee behavior is suggestive of different treatment of male and female offspring: For example, she noted, “data on young chimpanzees indicates that female chimpanzees spend more time observing their mothers termite-fishing and, in turn, are able to master termite-fishing using their mother’s technique at a younger age.” Researchers aren’t certain whether this is due to active socialization by mothers or an innate preference among female offspring to observe their mothers’ techniques. Even so, this observation is consistent with the idea of social determination of at least some, but probably not all, sex roles in chimps.
Flexibility in chimpanzee sex roles is not limited to the grooming patterns discussed earlier. Females occasionally participate in males’ political coalitions or go ranging with a mostly male group. Likewise, some males seem to prefer ranging with smaller groups of mostly females, or they spend more time interacting with infants than is typical for males. But scientists generally don’t consider this evidence of chimpanzee gender-bending. Rodrigues told me female-like behavior by male chimps is usually interpreted as a result of low rank—it’s not that the males prefer these feminine roles, it’s that they are relegated to these positions by dominant males.
“But,” Rodrigues said, “it may be that our existing frameworks for interpreting behavior are too focused on paternity and rank. I think one of the challenges in interpreting behavior is that our own social constructions color how we theorize and interpret data.”
(Now, you may be thinking, “What about bonobos?” Most of the evidence bearing on these questions comes from chimpanzees, who have been studied much more extensively than bonobos. That said, despite their many differences in behavior, chimpanzees and bonobos are still very closely related, and their cognitive capacities are likely very similar. If one species has something like gender, the other probably does too.)
So far, it’s not inconceivable that chimpanzees and bonobos might have something akin to human gender. But we haven’t yet touched on the other crucial criterion for gender: an internal, mental construct. How, if at all, do nonhuman animals think about sex and social roles? Scientists get at this question using cognitive testing—specifically, by testing animals’ concepts.
In psychology, “concepts” refer to mental categories. Round shapes vs. sharp shapes, light colors vs. dark colors, males vs. females—these are all concepts. Scientists have tried-and-true methods for getting at animals’ concepts, the most common being the match-to-sample testing paradigm: An animal is presented with a “sample” image, and then they must select the “matching” image among other options in order to receive a reward. For example, an animal might see a sample image of a female, then be rewarded for choosing a subsequently presented image of a female from alongside an image of a male. If the animal can learn to succeed at this task, it suggests that they possess a concept of “female.” This concept is, again, a mental category that allows the animal to recognize that some images depict a female and others don’t. In a few studies (like this one, this one, and this one) using this technique, monkeys have displayed concepts of male and female. In a similar study, where chimpanzees learned to match faces of individuals they knew to generic images of male and female behinds, the authors went so far as to call their findings evidence of a “gender construct.”
These studies are telling, but they’re not entirely conclusive. The subjects could have a full-blown, human-like concept of sex, but looking only at these tests, it’s also possible that the animals are simply learning to categorize images based on distinguishing features. Just as a sommelier learns to recognize different wines based on tannins, sweetness, and mouthfeel, subjects might be learning to recognize images of males and females based on depicted genitals, face shape, and body size rather than any social concept of the sexes.
Luckily, we don’t have to rely solely on cognitive testing; we can and should interpret the results of these tests in the context of natural social behavior in which there are plenty of examples of individuals seeming to distinguish between male and female groupmates. Alone, either of these lines of evidence—social behavior or cognitive tests—would be ambiguous, but taken jointly, they strongly suggest that chimpanzees have concepts of “male” and “female,” and, like humans, categorize individuals they know according to these concepts.
These concepts around the sexes are certainly an important part of gender, but they don’t equal a sense of gender identity—humans take these sex concepts and go further by applying them to how they think about themselves. Do our closest relatives do this? Direct evidence on this question is lacking, but some of the cognitive abilities that chimpanzees and bonobos have shown in unrelated contexts suggest that it’s possible.
Here it’s prudent to consider whether chimpanzees and bonobos have any sense of identity—or sense of self—at all. To find out, scientists have tested “mirror self-recognition”: the ability to recognize oneself in the mirror. As you might guess, chimpanzees and bonobos (along with other apes, dolphins, elephants, and some other nonhumans) show this ability, quickly realizing that the image in the mirror is a reflection of themselves and using the mirror to inspect their appearance. Scientists view this as evidence that an individual possesses an understanding of itself as an entity separate from the rest of the world. This understanding can be regarded as the foundation of a potential sense of gender identity.
A second question is: Do chimpanzees and bonobos understand that others are independent “selves” with their own internal mental lives? This understanding is really a set of abilities, collectively referred to as “theory of mind.” Chimp theory of mind is more controversial than mirror self-recognition, but the consensus view is that chimpanzees do possess this understanding, albeit probably not as fully as humans. (Again, because chimpanzees and bonobos are so closely related and have shown no major differences in cognitive abilities, we can assume the same is true of bonobos.)
So, chimpanzees and bonobos possess a sense of self and seem to understand that others, like them, have internal mental lives. And as we saw earlier, chimps seem to hold mental concepts of “male” and “female,” and categorize acquaintances accordingly. From there, I don’t think it’s implausible that chimps might apply those concepts not only to others but to their own sense of self. If—and this is a big if—that is the case, then chimpanzees possess sex roles that are not only flexible and potentially socially determined (as we saw earlier) but also tied to mental concepts that contribute to an individual’s sense of identity. If you ask me, that sounds a lot like gender.
It bears repeating that we lack direct evidence of an internal gender identity in chimpanzees, bonobos, and other nonhuman animals. But the question of gender in a nonhuman species has yet to be tackled in a comprehensive way, so perhaps a license to speculate a bit is warranted. If nothing else, it seems clear that gender in other species is entirely possible.
The more closely related two species are, the more likely it is that they share cognitive processes. And since chimpanzees and bonobos are our closest evolutionary cousins, the most scientifically sound approach may actually be to interpret ambiguous data as supporting, rather than challenging, the idea of human-like gender in our closest relatives. History has seen plenty of human-exceptionalist claims refuted. Much more research needs to be done, but in time, gender may turn out to be just one in a long list of attributes once thought to make humans unique.
Jay Schwartz is a Ph.D. candidate in the psychology department at Emory University.
Every scientist should be able to explain their research to someone who’s not in the field. Here’s my go.
Header image: Spectrogram of a rhesus macaque scream
Every scientist should be able to explain their research to someone who’s not in the field. I owe that much to my friends and family who have expressed interest in what I do, and the public whose tax dollars have gone to fund my work. With that in mind, this post is devoted to describing my dissertation, which is titled “Vocal Emotion Expression Across Contexts, Vocalization Types, and Species: Implications for General Processes of Vocal Evolution.”
I wanted to include all the background someone would need to understand why my dissertation research is important and what it all means, so this post is fairly lengthy; if you absolutely need the short version, skip ahead to the “what I’m actually doing” section. But I know that because you love me, you will read the whole thing…
Finally, as you might know, I am also working on a line of research investigating human screams—that research involves some of the themes I’ll cover here, but it is different enough from my dissertation that I’ve decided to cover it in a separate, future post. Stay tuned for that.
Animal Emotions, Feelings, and Evolution
I find emotions really fascinating; I want to understand what they are, what they do, and why we have them. And by “we,” I mean not just humans, but also other species. Charles Darwin believed that animals’ expressions betrayed an emotional life similar to that of humans. This idea caught on, leading some scientists to anthropomorphize animals in ways bordering on absurd (including examples I’ve talked about in a previous blog post). This had a chilling effect: for the better part of the 20th century, animal researchers were highly skeptical of the idea of animal emotions. While their skepticism was justified, they went too far in the opposite direction. The topic of animal emotion became taboo, and some scientists are still very hesitant to even talk about it. But the evidence is clear: advances in biology and neuroscience have shown that many nonhuman species (particularly mammals) do in fact have emotions, and these are very similar, biologically, to human emotions.
A tangential, but important point: by “emotions,” I and other scientists mostly mean neural and physiological activity: measurable things going on in your body. For example, heart rate increases, firing of neurons in your amygdala, and release of cortisol into the blood stream are all aspects of the emotion known as fear. Emotions do not equal our subjective experience of them—what we feel—which we call “feelings” (yes, that is the actual scientific term). The fact that nonhuman species have human-like emotions doesn’t mean they have human-like feelings. Scientific consensus is lacking on this. However, my professional opinion is that, because we share evolutionarily old emotions with other mammals, we should assume that feelings are also evolutionarily old, and therefore that other species have them. You might be thinking, “Of course! You just have to look into a dog’s eyes to know that.” I would respond by cautioning against relying so heavily on your intuition. We evolved to anthropomorphize others, that is, to look around and see other beings like ourselves, with minds and experiences like our own; just because we apply that evolved tendency to other species (and even inanimate objects!) doesn’t mean we are right to. That anthropomorphic bias is just that—a bias—and late-19th-century scientists going too far with that very bias is what caused later scientists to be overly skeptical of animal emotions in the first place. I believe that other mammals have feelings because the available evidence and reason suggest it is likely, irrespective of my own anthropomorphic biases.
I may be interested in emotion, but my roots are firmly in evolutionary biology, specifically ethology: the study of the evolution of animal behavior. So within the study of animal emotion, I am primarily interested in how animals’ emotions can and have influenced the course of behavior evolution. Broadly speaking, ethologists have acknowledged the emerging evidence of animal emotions with a collective shrug: “The question of emotions is interesting in its own right, but ultimately it concerns how an animal’s behavior happens. My research is focused on why, evolutionarily, the behavior happens, and so emotions are not really relevant to me.” I disagree. In fact, I believe that it’s impossible to fully understand how a behavior evolved, or is evolving, or will evolve, without understanding how the behavior occurs—including the role of emotions. I will come back to this.
The Sound and the Fury: Emotion and Vocalization
In addition to emotions (and some other things that won’t make it into this post), I’m also interested in sounds; that we perceive such complex sounds from mere vibrations in the air is one of the most mind-blowing things in the universe to me. So it comes as little surprise that my main area of research is how emotion influences the sounds that we (humans as well as other species) make.
Our emotions influence not only what we say, but also how we say it. When you are especially angry, especially happy, especially afraid, etc., certain changes happen to your voice: the pitch goes up, you talk faster, the tone changes, your voice gets louder, harsher, etc. These changes occur involuntarily, and often without conscious awareness. In the last couple of decades, scientists have begun to study whether other mammals’ emotions influence their voices in similar ways. Turns out, for the most part, they do. This suggests that these emotional changes in the voice are probably inherited, in humans and other mammals alike, from a common ancestor, likely one living over 100 million years ago. That said, this view is still tentative, and more research is needed.
What about emotions cause these changes in the voice? Our scientific understanding of this is murky, but we do know some things. Before getting into the specifics, it’s important to understand the effects emotions have on the body more generally.
Emotions involve changes not only in your psychological state, but also in your body. The best-understood example of this is the phenomenon of “arousal.” Contrary to colloquial usage, arousal doesn’t mean the experience of being turned on sexually, although that is a particular kind of arousal (called, as you might guess, “sexual arousal”). Arousal simply refers to a state of excitation, wakefulness, and alertness. Many emotions involve increased arousal, including joy, fear, and anger, while other emotions, such as sadness, are typically low-arousal. In fact, a prominent scientific view of the different emotions is that they are not actually distinct states, but just different points along the spectrum of arousal (along with 1-2 other spectra—no need to get into the details here).
Among other things, increased arousal causes changes to respiration, heart rate, and muscle tension. These changes can be helpful. Imagine an early mammal in the presence of a predator: sensing the predator nearby causes it to enter a fearful, and hence aroused, state. Increased heart rate, respiration, and muscle tension all prepare the mammal’s body to escape or fight for its life—the so-called “fight-or-flight” response. These bodily changes have, throughout time, helped our evolutionary ancestors and cousins survive and reproduce, and have therefore been favored by natural selection.
Importantly, the effects of arousal are not limited to the muscles involved in fight and flight, but are instead quite global—including, you guessed it, the muscles of the voice. Check out this illustration of the “vocal apparatus” (all the parts of your throat, nose, and mouth that contribute to the sound of your voice).
Increased arousal involves tension in many muscles in and around the vocal apparatus, and hence, changes to sound of the voice. Tension in the vocal fold muscles (formerly known as the vocal cords) can increase the pitch of the voice, while tension in the diaphragm can increase its volume, and changes in the pharynx can alter the tone of your voice, etc. There are dozens of acoustic variables like these that have been shown to change in association with emotional states, and many are associated with arousal.
We know how arousal and other aspects of emotions influence the voice in many species, but by no means all. And while there seem to be some similarities across species in terms of vocal emotion expression, we can’t assume that they’re all the same. Expanding our view of the relationship between emotion and vocal communication across species is important for a few reasons. First, there is the basic intellectual merit intrinsic in learning more facts about the natural world. Second, decoding the emotional significance of vocalizations in a species can allow human caretakers to better know what emotions an animal is experiencing, and therefore, help them provide better care. Finally, by studying more and more species, we can get a better idea of which aspects of the relationship between emotions and vocalizations are universal and which are unique. This influences our view of the evolutionary history of vocal communication, in ways that I’ll get into below. But first, it’s probably time I stop burying the lead and get into what I’m actually doing.
What I’m Actually Doing
For my dissertation, I’m exploring the link between arousal and vocalizations in rhesus macaques, an Old World monkey species native to South, Central, and Southeast Asia. These monkeys live in large, matrilineal social groups, meaning groups are structured around female families (matrilines)—sisters, daughters, aunts, and female cousins stick up for one another. Research institutions house more rhesus macaques than any other primate species, making it especially important to understand their emotions and behavior, so that scientists can care for them properly.
Rhesus macaques have a roughly average-sized vocal repertoire made up of around 10-20 distinct calls (depending on how you define a distinct call—that’s a whole can of worms). My research is focused on two of these calls, “coos” and “screams.” Coos sound like “hoo,” and range from jovial to forlorn in melody (these are very unscientific terms but they do the job here). Coos are an example of what’s called a contact call, meaning they are used in a wide variety of contexts, but mostly they function to convey that “I’m over here.” As you might guess, screams—which sound to me more like a screech than a human-like scream—are limited to intense situations, but unlike in humans, rhesus screams don’t usually have to do with extreme danger. Instead, individuals of this species scream when they are being threatened or attacked by a member of their social group. The function of screams is thought to be to let female kin know that “I’m being bullied, come get my back!”
My central question research question is, how does arousal relate to vocalizations in this species, and how consistent is this relationship across different vocalization types and different contexts? The first half of that is something that many researchers have looked at in a variety of species (although not rhesus macaques), but the second half is more unusual.
I’m addressing this question using a collection of coos and screams from juvenile rhesus macaques living at the Yerkes National Primate Research Center Field Station, located in Lawrenceville, GA. All my monkeys live in a social group of around 60 adults and their immature offspring, in a large (~180×180 ft) outdoor enclosure. I collected my body of screams by hanging out outside the enclosure and watching the monkeys through the fence with a pair of binoculars and a digital recorder and mic—every time I picked up a scream, I would note which monkey it came from and what the context was. The types of bullying that elicit screams range in intensity from a mere lunge forward to a full-on attack, and while all of these interactions probably evoke increased arousal in the victim, I’m comfortable assuming that an attack victim would experience a greater arousal increase than the victim of a mere threat. So, what I have amounts to a bunch of scream recordings representing a range of different arousal levels. This is going to allow me to examine the correlation between arousal and sound qualities of screams. To do this, I’ll be measuring around 20 sound-related parameters of all these screams, relating to pitch, tone, length, rate, etc.
My collection of coo vocalizations comes from a collaboration with the Stress, Obesity, and Diabetes project at Yerkes. One of the things these researchers are studying is the relationship between stress and behavior in rhesus macaques; to study this, they administered behavioral tests to monkeys, videotaped their behaviors, and measured their levels of cortisol, a hormone associated with arousal and stress. One of the behaviors you see a lot during these tests is cooing; as a result, I have a body of coos all associated with biological measurements of arousal. Like with the screams, I can examine the correlation between arousal and sound qualities of coos. And, as I mentioned above, I can compare the results with coos to the results with the screams, to see how consistent arousal’s effects on the voice really are in this species.
Finally, I’m also working on another project investigating how humans perceive arousal from rhesus macaque vocalizations—the same vocalizations I described above. As I live out of state, my fabulous undergraduate assistant Emma is currently running participants through this experiment. The question here is, do people know, just from listening, which of two coos or two screams represents greater arousal? Even if they’ve never seen or heard a rhesus macaque before? If so, that would suggest one of two possibilities: either we have some innate understanding of the emotional significance of the vocalizations of other species, or can use our learned understanding of emotion in the vocalizations of the species we are familiar with (humans, pets, etc.) and apply it to the vocalizations of an unfamiliar species. In either case, the takeaway would be that there is some cross-species evolutionary continuity in how arousal is communicated through vocalizations.
This idea makes sense if you think about it: the basic anatomy of the vocal apparatus is not that different across primates and other mammals, and neither is the way arousal works on muscles throughout the body, so it shouldn’t be shocking that we should see evolutionary similarities. But that remains a hypothesis that needs to be tested. By doing so in my dissertation, I’m extending the central research question—how consistent is the relationship between arousal and vocalizations across different vocalization types and different contexts?—to include different species as well.
Vocal emotion expression and evolution: Bringing it all together
It is tempting to ask, what are the evolutionary benefits of a particular sound in a particular situation? Perhaps a high-pitched call during an attack can startle a predator long enough to allow for escape? Maybe a harsher tone of voice in aggressive situations could intimidate potential rivals? Ethologists interested in vocal communication have made plenty of conclusions like these. My view is that these explanations, although perfectly plausible, are missing something—something that my research will help address.
This next part is going to be a little lofty and theoretical, but bear with me. Remember earlier when I said that you can’t understand how a behavior evolved without understanding how it happens? That holds true for vocal communication: I believe you can’t understand the evolution of the sound quality of a vocalization in a specific situation, without understanding how that vocalization is produced, including both the anatomy of the vocal apparatus and how it is acted upon by the animal’s emotions. To illustrate why, here is a hypothetical example: Imagine a population of early mammals. Some have this thing called arousal—when they are in intense situations, their whole body tenses up—and others have nothing of the sort. The individuals with this arousal thing are going to be better at escaping predators etc., and so natural selection will favor the tendency to become aroused in intense situations, right? Now, arousal may result in some bodily changes that have nothing to do with survival or reproduction—including, perhaps, changes to the voice (increased pitch, volume, etc.). These vocal changes don’t need to be beneficial, they could just happen to be consequences of the way arousal works. Nonetheless, as the link between intense situations and arousal is strengthened by natural selection, so too will be the link between those situations and these vocal changes. In other words, it could well be that natural selection has favored arousal in certain intense situations due to a few specific beneficial effects (e.g., muscle tension in the limbs), and then the vocal changes associated with arousal are simply coming along for the ride.
This process is referred to in evolutionary science as “correlated response to selection”: natural selection acts on one trait, and then as that trait evolves, other traits evolve as well even though they may not be beneficial in any way. We call the latter trait a “non-adaptive byproduct.” Correlated response to selection can only happen when the two traits are biologically linked in some way. In this case, emotion—specifically, arousal—is the critical link between the naturally selected trait (increased limb muscle tension in an intense situation) and the byproduct (changes to the voice in similar situations).
Taking the above idea further, another possibility is that any given vocal change in an intense situation might be a nonadaptive byproduct of natural selection on some other vocal change. As a hypothetical example, perhaps higher pitch is useful for escaping predators, so natural selection favors increased arousal in an intense situation, due to the effects of arousal on pitch. Because arousal also makes for a noisier tone, you might see that vocalizations during predator attacks are noisy. But it could be that the noisiness isn’t itself beneficial, but rather is a nonadaptive byproduct of natural selection favoring increased pitch (and hence increased arousal). As you can probably imagine, there are all kinds of possibilities like this; I’m tempted to keep giving examples, but I’ll leave it there as I think I’ve gotten the general idea across.
Geneticists and paleontologists have been talking about correlated response to selection for a long time—as you might guess, I think it’s one of the coolest ideas in evolution—but the concept has been much less common in ethology, and totally absent from the scientific literature around animal vocal communication. Why is that? Well, I believe that it comes back to ethologists’ tendency to dismiss the idea of emotion as irrelevant to our work. Remember, the critical link in this correlated response to selection idea is arousal, which is an aspect of emotions. If you think emotions are irrelevant, then it’s little surprise you wouldn’t consider the potential evolutionary significance of arousal.
So now I return to the central research question of my dissertation: how does arousal relate to vocalizations in rhesus macaques, and how consistent is this relationship across different vocalization types, different contexts, and different species? Another way of putting this is, how strong is the link between the different effects of arousal? Remember, this link is the critical piece in the hypotheses about correlated response to selection that I have outlined above. So among other impacts, my research will give us a much better sense of how likely this correlated-response-to-selection idea is for vocal changes, relative to the more common “oh, it must be beneficial in some way” types of conclusions that currently dominate the scientific literature in this area.
And there you have it! If you’ve stayed with me through this whole post, I commend your tenacity and thank you for reading. I hope that this has given you a better sense of what it is I’m doing, and I hope I’ve gotten you just the littlest bit excited about my research. Of course, please feel free to reach out to me with any lingering questions or comments you might have. Otherwise, see you next time.
Primatologists distinguish between three types of vocal learning: production, usage, and comprehension. Comprehension learning entails forming an association between other individuals’ usage of a call and an appropriate behavioral response (e.g., learning the meaning of a predator-specific alarm call). Conversely, usage learning is defined as experience-dependent changes in the contexts in which a vocalization occurs (e.g., learning which alarm call to utter for which predator). Finally, production learning is defined as experience-dependent changes in the acoustic structure of vocalizations. Put differently, vocal usage refers to when and where a vocalization is produced, while vocal production refers to how it is produced; vocal production learning, then, means that an animal learns how to produce a vocalization.
Human language requires a huge amount of learning at all three of these levels, as I can attest firsthand having watched my fiance’s nephew develop into a talking machine over the last several months. This real-time MRI video of the vocal tract during speech, posted by the Max Planck Institute just a few days ago, illustrates just how intricate the muscle movements we acquire through vocal production learning really are. Songbirds (as well as parrots, hummingbirds, elephants, and some marine mammals) also undergo vocal production learning: like human infants, young birds learn by listening to the adults around them. Certain species show geographic variation in song, similar to human accents or dialects. The strongest evidence for production learning in songbirds comes from lab-rearing experiments (many of which were conducted by Peter Marler, my academic grandfather): birds reared in the absence of singing adults (and hence without the opportunity to learn how to sing properly) grow up with impaired, simplistic songs. Remember this experimental paradigm—I’ll be coming back to it.
Nonhuman primates, on the other hand, are generally thought to lack vocal production learning, at least early in development. They certainly undergo vocal usage learning: an infant vervet monkey startled by a falling leaf might inappropriately utter a predator alarm call, before eventually learning when and where that call should be used. Adults usually ignore alarm calls coming from infants, due to their unreliable nature—one of many examples of vocal comprehension learning. But unlike songbirds (and humans) monkeys have been shown to acquire normal-sounding vocalizations despite social isolation, cross-fostering, and even deafness. Furthermore, the neural connections known to play a key role in vocal flexibility in humans and songbirds alike are absent from the (admittedly few) monkey species whose brains have been studied in depth. These findings have led most primatologists to the view that nonhuman primates are born with the production of species-typical vocalizations already hard-wired.
Enter the common marmoset. Marmosets (along with their cousins, the tamarins) are a group of New World primate species that have evolved a diminutive body size and subsist on insects, and sap which they extract while clinging to trees with tiny claws (an evolutionary reversal from the nails shared by most primates). You can listen to some of their vocalizations here. Due in part to marmosets’ small size, which makes them hard to study in the field, marmoset vocal communication has received relatively little scientific attention, until recently. Within the last several years, there has been a string of publications from researchers studying marmoset vocal behavior in the lab, using methods similar to those traditionally utilized to investigate bird song.
Earlier this month, Yasemin B. Gultekin and Steffen R. Hage at the Werner Reichardt Centre for Integrative Neuroscience, University of Tübingen, Germany, published the results of the latest in this string of studies, in the journal Science Advances. With the goal of examining whether young marmosets undergo vocal production learning, they emulated the experimental paradigm I described earlier: they analyzed vocalizations from three marmosets who had received only limited parental interaction, and compared them to the normal vocalizations of two parent-reared individuals. The limited-interaction marmosets grew up to produce “phee” vocalizations that were “immature,” sharing acoustic similarities with infant cries. This pattern persisted into the subadult stage of development, unlike in previous studies where vocal deficits due to limited parental interaction were temporary. The vocal differences are unlikely to be the result of general developmental stunting in the limited-interaction marmosets, because in other respects (body weight, and pitch of vocalizations), they seemed more mature than the parent-reared group (appropriately so—they were older).
These findings demonstrate that, like humans and songbirds, marmosets require social interactions with adults for the development of normal vocal production. This study provides some of the first evidence that vocal learning in nonhuman primates can go beyond usage and comprehension, to include production learning. The researchers suggest that marmosets may not be special among nonhuman primates in this respect. Rather, they argue, the widespread view of primates as lacking vocal production learning may be the result of insufficient acoustic and statistical analytical tools through much of the history of this research.
A couple of caveats. First, this study examined only five marmosets—two parent-reared and three limited-interaction individuals—which is an incredibly small sample. It’s difficult to know whether the results would hold true for other common marmosets, let alone any other species. Second, while the acoustic structure of the limited-interaction marmosets’ “phee” calls was off, they were still recognizably “phees.” This contrasts with the songs of socially isolated songbirds, which can be radically simpler than species-typical songs. Furthermore, other vocalizations (“twitters” and “trills”) did not sound significantly different in the limited-interaction vs. parent-reared marmosets. The degree of vocal production learning demonstrated in common marmosets in this study is far less dramatic than that which allows humans and some other animals to learn a huge variety of different sounds. Nevertheless, this study’s findings pose a serious challenge to the long-held view of primates as entirely lacking vocal production learning in early development.
Finally, those who care about animal welfare, as I do, may be concerned about the psychological consequences of separating young primates from their parents—which we know can be traumatic, thanks to the controversial and (I’ll say it) cruel maternal deprivation experiments conducted by Harry Harlow in the mid-20th century. Gultekin and Hage report that the three limited-interaction marmosets were not separated from their parents for the purposes of this study per se, but rather for practical reasons. One of them was rejected by his parents as a newborn, and after the other two had finished weaning, they were introduced to the rejected individual to form a new social group. My own research with howler monkeys suggests that facilitating social interaction among orphaned primates can lead to beneficial outcomes. Regardless, as long as this facility was going to limit these marmosets’ social interaction anyway, even the staunchest animal welfare activist would have a hard time objecting to these researchers taking advantage of the opportunity to investigate vocal development.
Next time someone corrects you on these terms, just respond “yeah well YOU’RE a monkey” – and know you’re not wrong.
Photo: Thomas Netsch
If you know anyone who is into primates, you know not to call an ape a monkey… or is it the other way around..? First, on behalf of all primatologists, we’re sorry. We experience a physiological reaction when you get it wrong, and can’t help but correct you. Personally, it’s all I can do to narrowly avoid barking gruff corrections at children at the zoo. Here, as an apology, I’m going to give you a sound evolutionary argument to rebut that annoying friend the next time they call you out on this. First, let’s review the commonly accepted usage of “monkey” and “ape.”
Monkeys and apes are both types of primates, as are lemurs, lorises, and tarsiers. Monkeys include a wide range of species, which we group into two categories: Old World monkeys like baboons and macaques, and New World monkeys like capuchins, howler monkeys, and marmosets (the names are self-explanatory–OWM’s are native to Africa and Eurasia, while NWM’s hail from the Americas). A good rule of thumb for distinguishing monkeys from apes is that monkeys don’t really get bigger than dogs. A better rule of thumb–the rule, in fact–is that all monkeys have tails. (So do lemurs, lorises, and tarsiers, so again, this rule is only good for distinguishing monkeys vs. apes.) Apes lack tails, are mostly larger than dogs, and are much less diverse: whereas there are dozens of monkey groups, there are only five groups of living apes: gibbons, orangutans, gorillas, chimpanzees and bonobos (they’re very closely related so they count as one), and humans. Yes, we humans are apes.
The categories delineated by the terms “monkey” and “ape” are defined by more than just superficial similarities like size and whether they have a tail; evolutionary relatedness also plays a role. In the evolutionary tree below, apes are in pink, while monkeys are in blue (Old World) and yellow (New World). We apes, as you can see, comprise what’s called a monophyleticgroup: a group of species that includes all descendants of a common ancestor. In this case, the ancestor is represented by the node labeled by the number 18, indicating that the apes’ common ancestor lived 18 million years ago.
The special thing about a monophyletic group is that all members are more closely related to each other than they are to any species outside of the group. For that reason, biologists prefer taxonomic terms to refer to monophyletic groups–this is called the principle of monophyly. In this case, because we apes are a monophyletic group, chimps, bonobos, gorillas, orangutans, and gibbons are more closely related to us than they are to any monkey. So calling a chimpanzee a “monkey” (I’m looking at you, small child at the zoo) is not only inaccurate, it can also be seen as perpetuating misconceptions and ignorance about how we are all related. If you’re gonna lump any ape in with the monkeys, then you had better be lumping humans in there too; otherwise it’s a double standard.
But that’s no excuse for rudeness. So, here’s what to say to that annoying friend when they get in your face about this. “Friend,” you say, “by your very logic, apes are monkeys!” Then pull out your copy of the primate evolutionary tree (which, I assure you, it is perfectly normalto keep in your pocket) and point out the relationships between apes, Old World monkeys, and New World monkeys.
If you look again at the evolutionary tree above, you’ll see that “apes,” “Old World monkeys,” and “New World monkeys” are all monophyletic groups, but “monkeys” is not. Apes and Old World monkeys split 25 million years ago–after their common ancestor split from the New World monkey lineage 40 million years ago. In other words, a baboon is more closely related to us apes than to any New World monkey. So your annoying friend is calling you out based on the principle of monophyly, but breaking that principle at the same time! A solution is to redefine “monkeys” to include all descendants of the common ancestor of Old World and New World monkeys (so, apes too), hence your quip “apes are monkeys.”
Quick caveat, savvy primatologists will realize that there’s an alternative option: stop using “monkey” to refer to both Old World and New World monkeys. If only one of these two groups is “monkeys,” then “monkeys” becomes monophyletic and all is well. Accordingly, some primatologists now reserve “monkey” for Old World monkeys, and instead talk about “New World primates.” However, as far as snappy rebuttals go, “by your very logic New World monkeys aren’t monkeys!” doesn’t quite have the same effect.
So now, if that explanation made any sense whatsoever, you have the tools to rebut your annoying primate-enthusiast friend. But first, take a moment to understand where they’re coming from. Part of it is monophyly, sure. But part of it, I suspect, is offense taken on behalf of the besmirched ape. When someone ignores the fact that a chimp is evolutionarily more like us than it is like a monkey, we can’t help but see it as a lack of respect. Now, of course the chimp doesn’t understand or care about that lack of respect. But for those who care about primates, especially given the context of humans’ extreme mistreatment of apes, it can touch a nerve. If we’re being rude, by all means, throw down the rebuttal I’ve provided above. Just please don’t take our reaction as mere semantics. It goes far beyond that. …Most of the time. Sometimes, we’re just being trolls.