New paper on vampire bat communication

Our newest paper is Common vampire bat contact calls attract past food-sharing partners in the journal Animal Behaviour. You can download the paper for free until June 12, 2016 here at this link:

It’s a simple playback experiment where we disentangled kinship and food sharing as predictors of a bat’s attraction to calls of different individuals. Subject bats chose between moving towards and spending time near two ultrasonic speakers pretending to be different bats. With the simulated callers were paired by kinship, we found that bats were biased to callers that had fed them more. But when callers were paired by sharing history, bats were not biased towards closer kin. The playback responses suggest that the vampire bats vocally recognized individuals, and this is a further illustration of how food sharing history can overshadow kinship in determining social bonds and behavior.

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Non-maternal allogrooming of pups

We have four new vampire bats. The bats here at the field station have been breeding in captivity, which is a good sign that they are doing well, and it ensures we have some highly related dyads for our experiments. My first intern, Jana, just took this neat video of a mother and her new pup being groomed by another female. I don’t know yet if this is a relative of the mom, but I will after I genotype everyone. This non-maternal (even non-kin) allogrooming of young pups is not uncommon in vampire bats, and I wonder if there’s an analogy to the common phenomenon of infant handling in primates.  I sped up the video except for some of the allogrooming.

The pups are born with open eyes and the ability to scramble around pretty well. The gestation period is about 7 months–pretty long! Pups grow quickly from about ~6 grams at birth to ~12 g in about 3 weeks, and ~24 g in 3 months. But the pups start out with big, almost fully-grown, feet! Adorable.

Pups also depend on their mothers for a longer period than other bat species. And for female pups, maternal care extends into an enduring mother-daughter relationship that can last many years.

In captivity, the mothers will carry their pups around while nursing them for 2 months, when the pups weigh roughly half their mother’s weight. At 4 months, the pups are flying around and drinking blood, and I’ve seen a 5-month-old juvenile regurgitate food to her mother!

Mothers stop providing milk to pups at about 10 months, after they have gradually switched them from milk to regurgitated blood. Wilkinson observed a mother regurgitating blood to her pup within minutes of its birth, which may inoculate the pup’s intestinal microbiome.  Pups are fed with regurgitated blood primarily by their mother, but increasingly also by other groupmates, especially maternal kin. Both Uwe Schmidt and I have even seen vampire bats fed by related males, such as a father or older half-brother.

Allonursing happens in captivity, although Jerry Wilkinson never observed it during his 400 hours of observation in the wild. Uwe Schmidt’s lab observed an orphaned vampire pup that was adopted by a non-lactating female. After a few days, the foster-mother began lactating and raised the adopted bat successfully [1]!

Female bats become sexually mature after their first year [2], and there is no strict reproductive season. A female can produce a new offspring about every 10 months, and individuals can live to be more than 30 years old in captivity [3]. There is even a record of a 37-year-old captive vampire bat [4]. In the wild, there are records of bats surviving at least 15 years [5], and some weaker dental evidence suggesting 18-year longevity [6]. This suggests that a female vampire bat could have have more than 20 descendants in her lifetime despite such an extreme life history strategy of high investment and slow reproduction.

Interestingly, at the captive colony I studied at Organization for Bat Conservation in Michigan, several females had very high reproductive success, while other females had no pups at all during the same 4-year-span. Given the huge geographic range of vampire bats, and given that these low-fitness females tend to come from different source populations, one explanation is that the males and females may have been sourced from wild populations that are too genetically distant.



  1. Schmidt, Christel. 1988. Reproduction. In: Natural History of Vampire Bats, edited by Greenhall, AM and Schmidt U. CRC Press. (And refs therein)
  2. Wilkinson, G. 1985. The social organization of vampire bats I and II. Behavioral Ecology and Sociobiology.
  3. Me. 2012. the oldest vampire bat. (and refs therein)
  4. On Oct 1, 2013, Guy Lichty, Curator of Mammals at North Carolina Zoological Park told me that a male vampire bat (#1272) was still alive and “records indicate that he was born 2 March 1976, which means today he is 37 years, 6 months and 29 days old.”
  5. Lord, R. D., Muradali, F., Lazaro, L., 1976. Age composition of vampire bats (Desmodus rotundus) in northern Argentina and southern Brazil. Journal of Mammology. 57, 573-576.
  6. Tschapka, M., Wilkinson, G.S. 1999. Free-ranging vampire bats (Desmodus rotundus, Phyllostomidae) survive 15 years in the wild. Z. Säugetierkunde, 64, 239-240.


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Q & A with our new interns

Every season, two volunteer interns will be assisting the vampire bat food-sharing project at the Smithsonian Tropical Research Institute in Gamboa, Panama. These are our first two interns for Spring 2016.


A month ago, eighteen-year-old whiz kid Jana Nowatzki (left) joined our project from Konstanz, Germany. Jana is a surprisingly self-motivated, bright, positive, and precocious student. Armed with an infectious enthusiasm and a curious mind, Jana has undertaken a project that involves focal sampling of bats to construct a social grooming network. While learning how to read scientific papers, she is also helping to translate some of the German literature on vampire bat research into English.


What are your interests?

I finished high school in June this year. Of course, I am happy with new possibilities for life experiences. That is why I really enjoy traveling, which allows me a closer look at the way of life in my country compared to other countries. I am really interested in the interface of human beings with their natural environment, the different attitudes of dealing with nature, what influences them, and how they arise. I am absolutely impressed by nature, ecosystems, and evolution.

What do you hope to gain from working on the vampire bat project?

I think my main idea is getting an impression of “the real working life” of research, which is hard to miss in Gamboa. Also, I wish to gain a basic understanding of how animal social structures work, why and how individuals act differently. Finally, I want to learn more about these strange little flying fur balls, and their amazing psychology and physiology.

What are your plans for the future?

I want to study biology. I’ve learned that plans change constantly, and at the moment it is hard to separate plans from dreams.


photoNext month, Yesenia Valverde (left) will be join our project from Brown University, where she is pursuing a degree in Conservation Science & Policy with a focus on tropical rainforests. After her volunteer work with vampire bats in Panama, she will serve as a research assistant at the Monteverde Cloud Forest Reserve in Costa Rica studying the ecology of epiphytes in order to predict their vulnerability to climate change.


What are your interests?

My entire life, I’ve grown up visiting family in Costa Rica and have always felt myself attracted to tropical rainforests. As I began to learn about current environmental issues, my admiration of nature evolved into a passion as I dedicated myself to learning more in order to join the effort to protect it. I’m especially intrigued by wildlife ecology and love to learn about and see new animals out in the field. When I’m not trying to save the rainforest, I really enjoy spending time with family and am currently really into improving my cooking skills!

What do you hope to gain from working on the vampire bat project?

I’m hoping to walk away from this experience with a lot of new knowledge about bats in general. Despite their bad reputation, bats are incredible animals that are vital to, among other things, the proper ecosystem functioning of tropical systems. I hope to gain valuable research experience during this time, especially learning how to mist net. I plan to mist net birds in Costa Rica for my senior thesis project later this year. I’m excited to further my development as a scientist as well by designing and carrying out behavioral experiments in order to answer fundamental questions.

What are your plans for the future?

After graduation, I plan on seeking field positions in wildlife research as part of my never-ending quest to gain more research experience. I want to explore different opportunities so that I can be sure of what it is that I want for the rest of my career. After that, I plan to go to grad school and pursue a PhD in Conservation Biology.


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An essay about caves and the origins of echolocation

Caves and the origins of echolocation

Imagine that you are in a cave, a very familiar cave, but with no light. Do you think you could collect information about your location by shouting or clapping and listening to the echoes? Would a large chamber sound different than a tight passage? Sound ridiculous? Try this experiment. Make a loud constant “shhh” sound as if telling someone to be quiet. Now close your eyes and move your hand back and forth in front of your face as if waving. Hear that? What you are now doing is very simple version of biosonar, or echolocation.

Bats do it way better. Most cavers know that bats use echolocation– a kind of sonar. But perhaps it’s demeaning to call what bats are doing “sonar” (an acronym for the human technology of SOund Navigation And Ranging), because comparing a bat’s echolocation to our military sonar is like comparing human vision to an earthworm avoiding light. Human-engineered sonar can detect submarines, but echolocating bats can detect miniscule flying insects, minnow fins protruding from a water surface, the tiny water ripples made by a calling frog, even the gossamer strands of spider webs. How do they do it?

Most echolocating bats produce a very loud chirp that sweeps across a precise range of frequencies. And when I say loud, I mean loud— louder than a smoke alarm or a circular power saw. Some bats actually disengage their ear bones to avoid deafening themselves. But the pulse is so brief and so high-pitch, your ears cannot perceive it. When the echoes reflect off an object and come back to the bat, she can form an image of what’s in front of her, whether it’s a cave wall or a moth. Note that a cave wall, especially a large flat surface, is much easier to detect than a moth, which is one reason why I think bat echolocation first evolved in caves.

Right: A bat flies through the dark of a cave with its mouth open. Photo by Brock Fenton.

In many ways, a cave is the perfect environment for a primitive form of echolocation to get started. Many animals take refuge in caves to avoid predation. If you’re a bat from the Eocene, caves are pretty safe and environmentally stable places to, literally, hang out. The problem is that deep inside where it’s safest, it’s also absolutely dark. But solve that problem and you can make a living deeper and deeper in a cave. The hard solid walls of some caves can produce crisp clear echoes compared to a cluttered leafy forest or open air. Actually, the easiest environment for echolocating would be inside a building with flat smooth solid walls. But alas, caves are the closest thing that Mother Nature gave bats. There are only a few natural environments where vision is absolutely useless, and the two that most easily come to mind are underground and under deep or murky water. And echolocation evolved independently in both these places. Bats and toothed whales both “invented” sonar in their own way.

Cave-dwelling animals have also independently evolved the ability to echolocate several times. This happened at least twice in two different groups of birds, oilbirds and swiflets, which both live in caves (Brinklov et al. 2013 Frontiers in Physiology). And as you’ll read below, it happened at least three times in cave-dwelling bats.

Do bats see with their ears?

The story of how people discovered bat echolocation is actually quite interesting. In 1793, an Italian scientist named Spallanzani found that flying owls would crash into obstacles in absolute darkness and they needed a bit of candlelight to avoid them. Bats on the other hand could avoid the same obstacles even in pitch-black darkness. He then did a rather unsavory experiment. He poked out the bats’ eyes and found that they could still somehow magically “see”. Further experiments showed that by plugging their ears, the bats could no longer orient and would crash into objects in total darkness. Spallanzani’s conclusion was simply that bats… somehow… see with their ears. Of course, this seemed ridiculous.

The puzzle was finally solved more than a hundred years later in 1939-1941 by a young biologist named Donald Griffin who realized, using fancy new microphones, that flying bats were making high-frequency ultrasound beyond the range of what a human was capable of hearing. So the story goes that when Griffin first presented his results at a scientific conference, his fellow scientists thought he was crazy (just like Spallanzani). In Griffin’s words,

“One distinguished scientist was so indignantly incredulous that he seized [my colleague] by the shoulders and shook him while complaining that we could not possibly mean such an outrageous suggestion. Radar and sonar were still highly classified developments in military technology, and the notion that bats might do anything even remotely analogous to the latest triumphs of electronic engineering struck most people as not only implausible but emotionally repugnant.” (quote from p. 35, Dawkins 1986 The Blind Watchmaker)


We now know that there are more than 1,300 bat species, and most of them echolocate. Different bats echolocate in very different ways. Most echolocating bats produce the echolocation sounds with their larynx, just like when you speak or sing. The local bats of the Eastern USA all scream loudly through their tiny mouths, but some bats in other parts of the world also send the signals out through their nose– which is one reason why their faces are often so weird-looking. To see what I mean, just do a Google image search for “bat noseleaf”.


Left: A bat that echolocates through its nose leaf

Natural selection creates two kinds of bat echolocation

This laryngeal echolocation later evolved into two distinct forms. The original “design” was to produce a series of brief pulses (each about 1/500th of second) separated by much longer pauses. For the bat, this is a bit like a strobe light, except the bat uses the timing of echoes to determine distances. So imagine a strobe light where you see near surfaces in first, then farther surfaces second, than even farther surfaces third and so on. (Now, for all I know, the bats might compile all these images into a single movie image, just like your brain puts together the illusion of a large visual field from a mosaic of your many focused eye movements.)

Some bats evolved a completely different kind of laryngeal echolocation, where they emit a long signal and then use Doppler shift to detect objects. That is, they use changes in frequency, not time, to detect objects and how they are moving. This is similar to how an ambulance that is approaching you sounds different than one that is driving farther away. This kind of bat echolocation is better for longer distances and hunting insects in open environments. Amazingly, it evolved twice independently in two completely unrelated groups of bats. (This is, by the way, not that strange: two different groups of bats also independently “invented” suction cups on their thumbs to cling to smooth surfaces. Google it.).

Anyhow, that’s all laryngeal echolocation. Laryngeal echolocation has become astonishingly sophisticated due to an evolutionary arms race with bat-detecting moths. But that’s a whole other story. Our story continues in a different direction. One group of bats, the Old World fruit bats, stopped living in caves, started hanging in tree branches, developed great vision, and started eating fruit. No one knows in which order those things happened. But they eventually developed big cute eyes and they are often called “flying foxes” because that’s what they look like. In fact, they look and act so different from other bats that biologists once classified them as something else entirely. (It’s possible that the ancestors of flying foxes never had echolocation and that their close relatives evolved it independently, but I think the more simple explanation is that the ancestors of flying foxes simply lost the ability to echolocate. That’s a common theme in evolution: use it or lose it.)

Echolocation by tongue

Ok, now comes a plot twist: one group of these non-echolocating flying foxes eventually moved from trees back into caves and re-evolved the ability to echolocate in a completely different fashion!

Instead of using their larynx, they click with their tongues (like those cool clicking languages used by some human tribes in southern Africa). Researchers use to think that tongue-clicking echolocation in bats was pretty unsophisticated, but work by the biologist Yossi Yovel showed that the bats actually steer and adjust their sonar beams in complex ways much like the more advanced laryngeal echolocators (Yovel et al 2010 Science, 2011 PLOS Biology).

There are some small flying foxes, like the flower-feeding Dawn bat, that don’t seem to echolocate at all, even though they live in caves. Or so we thought.

Echolocation by wing

In 1988, a biologist named Edwin Gould, who was mentored by Donald Griffin (the guy who first discovered echolocation), observed Dawn bats leaving caves in Malaysia and noticed that they made clicking sounds when in total darkness deep inside the caves but not near the lighted entrances. He did some experiments and concluded it was not tongue-clicking but wing-clicking. He then put paint on one wrist of some bats and found that it was transferred to the other wrist, so he hypothesized that the bats were actually clapping their wings together (Gould, 1988, Journal of Mammalogy). Nobody followed up on this work for 25 years, and at least some bat biologists thought this idea was a little crazy (sound familiar?).

temp4Left: A Dawn Bat. Photo by Alyssa Stewart.

Arjan Boonman, a postdoctoral researcher working with Yossi Yovel, now a professor at Tel Aviv University, followed up on Gould’s work and found that yes indeed—the Dawn bat does echolocate with its wings, along with two other supposedly non-echolocating fruit bats they tested (Boonman et al 2014 Current Biology and comment). The team showed that these bats were not actually clapping their wings to produce the sound, but they failed to figure out exactly how the clicks were created. So that’s one puzzle for the future.

But the team did clearly show that this rudimentary form of echolocation is pretty primitive. That is, it’s not great. The bats could detect and land on a flat solid surface that was one meter squared, but they struggled to detect smaller objects and they helplessly crashed into thin wires.

So, in summary, to navigate the pitch-black total darkness of caves, bats have evolved at least three different kinds of echolocation: wing-clicking (bad), tongue-clicking (good), and vocalizing (great).

There is one fact, however, that is a bit inconsistent with my simple story that caves created the necessity for the evolution of bat echolocation. Only the Dawn bat, one of the three wing-clicking bats that Boonman and his team discovered, was a cave dweller. It was the most frequent clicker and the best performer. But the other two species roost in foliage. What does that mean? I’m not sure. It’s still unclear how many kinds of Old Fruit bats can echolocate with their wings. One possibility is that all Old World fruit bats can echolocate with their wings, and that some of them later developed this ability further to varying degrees. Once a bat could do this well, it might be much easier to start echolocating using its mouth, because it’s already able to process the echoes. Boonman and his coauthors point out that primitive echolocation is more easily “evolve-able” than we once thought; it only requires that animals make some kind of sound (like clicks or squeaks) and then can make some sense of the echoes. So even though not all bats have sophisticated echolocation, perhaps many or all have rudimentary echolocation? After Boonman’s recent work, we may have to re-evaluate the whole evolutionary history of echolocation.

Echolocation for all (even humans)

Boonman and Yovel also pointed out that many mammals can probably be trained to echolocate. Humans are a prime example. Indeed, many blind people have learned the ability to echolocate to get around and do all sorts of useful things. Ben Underwood was a blind kid without eyes who could echolocate well enough to ride a bike and shoot a basketball into a hoop. Look it up on Youtube and have your mind blown. Daniel Kish is a blind echolocator who teaches echolocation to other blind people. He started a non-profit called World Access for the Blind and has teamed with researchers studying human echolocation who claim that in just 2 hours a day for 2 weeks a student can learn to detect large objects using tongue clicks, and in another two weeks, they can discriminate trees from walls.

So there you have it. If you ever want to have the ultimate backup to your headlamp, just remember that you can always train yourself to echolocate so you can tongue-click, hand-clap, or yelp your way through a cave without a light. But it might still take a few million years of living and evolving in the darkness before humans can reach bat-level and hear the soft echoes reflecting off a moth or a spider web.

Gerald Carter is a PhD Candidate at The University of Maryland a postdoc working with John Ratcliffe at University of Toronto and Rachel Page the Smithsonian Tropical Research Institute. He is not an expert on echolocation.

Feb 4, 2015. Written for The Speleograph, the publication of District of Columbia Grotto, a local chapter of the National Speleological Society. When I lived in Maryland, I went caving with the DC Grotto of the National Speleogical Society. I wrote an essay about caves and echolocation for their newsletter, the Speleograph, but I don’t think it was ever published. This essay came from a ‘what-if’ conversation I had with someone in a cave about humans using echolocation to navigate caves without a headlamp. I argued that it’s easier for us to hear echoes inside a building with hard flat walls, rather than say in a forest. And since caves were the only ‘indoors’ before humans existed, echolocation probably first evolved as a way for animals to live deeper in caves. So I thought I would write an article about that idea using some neat stories from bats. Now, a year later, I just found it sitting on my hard drive. 


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What can vampire bats teach us about human cooperation?

I have been asked this question several times by journalists and people during outreach events. So here’s my answer:

If you really want to understand human cooperation, you should study humans. Specifically, we should study how humans cooperate with each other under natural circumstances across a wide diversity of cultures. And we should manipulate the factors that we think drive natural forms of human cooperation to see if we can make cooperative behaviors go up and down. Such experiments must be smarter than the participants. It’s no good to merely ask experimental subjects “How much would you sacrifice to help a stranger in situation X?” and expect that the person won’t take into account the very real potential costs and benefits of being watched or judged, and the fact that the sacrifice is hypothetical and not real. The experiments must also not interpret confusion as choice. For those interested in evolved human nature, the experiments must also be natural enough to mimic scenarios that would have been important throughout our evolutionary past. Human brains might not be designed for one-shot anonymous economic interactions, just like we are not built for social isolation (or perceiving faces as inside out).

If you want to understand the evolution of cooperation more generally, you need multiple approaches, including both theoretical and empirical work with a diversity of organisms. Vampire bats are just an interesting piece of that complex puzzle.

Of course, studying vampire bats won’t tell us how humans cooperate. But they might give us general insights into how cooperation works in a long-term social relationship. This is simply because, compared with people and other primates, cooperative relationships in vampire bats are easier to measure and manipulate.

Vampires groom and share food with each other in a small dark corner of a cave or tree, which can be simulated in captivity. They are small, so you can easily house many in a lab space. In that way, they are a bit like highly cooperative lab rats. And do not underestimate what we have learned about humans from rodents! Obviously, we can do experiments with bats and rodents that we can’t do with humans. But by using simple tractable “model organisms” like monogamous voles, we have also gained extraordinary insights into the biological basis of complex behaviors such as romantic attachment and empathy.

Finally, and paradoxically,  I think it’s often easier to understand cooperation in a strange alien system like bats, because I am less self-deluded about how well I understand them. The study of humans always comes with the curse and blessing of being the subject. Like every human being, I have powerful intuitions about myself and about human nature: I think I understand human cooperation way more than I actually do. I think I know why I do what I do and why I care about what I do, but I don’t really. I’m overconfident that my subjective experience gives me some direct insight into my own and others’ cognition and behavior. We have intuitive folk wisdoms and ideologies about human cooperation that must be largely discarded before we can even think clearly about it.

With vampire bats, it’s easier to admit ignorance. I only have hypotheses about the function of their behaviors, not ideological beliefs. I have no idea how they think, or how similar or different they are to people. It’s a total mystery. The only windows I have are the experiments.





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  • Jan 14, Bambi Seminar, Smithsonian Tropical Research Institute, Barro Colorado Island, Panama — “The Reciprocity Controversy”
  • Jan 19, talk at University of Washington, Psychology Department — “Why do vampire bats share food?”
  • Jan 20, talk at University of Washington, Psych Dept, Animal Behavior Group — “Reciprocity with and without social bonding”
  • Feb 8-9, I’ll be at University of Toronto, where I was just hired as a Postdoctoral Fellow by Dr. John Ratcliffe. John is a cognitive ecologist, which means he studies how natural selection shapes cognitive traits. For example, he showed that, unlike other animals yet studied, vampire bats don’t form taste aversions because live blood can’t be adulterated or toxic, and he has done interesting work (with me and others) supporting the notion that a bat’s foraging ecology can shape how it learns. While working for John, I’ll remain at the Smithsonian Tropical Research Institute in Panama with my study colony of common vampire bats.


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Is the ingroup-outgroup bias just two points on a social distance spectrum?

Looking into the human literature on the evolution of cooperation, I feel that studies on humans are often conducted and interpreted poorly compared to studies of cooperation in ants, bacteria, fish, and other nonhuman primates. One point of confusion involves wrong assumptions about what individual humans should maximize and how well they should do it. But another (which I will discuss here) is an overwhelming focus in the social sciences on human groups as homogenous entities in conflict with each other.

According to much of the social psychology literature, humans conceptualize others into an “ingroup” and “outgroup”. This intuitive tribalistic categorization is considered a basic facet of human nature, and many research articles have been written about its causes and consequences.  I will give a couple of random examples. This paper in Science states,

Humans regulate intergroup conflict through parochial altruism; they self-sacrifice to contribute to in-group welfare and to aggress against competing out-groups. Parochial altruism has distinct survival functions, and the brain may have evolved to sustain and promote in-group cohesion and effectiveness and to ward off threatening out-groups.

A paper I just saw yesterday on learning and empathy,  states,

Deficits in empathy for out-group members are pervasive, with negative societal impact. It is therefore important to ascertain whether empathy toward out-groups can be learned and how learning experiences change empathy-related brain responses. 

An implicit notion is that empathy is essentially a dichotomous variable extended to the ingroup but not the outgroup. The prototypical experiment demonstrating the ingroup bias will divide participants into arbitrary groups like “red team” and “blue team” which then predictably leads to all manner of competitive attitudes and biases wherein the ‘reds’ are more judgmental towards the ‘blues’ and more generous and empathetic to fellow ‘reds’. One of the original classic cases is the Robbers Cave study.

This ingroup vs outgroup phenomenon is important, but it’s often over-emphasized to the point of distorting the complexity of the evolutionary design of human social cognition. For example, some influential authors believe that the bias towards cooperating with groupmates and aggressing against outgroupers amounts to a group-level altruistic trait that is only explainable by some form of group selection. A multi-level selection model would assume that, although ancestral humans spent much time competing with fellow groupmates for mates and resources– the real key to their survival and reproduction was successful competition with other tribes. The idea here is that competition between tribes was so severe that it led to heritable tendencies to put the average reproductive success of the group ahead of one’s own relative survival and reproduction within the group.

But humans are not eusocial insects! It is not our evolved human nature to give up one’s life for the group. Humans tend to help their group, because doing so simply helps their own direct fitness. Our tendency for human tribalism is mutually beneficial, not altruistic in the evolutionary sense*. I would hope everyone would agree on these basic points, and yet I’m not so sure when I read the literature.

(*One point of confusion here is that multi-level selection has a different definition of altruism, which allows this human tribalism to be called “altruistic” whereas the orthodox inclusive fitness definition means we can never be as “altruistic” as ants and slime mold. I think this is one additional reason that many social scientists prefer the multi-level selection terminology that most evolutionary biologists don’t prefer.)

Human evolutionary history is not a mere series of tribal wars. Selection continues on between these conflicts. Given that the majority of social behaviors occur ‘within-group’, it is unclear whether the ingroup fitness effects are overshadowed by the fitness effects of intergroup conflict (as assumed in the narrative given above). Within each of our ‘ingroups’, we are still closer to some group members than others (although one might not even detect this fact if one is focused solely on intergroup conflict). A machine well-designed for navigating a social world (i.e. a human’s social brain) would surely possess adaptations for dealing with cooperation and conflict within the group, and not simply sit idle waiting in anticipation for the next tribal war.

Upon closer inspection, a social group is really a social network. And upon even further inspection, the links in that network are weighted differently and change over time. A more nuanced individualistic approach is therefore to imagine that people place others on a spectrum of dynamic social distance, with close friends and family on one end and threatening strangers on the other.

So what happens when experimenters place strangers into red and blue teams? They are sampling two points on that spectrum, thereby creating the discrete group mentality that they claim they are revealing. A complex continuous variable is reduced to a simple discrete one.

A similar mistake is easy to make when we think about discrete personality types. For each personality trait, say extravert vs introvert, there is a continuous spectrum which probably approximates a normal distribution. Most of us are not truly ‘extraverts’ or ‘introverts’ but are rather just average and in the middle. But I’m always surprised by the number of people who think that people actually come in these distinct categories or types and that being moderate or average is therefore rare: “Are you an introvert or extravert? You must be one or the other!”

If you surveyed people who considered themselves definite extraverts or introverts, then placed them in two groups, and tested their behavior–you would see an exaggerated difference between these groups for two reasons. First there is the biased sampling. Second, there is a priming effect. The result would obscure the real complexity and flexibility of human personality often taken for granted: most of us are extraverted or introverted to a near optimal degree for the current social situation.

The same thing is true of human cooperation. Humans are obviously very good at flexibly treating others as either competitors, collaborators, or a mix of both. Of course it makes sense to act tribal when you are divided into warring tribes. But the real question is, how constrained are people in acting this way, when it would go against their own self-interest?

The ingroup-outgroup bias might be reducible to simpler cognitive biases. The first is that people generalize or stereotype people, places, and things into various categories. People prefer to chunk continuous measures into discrete types. It’s easier for us to think in chunks. This is one of the facets of psychological essentialism, where even young children intuitively assume that concepts like bird, female, or red have an underlying discrete reality, even without evidence. This pervades much amateur thinking about biology (e.g. “A virus must be alive or not, so which is it?”).

Another cause of tribalism is the acquisition and enforcement of social norms, which are themselves properties of groups. The difference between adherence to social norms and group loyalty can be subtle (or even identical) when groups are identified by social and cultural norms. Human social groups are often not defined by actually knowing the individuals in the group, but rather by markers such as dress (football jerseys, suits, or tribal garb), language or dialect, or by belief systems such as religions or political parties. In these cases, each individual could switch groups. So I could become a Dallas Cowboys fan, or join Islam, or become a citizen of Spain. But in other cases, group membership is more fixed, as when it is defined by biological traits like race or sex. But here again, the neat-and-tidy discrete lines between ingroup-outgroup may break down. A person might appear to be more “racist” towards a homeless black person than towards a black person wearing a suit and tie. In other words, being a certain race or religion is perhaps not an absolute cue of being an outgroup, it is just one of many cues that the person’s brain uses to decide how socially distant an individual is to them. The subconscious machinery might function to assess questions like: How likely is this person to be an ally to me in a conflict? If a tribal conflict breaks out, what will it be about? And will we be on the same side or opposing sides?

If I’m correct about this, then people’s ingroup-outgroup bias should itself be highly malleable. In a situation where I think I might be caught in a political war, I will suddenly see my country as my ingroup. But if I see a religious war brewing, then I might then see my religion as my ingroup. That is, our loyalties should be context-dependent. If you and I are different races/sexes/religions in one tribe, we will largely forget that when we join forces to fight an army from a completely different tribe. Then when the tribal war is over, those race/sex/religion issues will re-emerge.

But I might be wrong. Perhaps in our evolutionary history, groups were so stable and homogenous that such strategic social positioning was never necessary. Perhaps social group identity is more real and stable than I am imagining. I don’t know. Do people have one set of social instincts for regulating inter-group conflict and another set for regulating intragroup conflict? Or do we have just one set for both situations?

Is the ingroup-outgroup bias a separate evolved cognitive bias serving its own evolutionary function? Or is it merely a byproduct of intuitive essentialism and adherence to social norms? (Or is it somewhere in the middle? 😛 )

One way to answer these questions would be to describe the distribution of perceived social distances in a person’s social network. Are there two discrete humps? One for ingroup or one for outgroup? Or is there a smooth curve? How does this distribution differ in different societies around the world? Social distance itself could be measured either objective or subjectively– and that difference itself might be interesting. Perhaps this has all been done already? Let me know in the comments.

I need to read more of the literature about human social networks. But so far I don’t see people answering the questions that fill my mind.

My own concern is more that the ingroup-outgroup distinction sadly oversimplifies the conversation about human cooperation. If you give people simple questions (ingroup or outgroup?) you will get simple answers. But real social life is full of nuance, as we all recognize simply from being human. Ponder the strategic logic underlying these phrases:

“A friend in need, is a friend indeed.”

“The enemy of my enemy is my friend.”

“A friend to all, is a friend to none.”

There are even less obvious facets to the adaptive design of human friendships. For example, two friends who are better friends with each other than with you, are both less ‘valuable’ to you than two friends that don’t know each other (see the evidence). Even nonhuman animals appear to make strategic social decisions about bonding that are contingent on their shifting place in a social network. In baboons, females are more willing to make new friends when a close relative dies. In vampire bats, those individuals who form more non-kin relationships do better when their kin partners are unavailable to help them.

Many primates not only create and manage their own ‘ingroups’ but they probably manage each social relationship individually according to what is happening in all their other relationships (much like in a market). And moreover, in each population, there might be different social management strategies that are under frequency-dependent selection. Rather than focusing so much on ingroup-outgroup bias in humans, these are the kinds of design principles of social cognition that, in my opinion, would be more illuminating to investigate.

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