Social benefits of non-kin food sharing by female vampire bats

My new paper just came out in Proceedings B. For now, it’s freely available to download at the journal website here.

The paper describes an experiment that ‘failed’ in one sense but yielded another very neat finding nonetheless. The main goals was to detect for contingent reciprocity between close relatives. I kept several pairs of mothers and daughters from feeding each other for 3 weeks. The experiment should have been longer than this, but it was cut short. The positive result would have been evidence that all the bats began strengthening their relationships with other females at the expense of investments in the partner targeted by the experiment (a subtle form of partner switching). The negative result would be no response. What I found was that about half the bats did what I expected, and the other half actually invested more. You can read about that in the interview below. Basically, I consider that result a ‘failure’ because it needs further examination, and I can only speculate as to what it means. So I still don’t know if you can get evidence for partner switching among close kin. You need a better, longer experiment.

The other neat result came out exactly as predicted. When I took away a primary donor, the bats that had fed more non-kin females in previous years coped better: they had more donors and hence were fed more. This is what you would expect if bats feed non-kin because it increases their social safety net of reciprocal benefits. Both these results are shown visually in my previous post with slides from a conference talk.

There’s also a Nat Geo article about it here. The original unedited email interview by the journal article is below:

Hi Michael,

1. How did you first get involved with studying bats? And of all bats, why vampire bats?

I have been a bat fanatic since I was two or three years old. One of my earliest memories was seeing a large flying fox up close in the Philippines (my mother is from there) when I was just a toddler. Ironically, this is because the bats were being eaten!

I have been fascinated by bats from grade school to college to grad school. I discovered vampires during my undergraduate at Cornell. I took on a project to sequence prey DNA in vampire bat feces. When I started working with vampires, I immediately became enamored with their intelligence and their unique biology. Just about everything about vampire bats is interesting.

2. Prior to you and Jerry Wilkinson doing the work described in this study, what did we know about vampire bats’ feeding behavior?

I assume you mean food sharing behavior–
Jerry Wilkinson (my doctoral advisor) did this now-classic multiple-year field study on vampire bat food sharing, which was popularized in many textbooks and popular books such as the The Selfish Gene (Second Edition) by Richard Dawkins. He did this study for his PhD around the year I was born. I came to University of Maryland specifically to work with him and follow up on his work.

Basically, what Wilkinson found is two key things. First, vampires regurgitate blood to feed other adults (just like mother birds feed chicks). These blood “donations” are predicted by co-roosting association and by genetic kinship. Each of these factors was predictive, even when you controlled for the other. Second, he put created a captive group of non-relatives and simulated a situation where each failed at getting blood; he fasted one bat per night. The bats reciprocated donations more than expected by chance. From these findings, he suggested that bats help others, not only because of genetic kinship, but also because it would encourage reciprocal donations. This was of the first studies suggesting that reciprocity could be occurring even among related individuals. The idea that genetic relatedness can lead to altruism is one of the most well-supported findings in evolutionary biology. So when biologists see cooperation between animals that are genetic kin, it’s simple to say, “they cooperate because they are related” but of course it might be more complex.

Over time, Wilkinson’s conclusion that vampires share food because of reciprocity has drawn a lot of attention and also controversy. That’s why I wanted to look at it in more detail.

In a previous study we published in 2013 (in this same journal), we fasted a lot more vampire bats on a lot more nights. These were related and unrelated adult bats that were all familiar with each other. Under these circumstances, the bats’ decisions to help were based on their past social experience (things like receiving food or grooming in the past) not genetic relatedness. The best predictor of sharing from bat A to B was the sharing rate from B to A. To me this suggested that kin selection could not be the whole story for vampire bat food sharing.

3. Walk me through how you raise vampire bats in captivity. How do you keep them? Where do you get the blood? Give me a sense of what it’s like to stand in the enclosures and do your work.

I have worked with captive-born bats from zoos and with wild bats captured in Mexico, Belize, and Trinidad. The bats in this study are cared for by the Organization for Bat Conservation in Bloomfield Hills, Michigan. This is a great organization that supports conservation and research on bats. The advantage of these captive bats is that we know their maternal pedigree and their social history (every other vampire bat they have ever interacted with). The blood they drink is donated from an organic hormone-free slaughterhouse.

[NOTE: The article (and perhaps my response here) makes it seem like I started this colony, but I didn’t; I came to work at OBC’s vampire bats because I was invited by Rob Mies who saw me talk at a conference.]

When you walk into the enclosures, the older vampires all open their mouths wide to echolocate, so they can “see” you with their biosonar. A few of the younger bats will actually fly over, land on me, and run all over my body. That’s only because they were used to me being in there. This is something vampire bats will do to a large prey animal like a cow or horse. I really like it when they do that. They have never bitten me to get blood, but if I stayed still long enough in the cage or fell asleep, I think they would try to feed on me, even though these guys drink cow blood out of little spouts.

Most of my time is spent crouching on the floor outside the cage and watching them socialize using an infrared-sensitive camcorder. But I have it easy. Jerry Wilkinson spent hundreds of hours laying on his back amidst cockroaches and bat guano, directly below a wild vampire bat colony.

4. If I read your paper right, females that fed more bats who weren’t her kin received more food in turn, and they had more donors — but this doesn’t hold for males, or related bats of either sex. Why do you think that the non-kin female relationship is special?

It’s not that the non-kin female relationship is special, it’s just that if you want to have a wide social network, those are the relationships you need. If you’re a female vampire bat, you have a pretty limited number of maternal relatives, but many other non-relatives or distant relatives that you can form bonds with.

In the wild, male bats don’t form long-term social bonds. They leave their natal social group once they reach reproductive age. Food sharing is really more of a female behavior in vampire bats. Males sometimes feed females and vice versa, but males almost never feed males.

5. Figure 4 struck me for the starkness of the bimodal response: donor bats either dramatically upping or decreasing their donations once they were able to donate blood once again. What’s going on there?

I’m not 100% sure yet. I would like to repeat this experiment for a longer period and with more bats. One possibility is that only some of the bats are starting to shift their investments over to alternative partners, as we expected if the bats are basing their cooperative investments on their returns (gradual “partner switching”). And perhaps the other bats are actually sharing more with each other because they could not before, just like how you might be especially generous to a friend if you were unable to help them for a long time. I think we called this “relationship repair”. At this point, it’s still very speculative. I would need to see more data before I drew any strong conclusions.

6. Outside of feeding behavior, how else do vampire bats display sociality?

They groom each other. They cluster for warmth. They produced vocalizations (‘contact calls’) when they are isolated from the group. They might also follow each other to feeding sites. I have seen two vampire bats feed from the same wound. In fact, I have even seen one vampire bat crawl unto the back of another to reach a high wound on the leg of a chicken.

7. At the risk of oversimplifying your paper, it strikes me that socially speaking, female non-kin bats seem to play a long game, with feeding events affecting reciprocal behaviors weeks after the fact. How does that change our understanding of bat sociality — and influence our understanding of sociality as a whole?

Yes, I do think vampire bats are playing “the long game”. They can live to be more than 30 years old, and even in the wild, we have seen the same female vampire bats roosting together after more than a decade. Vampire bats were often the textbook example of reciprocity as “tit-for-tat” in the prisoner’s dilemma (an important game theory scenario where cooperation is difficult). The idea is that two animals might take turns helping each other, even though they suffer a risk at being exploited if the other doesn’t reciprocate. Reciprocity across repeated interactions is a good, stable strategy for cooperation (even in the prisoner’s dilemma). But in nature, It’s hard to find any scenarios in nature that really match all the assumptions of a prisoner’s dilemma. Cooperation rarely occurs in discrete rounds, and animals often have more than one social partner. Most importantly, they might have a foundation of past social experience– a social relationship.

When researchers studying cooperation have wanted to test the hypothesis of reciprocity in some animal species, we typically test whether two individuals will take turns pulling a lever to deliver a food gift to a partner (or something like that). If you and I are playing a cooperation game, the expectation is that I will reply to what you did to me in the last round. If you cooperate, I cooperate. If you defect, I defect, etc.

But what if you and I have been friends a long time? Well, it might take much longer for you to see me begin to stop cooperating with you.

I think we will find that many long-lived social animals, like cetaceans, elephants, vampire bats, ravens, and of course nonhuman primates, have cooperative social relationships that are functionally analogous to human friendships. I think we will find that they are reciprocal in the long-term but not strictly alternating. And the cooperative services are varied and diverse, like a human friendship. Friends are, in many ways, the family you choose. And as Aristotle said “A friend to all is a friend to none.” Human friendship is not a equal group-level behavior, you invest in certain relationships to the exclusion of others. These are important properties for when thinking about how cooperation might work in other species.

[Two more general questions]

Yikes, I have run out of time and I’m late for something. So I’m going to skip this question and the next one for now. I will write more later if I have time.


Thanks for your interest in my article.

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