Vampire bats will feed trapped individuals through a cage wall

Cooperative behaviors, such as food sharing, can be studied under a spectrum of conditions from completely natural to very artificial. For example, a careful observer in the field can measure the extent to which male chimpanzees will share meat in the wild after a hunt, or an experimenter in the lab can test the extent to which two chimpanzees in separate cages will pass food to each other by operating a simple mechanical device (like pulling a tray that delivers food to another individual).

There are clear pros and cons to each of these approaches, and each will give you different sorts of information about the animals. For example, field observations of cooperation, albeit natural, won’t allow you to manipulate the situation very much, and often cannot show you what enforces cooperation (i.e. what protects it from freeloading). On the other hand, controlled studies in captive settings won’t necessarily elicit the natural evolved behaviors in which we are all interested. So, for a given helping behavior in a given species, it would be interesting to see what happens when natural forms of cooperation are tested under artificial conditions.

I asked this question in vampire bats by testing if vampire bats would still share food if they had to pass it through a barrier– the metal bars of a cage. This is, of course, a completely unnatural situation. Normally, the bats groom each other awhile before regurgitating food. But if the bats are motivated to feed another individual and not just responding, perhaps they will adapt their food sharing behavior to the novel situation.

After first observing that wild vampire bats will feed unfed individuals inside their roosts, Wilkinson began inducing food sharing by keeping a bat from feeding overnight, first in the wild and then in captivity. For the past couple of years, I have been doing similar trials many times with captive vampire bats at the Bat Zone in Bloomfield Hills, Michigan. This has allowed me to map out the food-sharing network in a group of common vampire bats with varying relatedness but that all know each other equally. One of the things that surprised me is that the majority of the donors approach the hungry bats, not vice versa. This led me to believe that food sharing could not be explained as merely a response to harassment, something which you sometimes see in primates.

In the last couple of weeks, I’ve been placing the hungry bat first inside a small cage in the corner of the larger walk-in colony flight cage, before releasing it into the colony cage. Not only is having to feed a trapped bat across a barrier a novel situation, but the donor and recipient would not be able to groom each other beforehand. This is relevant because Wilkinson speculated that social grooming might allow the bats to detect cheaters by feeling the fullness of a partner’s belly. Also, a trapped hungry bat would not be able to beg or solicit a food donor. Any donating bat would have to willingly fly across the flight cage in order to feed the hungry trapped individual. For all these reasons, I was highly skeptical that this would work, but I thought it was worth testing (my main goal here was to record social calls and get fresh DNA).

Much to my surprise however, when an unfed bat was trapped in a wire cage, other individuals would sometimes come down and regurgitate food to this hungry bat by pressing their face against the metal bars. It was not so surprising that mothers sometimes did this for their grown offspring, but I was more surprised that the adult offspring would also sometimes pass their food across the cage wall to their mothers. Also noteworthy was that the food sharing across the cage wall barrier was much less common than the more natural form of food sharing– hanging belly to belly with copious social grooming before and after. Whereas the natural form occurred for almost every subject, only a few bats regurgitated small amounts of food across the metal bars of a cage.

To me, this demonstrates that even though a given species might often share food in nature, if you make the sharing behavior more difficult or artificial in a lab environment, they might not cooperate as much or even at all. It may also be true that animals that normally would not share food under natural circumstances, might then actually do so under certain lab conditions, for example, if they learned how to obtain a food reward by cooperating during the course of the experiment. This would be simply operant conditioning, not a reflection of whether this species was particularly social or predisposed to cooperation. So caution must be used when interpreting the results of such experiments.

7 thoughts on “Vampire bats will feed trapped individuals through a cage wall

  1. Fascinating. Though always so much work it would be interesting to know what sounds were emitted by the recipient. What if there was a barrier that only permitted sound. What bat (s) would approach? Relatives with full stomach?


    1. Based on my preliminary observations, it seems that the bats are using lower frequency social calls to recruit roostmates over to them. I’m recording sound for the entire session. I’ll be doing some kin discrimination playback experiments later with relatives that are both familiar and unfamiliar…


  2. Gerry, I’ve been reading the Social Conquest of the Earth, so I’m really curious about the role of kin selection vs. group selection here. It sounds like related individuals might be more apt to feed across a barrier, but perhaps relatedness is not as much of a factor when contact is possible. Do you think the donor gains something from the recipient when they have contact? Maybe just the “promise” of a future donation?


    1. I feel insanely arrogant saying this, but it’s true: E. O. Wilson doesn’t understand kin selection. Even as far back as Sociobiology (1975), he defined kin selection as not including parental care (which makes one wonder if kin selection can even occur in asexual species). Kin selection vs group selection is a false dichotomy. The theories only seem different because they use different definitions of “altruism”. This is an extremely common misunderstanding. I think, it’s because the term “kin selection” is so confusing. Maynard-Smith coined the term “kin selection” as an alternative to group selection, after reading Hamilton’s inclusive fitness theory. But “kin selection” is not really a kind of selection, it’s just a more clear understanding of what adaptations are for (increasing copies of genes, not just offspring, as Darwin originally thought). Here are 4 key papers which clarify this topic: 1, 2, 3, 4. One species where competition between groups appears to be important is Phyllostomus hastatus. I would not call this “group selection” though, but rather group augmentation, which has a more precise definition. I wrote a review about this specific for bats here.


      1. Gerry, thanks so much for sharing your book chapter, which I really look forward to reading. If anything, reading Wilson’s book has gotten me thinking more about social organization in bats. I have a student who will be studying social calls in Indiana bats now, so my horizons are already being broadened. With regards to Wilson’s ideas, perhaps his arguments are evidence for the need to clarify what kin selection really means as you’ve done above. Keep at it. I enjoy reading about your work!


  3. Gerry, I’ve been reading The Social Conquest of the Earth, wherein E.O. Wilson argues that kin selection is not the main driver behind eusociality. Thus, I’m particularly interested in the idea of kin selection vs. group selection here. It seems that related individuals are more likely to feed across bars. Do you think there is some benefit to the donor in having physical contact with the recipient whether or not they are related?


    1. Oh, yes, to actually answer your questions 😛 ….

      I think the bodily contact makes cooperation more likely in the same way that we are more likely to make a donation to a charity to help starving children when we see a picture of a starving child or when we actually meet the people we will be helping. I think prosocial inclinations are triggered by a set of cues, and by putting animals in an artificial scenario, you remove some of them. There is much evidence that subtle cues or changes in context can have big effects on cooperation. Also, I do think there’s a benefit to the donor in approaching, contacting, and grooming the receiver, since the donors usually start this process. I have my own opinions about how it benefits them (in that review paper linked above), but there still a lot of work to be done to understand the bats’ decision-making process.


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