Common vampire bats regularly share food in order to survive and it appears that their social bonds with food-sharing partners take priority even over their own kin.

Vampire bats may be portrayed by Hollywood and pop culture as terrifying creatures of the night or as Draculas in disguise, but in reality these furry little mammals are far more benevolent and affectionate than many of us know.

In fact, common vampire bats (Desmodus rotundus) regularly form stable social relationships with a few close friends. These bosom companions will roost together, groom each other, and (most importantly) share precious meals of blood.

Hunting can be hard work for these little guys and a vampire bat will starve to death after missing only two or three nights of feeding. Yet if one bat returns to the roost after a successful meal and notices one of his or her companions seems poorly, the satiated bat will gallantly regurgitate some blood to keep his or her pal going—after all, what are friends for? When by chance their roles are reversed someday, the favour will be returned, and thus the bats forge a tight relationship based on mutual food-sharing generosity. Blood brothers, one might say.

Evolutionarily speaking it’s sometimes tricky to pinpoint how altruism can develop in a species when being greedy and selfish is almost always the best way to get ahead, and so when cooperation does evolve it’s often because the individuals are genetically related. One might be willing to invest time and effort in assisting a sibling or cousin because some of their genes are identical to yours and therefore by aiding a relative, you are indirectly helping yourself (or at least your genes) to survive and reproduce.

So, one would logically expect common vampire bats to prefer to share food with their kin over non-kin. Yet it’s been observed that these food-sharing partners-in-crime are often related, but not always.

Recently, scientists Gerald Carter and Gerald Wilkinson at the Smithsonian Tropical Research Institute in Panama wondered which trait is a better predictor of which individuals vampire bats will choose to associate with—past food-sharing experience, or genetic relatedness? In other words, do social bonds take priority over kin in the minds of vampire bats?

To answer this question, Carter and Wilkinson first recorded the “contact calls” from thirty-one captive common vampire bats whose social dynamics and genetic relatedness was known. Every bat’s “contact call” has a unique vocal signature and the bats are known to use these calls to identify and locate each other.

Then the researchers set up two experiments. In the first experiment, each bat was exposed to the contact calls of two of its fellows and allowed to choose which one to seek out. The two contact calls both came from bats that were equally genetically related to the bat being tested, yet had different social experience with that bat. One of the two calls always came from a bat that had been a food-sharing buddy with the test subject, while the other call came from a bat that had rarely or never shared food with the bat being tested.

Unsurprisingly, the tested bats gravitated towards the calls of their food-sharing friends while ignoring the calls of the bat they had no social connection with. Yet in the next experiment the setup was tweaked slightly. This time the bats were given a choice between two contact calls that both came from food-sharing buddies, with the difference being that one call came from a close relative to the test subject and the other call came from a nonrelated or distantly-related bat.

Clearly, vampire bats are drawn to the voices of their familiar companions; but when they have an equal social connection to both of the bats calling out to them, will they fly to the aid of a sibling over a third cousin?

In fact, the answer was no—not necessarily. Despite evidence from past studies suggesting bats can tell if they are related or not, the bats in Carter and Wilkinson’s study did not seem to show any particular preference between kin and non-kin when food-sharing history was equal between them (with the notable exception of a few female bats that were consistently drawn to the calls of their adult children).

So what can be gleaned from these results? This study confirms yet again that vampire bats can distinguish between other individuals based on their unique vocalisations alone, and also that they are intelligent enough to remember all of their personal social relationships with each individual. According to some studies vampire bats can even remember how much blood each individual friend had been willing to spit out for them so that they can repay their partner in equal measure! This is all indicative of fairly advanced social awareness usually attributed to such well-known cooperators as canines or apes.

Additionally, Carter and Wilkinson’s study was able to determine whether social history or genetic relatedness takes priority when vampire bats are choosing whom to hang out with (no pun intended) and it appears that lingering maternal instinct aside, vampire bats are far more concerned with the social bonds formed through mutual grooming and reciprocity of food-sharing than they are with worrying about who is related to whom.

Cooperation in animals has been an interesting and exciting subject for behavioural scientists for a very long time and there have been many suggested answers to the fundamental evolutionary question, “Why would an individual spend energy assisting anyone else, especially when it risks one’s own health or survival?” In the famous examples of ape troops or dolphin pods, cooperation can be best explained by kinship and the principle that what’s best for your family is best for you and your genes.

And that’s exactly what makes species like the common vampire bat so intriguing—they are willing to selflessly share their precious food resources with no real guarantee that they will ever be reimbursed (there’s always the chance that your friend will get eaten by an owl before they can repay you, after all) and they don’t choose recipients of this charity based solely on kinship. Although there’s indication that vampire bats do recognize kinship to some extent, it seems their decisions on whom to split a black pudding with are based much more on friendship and reciprocal loyalty.

As researchers Carter and Wilkinson observed, “Food donors often initiate and approach hungry bats to feed them. In fact, they will sometimes fly to trapped hungry partners and even feed them through cages bars, suggesting that vampire bats can recognize a needy partner from a distance.”

This could explain why it’s so crucial for vampire bats to be able to recognize each other based on contact calls alone—so that bats can find their buddies and make sure they roost together, despite their otherwise fluid social dynamics in which groups are constantly split, combined, or reshuffled when bats switch roosts.

Common vampire bats are driven to form social bonds with partners with whom they can share food when in need, and it appears that the survival benefits of fidelity to their friends may even outweigh their genetic link with their family. As it turns out, blood may not be thicker than water after all.

Reference

Carter, G.G. and Wilkinson, G.S. (2016) Common vampire bat contact calls attract past food-sharing partners. Animal Behaviour 116: 45-51. dx.doi.org/10.1016/j.anbehav.2016.03.005

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