MSc student Rachel Crisp, Laurent Brent, and I published a paper today in Royal Society Open Science, entitled “Social dominance and cooperation in female vampire bats”
We did this study because, although I’ve worked on cooperation in female vampire bats for years, but we had no idea about the role of competition among females. We only knew that males competed for status and access to territories. But interestingly, male vampire bats are smaller and may be subordinate to females. For dominance among females, we just didn’t know much of anything.
Now, we do know a lot about female rank in other species. In many cooperative group-living animals, female dominance is hugely important. In cooperative breeders, only the dominant female breeds and the other females just help raise her offspring. In meerkats, a female even grows way bigger as she gains her alpha status, so she can intimidate and beat up the others. In many primates, all females breed but dominant females have priority access to food and other resources. The dominance hierarchies often follow matrilines (so a daughter inherents high rank from her mom). There’s a clip of a BBC animal documentary I sometimes show in talks of a juvenile macaque just stealing from the mouth of an adult, and the juvenile can do that because her mother is socially dominant over the adult. In many group-living animals, you can often see dominant females harassing and bullying subordinates. Does this kind of thing happen in female vampire bats?
To answer this question, we looked at conflicts over food among captive female vampire bats from two different wild sites. Rachel Crisp scored over 1000 observations of bats pushing or displacing each other to access food (spouts of blood) while working with me as a research assistant at the Smithsonian Tropical Research Institute in Panama. She then worked as a MSc student with primatologist and social behavior expert, Lauren Brent, at University of Exeter. Rachel found evidence for dominance hierarchy, but it was very subtle and it didn’t appear to correlate with physical traits like body size or with cooperation. For instance, we didn’t see evidence that females preferentially groom or share food with more dominant individuals, like we see in some primates. Nor did they clearly prefer to groom others that were close in rank, which is something else we see in primates. We compared the amount of dominance structure in their society to female groups in 15 other mammals, and they ranked near the bottom using different metrics.
This finding does make sense because female vampire bats often groom or share food “voluntarily” (not from pressure or harassment) and reciprocally (not asymmetrically “up” or “down” as expected from a strong dominance hierarchy). Also, if females from the same group were really in strong competition over food at night, why would they even feed each other the next morning? It might be that females from the same roost don’t often compete over food in the wild. Instead, they might compete more with strangers from other groups.
There are also a number of caveats we discuss in the paper. For example, perhaps female vampire bats compete in ways that we did not observe (like by using social calls to attract and repel each other). Or perhaps, food was not limited enough to evoke competition. Rather than completely resolving anything about female dominance with certainty, I think this paper establishes a firm foundation to build on. This study is only the first step in understanding female conflict.
Here are some videos of vampire bats squabbling, fighting, and tolerating each other.