Social structure of frog-eating bats

There’s a new a paper out today entitled “Social structure and relatedness in the fringe-lipped bat (Trachops cirrhosus)” in Royal Society Open Science, by Victoria Flores, me, Tanja Halczok, Gerald Kerth, and Rachel Page (who was an advisor for Victoria as a PhD student and for me as a Postdoc). The fringe-lipped bat is an eavesdropping predator also often called the frog-eating bat (Trachops cirrhosus see picture left and below), but it eats more than just frogs. The ‘fringe-lipped’ name is for the wart-like protrusions around the chin and mouth of Trachops, which people have speculated might help them detect poison on the skin of frogs, but behavioral observations did not provide any clear evidence for this (yet another mystery of a bat’s strange face).

This study was based on six years of data collected by the Rachel Page Lab in Gamboa, Panama. Since 2012, the Gamboa Bat Lab has been capturing, sampling, and marking Trachops roosting in the drainage tunnels under a trail in Soberania National Park’s Pipeline Road (apparently a famous destination for birders). More than a hundred times over six years, the lab captured roosting groups of these bats to test them in a series of behavioral experiments on sensory ecology, social learning, and cognition. Although they are known for for their social learning, almost nothing is known about their social structure.

For most classic studies of bat social structure, researchers take repeated visual observations of associations between banded individuals at several roost sites over several years, allowing the creation of a social network of the overall association rates between all individual bats. Over the years, data on social associations have become much faster, easier, and more reliable with the ability to automate roost attendance using technologies like PIT tags. However, what we had for Trachops was a much smaller and messier sample of observations where we knew individuals sometimes escaped. We did not have enough data to map out an accurate social network, so the question was: Could we infer anything about the social structure of this species from these opportunistic capture data?

A group of Trachops in a nice little cluster. Photo by Christian Ziegler.

It turns out we could learn quite a bit. Using existing genetic data, we could confirm in the recapture data that, female frog-eating bats (Trachops) were philopatric, whereas males dispersed at reproductive maturity, similar to vampire bats. Also like vampires, adult female Trachops formed matrilines and tended to roost with their mothers and daughters, across many different roost tunnels. Like most forest-dwelling bats, Trachops did not appear to live in completely stable groups and they often switched roosts. Yet both males and females had preferred roosting partners, even when controlling for the effects of time and location. The data permutation tests we did here were also a bit more careful and constrained than the ones we did in another recent study using several species of bats. The more social data we get from more bat species, the closer we get to making comparisons that might show the influence of phylogenetic and ecological factors in shaping bat social structure. One main challenge for bats is that there are more students willing to watch primates and birds for hours compared to crawling into caves and trees to try and see some bats, but technological advances are revolutionizing our ability to assess social structure in bats.

Page Lab members Basti, Victoria, and Luisa, back when people wore masks to enter a bat roost rather than the grocery store
Victoria retrieves a bat from a net

Victoria Flores, the lead on this project, also discovered an interesting behavior in reproductive males. They create a smelly ‘crust’ on their forearms from what seems to be a mix of saliva and bodily secretions. Mariana Muñoz-Romo in The Page Lab is continuing research on this topic (see video below).

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