There is a vast global enterprise collectively called the “social sciences”. It includes all of economics, sociology, anthropology, linguistics, psychology, and so on. The goal of this massive undertaking is to understand the social lives of humans. The major scientific contribution of my life is to better understand the social lives of bats.
Bats live in a world of sound, so their use of vocal communication is one of the best insights we have into their social lives. Our latest paper entitled “Calling to the collective: contact calling rates within groups of disc-winged bats do not vary by kinship or association” was led by my friend and colleague Gloriana Chaverri. She has spent much of her life studying disc-winged bats (Thyroptera tricolor), a species with one of the most unique and interesting social structures and roosting behaviors among bats. Small groups of these bats roost inside the furled developing leaves of tropical Heliconia plants. They have suction cups on their thumbs which allow them to stick to the smooth leaf surface. Each day the leaves gradually unfurl which means the bats need to find a new furled leaf to roost within. To do this, they use a contact calling system. A bat in search of a leaf roost will make “inquiry calls” while in flight. Another bat that finds and enters a furled leaf will make “response calls” from inside the leaf. These vocal exchanges allow the entire group to switch roosts on a nightly basis.
There are two interpretations of these vocal exchanges. The first interpretation is that each bat is calling to its entire group. One bat finds a roost and she calls in all her groupmates. The calls and responses of each bat don’t depend on who the exact groupmate is, they only care that someone in the group responds. Interestingly, the calling rates of individuals vary quite dramatically with some bats not calling often or at all. The reasons for this variation remain a mystery, but according to this interpretation, it is the caller that explains the calling rate.
The other interpretation is the one that I favored. If Thyroptera are like vampire bats, then each bat would have individually differentiated relationships within their groups. These relationships would be predicted by genetic kinship and by dyadic rates of association (roost-sharing) or interaction (allogrooming). We might expect the bats to preferentially respond to calls from specific groupmates. If so, then the variation in calling rates might be explained not only by the caller’s identity but also rather the vocal exchange partner. They are calling in hopes of getting replies from specific individuals. Based on my work with vampire bats, I assumed that this might be what Thyroptera were doing, and I suggested that Gloriana test it.
It turns out I was wrong. Gloriana and I discussed a project that would test these ideas and she and her colleagues did the exact experiment I was imagining. Random pairs of groupmate bats were recorded in a large flight tent–one flying bat in the roost-searching role and the other bat (inside the leaf) as the roost-advertising role. We then asked the simple question: Do the bats produce more contact calls towards groupmates that are more related, towards groupmates with higher association rates, or neither? The answer was neither. Calling rates were based on who was calling and not who was receiving and replying. Of course, response call rate was driven by inquiry calling rates (otherwise we wouldn’t call them responses), but neither calling rate was higher when the interacting pair had higher kinship or association. For both call types, about half of the variation in the rate of calling was explained by the caller, much of it was unexplained by the factors we measured, and the none of it was explained by the receiver.
Our results were consistent with the idea that contact calls are produced for any or all group members rather than for specific individuals. A second, I think less likely, explanation is that vocal recognition might simply be too difficult or unreliable due to the reverberations of the sound in the leaf so they call and respond to anyone until they get closer to gather more information. It’s also possible, and likely, that variation in calling rate from the same caller is explained by factors we didn’t measure such as the bat’s energetic or motivational state. I’m very excited to do more work on these interesting (and very cute) bats. One obvious question is whether bats that are more vocal are also more socially connected. We actually have that data…I should probably do that analysis right now.
My PhD student Julia Vrtilek is doing similar experiments with vampire bats right now. The counts of contact calls she sent me today look quite similar to call counts of Thyroptera in that there are some bats that call a lot and others that almost never do. We are asking the same question but we also want to know if the bats match the calls of their partner, especially in cases like the white-winged vampire bat where they clearly match the calls in time. There is evidence that this “vocal labelling” happens in dolphins and parrots. Someday soon we will put some microphones on free-flying wild vampire bats as well to truly track their vocal interaction networks.
Carter Lab 