Animals that live in stable social groups often cooperate to find and defend limiting resources, such as food or sites for reproduction. When social groups also control access to these limiting resources, membership in a social group can be an essential component of individual fitness. Indicating group membership and discriminating group mates from others should be favored under such circumstances. Signals that indicate group membership are shaped by the function they serve; they should vary among social groups at the individual or group level, and individuals should be able to discriminate between signals given by group mates and others. To retain reliability, group membership signals should also be difficult or costly for outsiders to imitate.
Female greater spear-nosed bats, Phyllostomus hastatus, use audible frequency calls, termed screech calls, to coordinate foraging among long term associates. Field observations imply that females use screech calls to identify members of their social group who not only live but forage together (Wilkinson & Boughman 1998). We showed that females benefit from group foraging by improved food finding and better defense of rich resources. Benefits are limited to females who forage with social group mates (Wilkinson & Boughman 1999). Our work shows that group signatures are an important mechanism to facilitate cooperation among these unrelated females. Partly because of the difficulty in gathering data, bats are a woefully understudied taxon. Thus, this is one of the first studies that determined call function in a natural population. It is especially interesting because of the insights it gave us into this species’ social and cooperative behavior, and how they communicate.
My dissertation work determined call function more precisely and showed that screech calls are group signatures. I characterized the acoustic structure of screech calls recorded in the field and lab, and tested which aspects of identity are encoded in calls. Results demonstrated that screech calls are structured to convey the group membership of the caller, but not individual identity (Boughman 1997). I also found that calls differ between cave colonies. I used field and lab playback experiments to test call function, and found that bats can detect colony and group differences but do not differentiate among individuals based on screech calls (Boughman & Wilkinson 1998). These results were surprising because of the ubiquity of individually distinct vocalizations. This implies that, within the context of cooperative foraging, group membership is essential and individual identity less crucial. Combined with results on the composition of foraging groups, these results indicate that group foraging is mutualistic, and argue against alternative explanations such as reciprocal altruism.
The members of social groups cooperate in foraging and give calls very similar to one another, yet these females are not closely related. Thus, kin selection is unlikely to explain cooperative foraging and vocal similarity does not arise from genetic similarity. To test the hypothesis that vocal convergence is due to vocal learning, I transferred bats reciprocally between social groups. Transfers and residents showed dramatic convergence in call structure. This experiment provided strong evidence that group differences arise through vocal learning. Comparisons of transfers with age-matched half-sibs indicated that call changes are not simply due to maturation, the physical environment, or heredity. Social modification of calls allows females to adjust their calls when social group composition changes. Call modification is not immediate, and the time required for individuals to match a new group could provide protection against outsiders who might feign identity to obtain access to resources controlled by social groups. This work provides important and rare experimental evidence of vocal learning in a mammal (Boughman 1998). It suggests that we should expect vocal learning when call similarity is essential for call function (Boughman & Moss 2003). Future work will test hypotheses developed in this review on the factors that favor vocal learning in mammals.
In a collaborative project, we followed up on these results by investigating auditory perception, and matching of perception and call structure (Bohn, Boughman, Wilkinson & Moss, 2004). We found close matching between auditory sensitivity and frequency of social calls that facilitate mother-pup interactions. We did not find close matching between sensitivity and screech calls. Future work will continue to explore the interaction between perception, communication, and social behavior in this and other bat species. Primary questions to be addressed revolve around understanding selective forces shaping communication signals, and how communication mediates cooperation and conflict.