Pup Guarding in Greater Spear-Nosed Bats

Greater spear-nosed bats (Phyllostomus hastatus) are a large omnivororous species that inhabit the neotropics. These bats are of particular interest because they have a unique social structure: unrelated females reside in extremely stable social groups.

A P. hastatus social group of females and their pups in their roost site. Photo by Gerald Wilkinson

Not surprisingly, cooperative behaviors are important in maintaining these long-term associations. For example, P. hastatus social groups forage together using learned group-specific vocalizations illustrating the link between social and vocal complexity. Recently, we discovered that these bats also engage in another fascinating cooperative behavior: pup guarding. In this species all adults give birth only once per year and pups are unable to fly for their first six weeks. During this period pups frequently fall from roost sites in the cave ceiling to the cave floor. Once they have fallen to the floor they cannot get back to their roost sites and likely die (either through predation or starvation) unless retrieved by an adult. Thus, pup retrieval is likely a highly important factor for the reproductive fitness of adults. In the context of a pitch black cave, females are alerted to fallen pups through the pups’ vocalizations, termed “isolation calls”. When pups call they attract adults.

Bats were never on the walls of the cave, until a pup fell and started calling, then many adults would land nearby and interact. This was an odd observation because the isolation calls are individually distinctive. One would expect each pup to be visited and then retrieved by only a single female-the mother, instead we observed on average 17 visits per pup with over 300 visits to a single pup! We examined the behavior of visiting females using infrared video and the main conclusion of our research is that adult group mates leave their own pups in their roost sites to fly down and guard pups from adults from other groups that attack and sometimes kill pups.

We also show using genetic markers that females are unrelated to group mates and unrelated to the pups they guard. We also tested and rejected the most common models of cooperation among non-kin: reciprocity and mutualism. Females are not simply reciprocally guarding each other’s pups nor do they gain immediate benefits by guarding others. Considering this species is remarkably long-lived for a small mammal (living 20 years or more) and the social groups are highly stable we conclude a more complex model of cooperation, one that likely incorporates other cooperative behaviors is likely operating.