Social organization and kinship in the polygynous bat Phyllostomus hastatus

1. Social behavior in the bat Phyllostomus hastatus was examined in Trinidad, W.I. over a 26-month period. The studies included (a) long-term observations on marked individuals, (b) the use of allozyme polymorphisms to estimate paternity and the genetic relationships among individuals in social groups, and (c) the investigation of foraging behavior by radio-tracking. 2. Day-roosting cave colonies of this bat are subdivided into highly stable, compact clusters of adult females (mean cluster size=17.9±5.1 females) and less stable bachelor groups. Female clusters are always tended by a single adult male and genetic tests demonstrated that these harem males father most or all of the babies born to the females in clusters. Harem males actively defend female clusters from other males in the roost. 3. Membership in female clusters is extremely stable and the same individuals roost together for years at a time. Harem male tenure can also be very long and several males retained residence in harems through a minimum of three annual reproductive periods. However, harem male turnovers were also frequently observed and in no instance did a change in males result in disruption of the female roosting cluster. Experimentally disrupted harems reconstituted their original memberships even in the absence of males. 4. These stable female clusters are not matrilineal kin groups. Juveniles of both sexes disperse and are not recruited into parental social units. New stable female clusters were formed as cohorts of yearling females drawn from different harems and cave colonies. This was corroborated by independent studies on the age structure of harems. Genetic tests showed that the females comprising any cluster were a genetically random sample of the total adult population. 5. Radio-tracking revealed that each individual has its own foraging area that it utilizes throughout the year. The foraging areas of females belonging to the same cluster were adjacent to one another and apparently segregated into larger cluster-specific areas. Females occasionally foraged within the areas of other females in their roosting cluster, but females from different clusters did not forage near one another. Harem males did not defend female foraging areas and in fact foraged in areas well apart from their harem females. 6. These compact and stable female roosting clusters are the fundamental unit of P. hastatus' social organization, and apparently they facilitate a male strategy of female-defense polygamy. We hypothesize that these stable clusters arise from active cooperative interactions among females on the foraging grounds. The most likely form of cooperation is the sharing of unpredictable food sources among cluster members. In P. hastatus, such behavior has not evolved through nepotism or processes of group selection that require high group stability. All of our observations and the basic biology of this bat suggest that reciprocity is the most likely means by which the system operates. Mutualism and weak altruism may play secondary roles but cannot be evaluated without additional field study. 7. Bachelor groups appear to be much less structured than female clusters, and the possibility of cooperation among bachelors is still open to study.