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1. Social behavior in the bat Phyllostomus hastatus was examined in Trinidad, W.I. over a 26month 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.

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Mate choice cues in sage grouse were reinvestigated by analyzing relationships between male mating success and a range of suggested cues. Display cues were implicated by significant relationships between mating status (whether or not a male mated) and lek attendance, display rate (corrected for effects of female proximity and time of day), and an acoustic component related to temporal and frequency measure of a whistle emitted during the strut display. Although display rate and the acoustic component were intercorrelated, both exerted significant partial effects on mating success in multivariate analyses. These display measures also differed significantly between males. In contrast, mating success was not significantly related to measures of territory characteristics, including size and proximity to the lek center, or to body size. These results resolve discrepancies between previous studies and provide a basis for experimental analysis of the role of female choice in this lek system.