Behav Ecol Sociobiol (2003) 54:285–293 DOI 10.1007/s00265-003-0634-z ORIGINAL ARTICLE Sharon T. Pochron · Patricia C. Wright Variability in adult group compositions of a prosimian primate Received: 23 January 2003 / Revised: 21 April 2003 / Accepted: 29 April 2003 / Published online: 27 May 2003  Springer-Verlag 2003 Abstract Many species fall into specific mating-system categories, and that category is usually associated with a suite of behavioral and morphological characteristics. Several lemur species, including Propithecus diadema edwardsi, have been labeled “idiosyncratic” because variation in socionomic sex ratios among groups is consistent with wide variation in social structure. We used several hypotheses founded in behavioral ecology to assess variability in P. d. edwardsi. First we examined 46 group-mating seasons to quantify variability. We then tested predictions that the number of males per group would increase as the number of adult females increased, and the number of males would increase as female mating synchrony increased. Examining variation in offspring survival relative to the number of adult males in a group may tell us which composition is likely to persist into the future, so we also hypothesized that as the number of males in a group increased, fertility and offspring survival would increase. We found an equal distribution of polygynous, polygynandrous, pairs, and polyandrous groups. Furthermore, female distribution and mating synchrony did not predict the number of males, and offspring survival was not correlated with the number of males. Since infants survived equally well in groups of all compositions, sifakas experienced no pressure to maintain a particular number of adult males per adult female. The small number of adults per group (mean=3.2) may result from balancing feeding competition against predator detection. Augmenting the mate pool available from the group with mates from neighboring groups may promote the notable variability seen in the adult group composi- tions of sifakas. Keywords Propithecus diadema edwardsi · Mating system · Sex ratio · Group composition · Group size Introduction Socionomic sex ratios have been used to sort vertebrate mating systems into categories (Krebs and Davies 1993): solitary, polygynous, polyandrous, polygynandrous, and monogamous. A primary goal of vertebrate behavioral ecology concerns understanding the selective forces that drive variation in these systems (Crook and Gartland 1966; Eisenberg et al. 1972; Davies and Lundberg 1987; Nunn 1999; Heymann 2000), and the category into which a species falls provides an important predictor of the intensity of mating competition (Kvarnemo and Ahnesjo 1996; Mitani et al. 1996b; Kappeler 2000), morphology (Harvey and Harcourt 1984; Plavcan et al. 1995) and variance in male reproductive success (Pope 1996, 2000; Nievergelt et al. 2002; Wimmer and Kappeler 2002). Socionomic sex ratios—and mating systems—are influenced in part by patterns of association between adults, currently modeled with a two-step process (Emlen and Oring 1977; Andelmann 1986; Ostner and Kappeler 1999; Altmann 2000). Ecological factors like food distribution and predation determine the distribution and grouping of females, and the distribution of males follows (Dunbar 1988). Comparative analyses in group-living primates show that the number of males per group increases with the number of females (Andelmann 1986; Mitani et al 1996a; Nunn 1999). Female mating synchrony, which influences sex ratio because of its effects on male ability to monopolize females, also predicts the number of males, after controlling for the number of females per group (Ridley 1986; Nunn 1999). We apply the principles of Nunn’s comparative study to a single species. If mating syn- chrony influences mating patterns, then as females Communicated by D. Watts S. T. Pochron ( ) ) · P. C. Wright Department of Anthropology, State University of New York, Stony Brook, NY 11794, USA e-mail: spochron@ms.cc.sunysb.edu Tel.: +1-631-6321568 Fax: +1-631-6327692 P. C. Wright ICTE, Stony Brook, New York, USA