Variability in adult group compositions of a prosimian primate

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 compositions of sifakas.