The 84th Annual Meeting of the American Association of Physical Anthropologists (2015)

Determinants of primate sex ratios: an analysis of demography in Verreaux’s sifaka (Propithecus verreauxi)


Department of Anthropology, University of Texas at Austin

March 26, 2015 , Archview Ballroom Add to calendar

Sexual selection predicts that males strive to monopolize paternity by residing with few reproductive competitors, while females benefit from the presence of many males. Primate groups are often disproportionately comprised of females. However, lemurs deviate from this norm with even or male-biased sex ratios. We used demographic data from five groups of Verreaux’s sifaka (Propithecus verreauxi) in the Kirindy Mitea National Park from 2007-2014 to (1) describe variation in group size and composition, (2) examine proximate determinants of sex ratio, and (3) explore the hypothesis that unusual sex ratios in lemurs are ultimately driven by infanticide risk. Mean group size was 6.2 individuals (range: 4.9-7.4) and 80% of groups were unexpectedly female-biased. Neither birth rates nor survival to subadulthood differed between the sexes, but males dispersed at higher rates than females and rarely delayed natal dispersal. Contrary to expectations, infant disappearances were not significantly associated with entrances by extra-group males. Thus male-biased migration had a greater proximate effect on adult sex ratio than sex differences in birth or survival in this population. Our results are consistent with low overall infanticide risk, which may influence sex ratio by lessening constraints on female group size and reducing the benefit of surplus males for takeover defense. Sifaka have been suggested to live in neighborhoods with flexible mating boundaries, so entrances by neighboring males may pose little threat because adjacent groups might contain their offspring. Interestingly, while infant disappearances associated with entrances occurred very infrequently, most involved an immigrant from outside the study population.