The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

Birth seasonality in relation to rainfall and food in wild geladas (Theropithecus gelada) at Guassa, Ethiopia: Variation within and between one-male units


Department of Anthropology, California State University Fullerton

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Primates occupying seasonal habitats cope with predictable fluctuations in food supply by varying what they eat and how they obtain these food items, decisions which, in turn, impact female reproductive decisions and life histories. We investigated patterns of within- and between-group (i.e., population-wide) birth seasonality in relation to rainfall and primary food productivity in 15 social units of wild geladas (Theropithecus gelada) at Guassa, Ethiopia from January 2007-July 2011. Geladas occupy a higher altitude and colder climate than any other African primate, and are expected to be especially responsive to seasonal fluctuations in climate and food availability. We provide evidence (A) of within- and between-group birth seasonality among geladas at Guassa, and (B) that the birth peak at this site is timed to coincide with seasonal increases in rainfall and food abundance. Using circular statistics, we found that this population of geladas reproduces seasonally, with a protracted peak (69% of 99 births occurred from July-December, mean month = October) encompassing the wettest months of the year which were also characterized by high food abundance. In 12 of the 13 social units with multiple births, most births occurred during the population-wide birth peak. The birth peak appears timed to ensure high food availability during late gestation and early/mid lactation. Our results suggest that geladas at Guassa are pursuing an income II breeding strategy, wherein the most energetically expensive phase of reproduction for females (i.e., lactation) is timed to coincide with annual peaks in nutrient or energy availability to maximize maternal survival.

This study was funded by Pittsburgh Zoo, Margot Marsh Biodiversity Foundation, Primate Conservation Inc., and Cleveland Metroparks Zoo.

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