The Effects of Seasonality on Reproductive Skew and Mortality
Gogarten, Jan Frederik
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Primate habitats differ in the degree to which their environments are seasonal. Here I examine how differences in seasonality impact two key aspects of primate life history and fitness; the distribution of male reproductive success across members of a group and mortality patterns. Among non-human primates, the ability of a male to monopolize fertilizations within a group appears to be primarily affected by the number of males in the group and the degree to which receptivity is synchronous in females. These factors have both been assumed to depend on the length of the breeding season (i.e. reproductive seasonality), with shorter breeding seasons leading to greater numbers of females mating simultaneously (i.e., female synchrony), which in turn reduces the likelihood of male monopolization of fertilizations. In chapter two I tested the relationship between reproductive seasonality, female synchrony, and male reproductive skew (i.e., the degree to which male reproductive success is monopolized by the dominant male) among primates by compiling data from the literature on genetic paternity, the number of males per group, the degree of female receptive synchrony, and the seasonality of reproduction for 28 populations representing 19 primate taxa living in multimale groups. I analyzed these data using hierarchical regression by sets and controlled for the effect of evolutionary relationships using phylogenetic generalized least squares. Results indicated that as predicted, male reproductive skew decreased significantly with increasing number of males per group and female receptive synchrony. However, in contrast to my prediction, there was no strong association between reproductive seasonality and synchrony, suggesting that reproductive seasonality has limited use as a predictor variable for female synchrony or reproductive skew. In chapter three I tested whether greater seasonal variation in rainfall is associated with greater seasonality in primate mortality patterns, and also whether deaths occur more frequently during food-scarce periods of the year. To do this, I compiled data on adult and juvenile mortality from nine wild non-human primate taxa, including prosimians, New and Old World monkeys, and apes. Results indicate that mortality was seasonal in some (six of ten) primate species, although the effect of seasonality on mortality was considerably less than on other seasonally influenced life history variables, such as the annual distribution of births. When seasonal mortality occurred, it was tied to rainfall seasonality more often than food availability, with deaths occurring more frequently during the wet season, rather than in food scarce periods. In contrast to my predictions, mortality was not more seasonal in more seasonal environments. Finding that deaths do not occur most frequently during food scarce periods for the majority of primate taxa has implications for the interpretation of the hominin fossil record, since there is now no reason to predict a priori that deaths should occur more often when animals are feeding on fallback resources. The absence of a single factor explaining mortality patterns is likely a result of the myriad of causes of death (e.g., predation, disease and injury), each of which may be influenced differently by environmental variables.