American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Lifetime fitness in wild baboons: tradeoffs and individual heterogeneity in quality”

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Emily M. McLean, Elizabeth A. Archie, and Susan C. Alberts (Dec 2019)

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Life history tradeoffs in female baboons evident after accounting for heterogeneity in individual quality

Abstract

Understanding the evolution of life histories requires information on how life histories vary among individuals, and how such variation predicts individual fitness. Using complete life histories for females in a well-studied population of wild baboons, we tested two non-exclusive hypotheses about the relationships among survival, reproduction, and fitness: the quality hypothesis, which predicts positive correlations between life history traits, mediated by variation in resource acquisition, and the tradeoff hypothesis, which predicts negative correlations between life history traits, mediated by tradeoffs in resource allocation. In support of the quality hypothesis, we found that females with higher rates of offspring survival were themselves better at surviving. Further, after statistically controlling for variation in female quality, we found evidence for two types of tradeoffs: females who produced surviving offspring at a slower rate had longer lifespans than those who produced surviving offspring at a faster rate, and females who produced surviving offspring at a slower rate had a higher overall proportion of offspring survive infancy than females who produced surviving offspring at a faster rate. Importantly, these tradeoffs were evident even when accounting for: (i) the influence of offspring survival on maternal birth rate, (ii) the dependence of offspring survival on maternal survival, and (iii) potential age-related changes in birth rate and/or offspring survival. Our results shed light on why tradeoffs are evident in some populations, while variation in individual quality masks tradeoffs in others.