Abstract
Individual variation in reproductive success is a key feature of evolution, but also has important implications for predicting population responses to variable environments. Although such individual variation in reproductive outcomes has been reported in numerous studies, most analyses to date have not considered whether these realized differences were due to latent individual heterogeneity in reproduction or merely random chance causing different outcomes among like individuals. Furthermore, latent heterogeneity in fitness components might be expressed differently in contrasted environmental conditions, an issue that has only rarely been investigated. Here, we assessed (i) the potential existence of latent individual heterogeneity and (ii) the nature of its expression (fixed vs. variable) in a population of female Weddell seals (Leptonychotes weddellii), using a hierarchical modeling approach on a 30-year mark–recapture data set consisting of 954 individual encounter histories. We found strong support for the existence of latent individual heterogeneity in the population, with “robust” individuals expected to produce twice as many pups as “frail” individuals. Moreover, the expression of individual heterogeneity appeared consistent, with only mild evidence that it might be amplified when environmental conditions are severe. Finally, the explicit modeling of individual heterogeneity allowed us to detect a substantial cost of reproduction that was not evidenced when the heterogeneity was ignored.
Highlights
Differences in the reproductive and survival abilities of individuals in a population have long been considered a key feature of life (Darwin 1859)
As suggested elsewhere (Cam et al 2002), individual heterogeneity in fitness components likely concerns both reproduction and survival, here we focus on reproductive rates because we expect them to display more heterogeneity given that, at the population level, they vary much more than do adult
Encounter histories from 954 females provided a total of 6792 known-state observations (i.e., “individual-years”), with 954 observations in started at the first reproductive event (state F), 3667 in state E, and 2171 in state S
Summary
Differences in the reproductive and survival abilities of individuals in a population have long been considered a key feature of life (Darwin 1859). The investigation of the prevalence, nature, and underlying mechanisms of such individual heterogeneity in fitness components is relevant to both evolutionary ecology (Wilson and Nussey 2010; Bergeron et al 2011) and population dynamics (Lomnicki 1978; Kendall et al 2011). Individual heterogeneity may be explained by variations in conditions and resources during early development (Lindstro€m 1999; Festa-Bianchet et al 2000; Metcalfe and Monaghan 2001; Lummaa and Clutton-Brock 2002; Hamel et al 2009b), in environmental conditions throughout life (Landis et al 2005), in parental care
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