Climate fluctuations interact with local demography and resources to predict spatially dynamic adult sex ratios in a megaherbivore

Abstract

Adult sex ratio (ASR) is a fundamental concept in population and evolutionary biology, with implications for management and conservation. Although ASR is typically measured at the population-level, local mate competition points toward spatial variation in ASR within populations, the causes of which remain unclear. Over five breeding seasons (2008-2012), we tracked the life histories and movements of all male and female feral horses known to be alive (n  721) on Sable Island, Canada, to investigate determinants of spatially explicit ASRs. We show that local demographic traits (density, adult female abundance, and abundance of unpaired males (e.g. floaters, adult bachelors)) operate together with inter-annual changes in weather to determine asymmetrical ASRs across time and space that deviate from the population-level mean. While accounting for possible confounding effects of unpaired male movements and weather, we also show that local demographics are best explained by different responses to an environmental gradient (distance to surface water). Our results demonstrate that local demographic traits operate as mechanisms by which environmental gradients and weather can shape spatial variation in ASR within wild populations, which has important implications for predicting how opportunities for sexual selection may follow from changes in resource availability and climate.

Environmental and demographic drivers of male mating success vary across sequential reproductive episodes in a polygynous breeder

“…We modeled variation in mating success among dominant males attributed directly to local ASR for each selection episode using hierarchical mixed‐effects linear models (LMM). For each dominant male, we estimated local ASR (ASR ij ) as the mean number of adult males/(adult males + adult females) with ≥1 buffered locations that overlapped ≥1 locations of the i th band over the course of the field season in the j th year (Manning et al, ; Parker & Simmons, ). This relative ASR was spatially and temporally linked with its corresponding estimate of I , included unpaired adult males, and ranged from 0 when only females were ready to mate, through 0.5 at equality, to 1 when only males were ready to mate.…”

“…We expected resource availability effects on male mating inequality because male horses do not possess ornaments and there is little or no sexual dimorphism (Berger, ), indicating the importance of resource quality in female mate choice and the presence of the EPP. We expected density effects on male mating inequality in both selection episodes, as density has been linked to ASRs in this system (Manning et al, ). We predicted that limited freshwater availability would increase male mating inequality in the second episode because increased water requirements during reproduction, pregnancy, and lactation would heighten female mate choice under these conditions (Contasti, Tissier, Johnstone, & McLoughlin, ; Richard, Simpson, Medill, & McLoughlin, ).…”

“…Understanding the interplay between demographic and environmental processes that shape mating systems has long been a major focus of evolutionary ecology (Kokko, Klug, & Jennions, ; Kokko & Rankin, ; Manning, Medill, & McLoughlin, ; Simmons & Kvarnemo, ; Székely, Weissing, & Komdeur, ). In particular, the social and ecological environments have been presumed to influence the potential of males to monopolize mates (Emlen & Oring, ).…”

Island tameness and the repeatability of flight initiation distance in a large herbivore

“…Alternatively, changes in behaviour could reflect shifts in physiology, such as a steady decline in plasma cortisol concentrations in early life (Panzani et al 2009;Comin et al 2012). Similarly, spatial variation in FID could be due to important west-east gradients in horse habitat quality (Contasti et al 2012), social environment (Manning et al 2015), and genetic diversity (Lucas et al 2009) that are known to occur along the length of the island, rather than (or in addition to) human presence. For example, FID could be related to spatial variation in band size or local sex ratio (as mentioned earlier) or diet (Redondo et al 2009;Han and Dingemanse 2015).…”

“…In addition, because risk-taking can vary between the sexes (Samia et al 2015), we tested if FID varied between males and females. Sable Island is characterized by important west-east biological gradients that could impact FID, including human activity but also horse habitat quality (Contasti et al 2012), horse social environment (Manning et al 2015), and genetics (Lucas et al 2009); thus, we also tested whether FID varied along the length of the 49 km long and narrow (1.5 km at its widest) island. Finally, we tested for the repeatability of FID.…”