Climatic conditions cause spatially dynamic polygyny thresholds in a large mammal

Abstract

The polygyny threshold (PT) is a critical transition point in the sexual selection process for many organisms in natural populations, characterizing when females choose to mate with an already mated male over an unmated one to improve fitness. Understanding its causes and consequences is therefore of high interest. While both theoretical and empirical work suggest that the degree of polygyny within a species is plastic and a function of male inequality, the functional relationship between underlying availability of resources occupied by breeding males under variable climatic conditions and the dynamics of PTs across space and time has received less attention. Here, we use a standardized measure of male mating inequality as the culmination of female mate choices to analyse how spatially dynamic PTs in a naturally regulated feral horse (Equus ferus caballus) population emerge along a geographic gradient in a known, limiting resource (freshwater) each year from variable climatic conditions. Polygyny threshold distance from permanent freshwater increased with increasing precipitation during the breeding season of each year, suggesting a relationship between annual resource availability and female mate choice. The mechanism by which climatic conditions underpin the spatial dynamics of PTs was likely through precipitation providing ephemeral freshwater sources across the study area that effectively weakened the gradient in availability of permanent freshwater, thereby providing mating males that occupied home ranges far from permanent water with access to this limiting resource and enabling them to attract and retain females. Increased precipitation also coincided with a decreased proportion of males in the population that experienced sexual selection pressure attributed to female mate choice in relation to the acquisition and/or defence of freshwater sources. Climatic conditions caused spatial shifts in PTs annually along the geographic gradient in resource availability. Our findings reveal that such environmental gradients may either buffer or amplify impacts of climatic variation on selection pressure operating in natural populations, and emphasize the importance of integrating spatially explicit PTs with atmospheric fluctuations when predicting the effect of climatic change on selection processes within populations that occupy environmental gradients.

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

“…As demonstrated with our other work in this system (see Manning et al, ; Manning & McLoughlin, ), some horses were observed repeatedly over the study while others were observed only once in a season, leading to samples that were unequally distributed among the dominant males. Repeated counts of individuals within each year were accounted for by first using the sum number of unique overlapping individuals in a given year to calculate annual measures of each selection metric associated with a dominant male, and also including individual male identity as the random effect intercept term in each model (Laird & Ware, ).…”

“…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, ). Lastly, in line with the EPP hypothesis, we predicted that a freshwater availability effect on male mating inequality would vary among years, as the annual amount of rainfall that recharges the island's water table and influences freshwater availability has been shown to underlie spatially dynamic polygyny thresholds in this system (Manning & McLoughlin, ).…”

“…Lastly, in line with the EPP hypothesis, we predicted that a freshwater availability effect on male mating inequality would vary among years, as the annual amount of rainfall that recharges the island's water table and influences freshwater availability has been shown to underlie spatially dynamic polygyny thresholds in this system (Manning & McLoughlin, 2017).…”

“…This situation would create equally attractive males with respect to water access during mesic summers, thereby Figure 5a). Climate variables included in models were based on previous work on horses in this system (e.g., Contasti et al, 2012;Manning et al, 2015;Manning & McLoughlin, 2017 2008, 2011, and 2012), when a disproportionate availability of small, ephemeral water sources emerges across the island, did the effect of differences among males in their ability to monopolize these small ephemeral water sources and the local females that depend on them peak and reach an asymptote in I mares|harem in relationship to summer precipitation (Figure 5b). This finding raises interesting questions regarding the potential for selection thresholds in relation to resource availability under the EPP hypothesis.…”

“…For example, emerging evidence suggests that mate choice can be highly variable and dependent upon the prevailing weather conditions (Robinson, Doorn, Gustafsson, & Qvarnström, 2012). Weather has also been shown to interact with resource availability along environmental gradients to drive spatially dynamic adult sex ratios (ASRs) and polygyny thresholds (Manning & McLoughlin, 2017;Manning et al, 2015). Additionally, variation in population density may have sex-specific effects (see Kokko & Rankin, 2006), and density-dependent processes have been shown to be strongly scale-dependent (Ray & Hastings, 1996).…”