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

Abstract

Ecological and social factors underpinning the inequality of male mating success in animal societies can be related to sex ratio, sexual conflict between breeders, effects of nonbreeders, resource dispersion, climatic conditions, and the various sequential stages of mating competition that constitute the sexual selection process. Here, we conducted an individual‐based study to investigate how local resource availability and demography interact with annual climate conditions to determine the degree of male mating inequality, and thus opportunity for sexual selection across two sequential reproductive episodes (harem and subsequent mate acquisition) in a naturally regulated (feral) horse population in Sable Island National Park Preserve, Canada. Using a 5‐year, spatially explicit, mark‐resight dataset and hierarchical mixed‐effects linear modeling, we evaluated the influence of adult sex ratio (ASR) on mating success and then tested for effects of freshwater availability, density, unpaired male abundance, and precipitation during each breeding season. Unpaired male abundance, freshwater availability, and ASR differed in their effects on male mating success according to year and selection episode. Opportunity for sexual selection in males associated with harem acquisition increased with ASR, and unpaired male abundance further explained weather‐related interannual variation after accounting for ASR. In contrast, once a harem was secured, ASR had little effect on male mating inequality in regard to acquiring additional females, while interannual variation in mating inequality increased with decreasing freshwater availability. Our findings show that local demography, resource availability, and weather effect opportunity for sexual selection in males differently depending on selection episode, and can attenuate or accentuate effects of ASR.

Bacterial dispersal and drift drive microbiome diversity patterns within a population of feral hindgut fermenters

“…although bands as large as 16 horses have been observed (Manning & McLoughlin, 2017). Band memberships are stable across years but 67% of adult females have been observed to disperse to a different social band at least once during a 7-year period (Debeffe et al, 2015).…”

Harem size should be measured by more than the sum of its parts: Phenology‐based measurements reveal joint effects of intrinsic and extrinsic factors on a polygamous herbivore under non‐stationary climatic conditions

“…Additionally, we calculated the within‐year dispersion of harem sizes as the coefficient of variation in mean monthly harem size each year ($$$ {\mathrm{CV}}_{\mathrm{mmhs}}=\left(\mathrm{SD}/\overline{x}\ \mathrm{monthly}\ \mathrm{harem}\ \mathrm{size}\right)\times 100 $$$) (Figure S1). We assumed that years with relatively greater dispersion reflected an increased degree of male mating inequality and opportunity for sexual selection (Emlen & Oring, 1977), which has been shown to emerge in feral horses from factors including local demography and drought conditions (Manning & McLoughlin, 2017, 2019). Lastly, because harem size is expected to vary within and among years, we calculated a monthly harem size metric for each year by first measuring the difference in mean monthly harem size (the i th month of the j th year) from the long‐term average monthly harem size over the 15‐year period (mean harem ij – 15 year monthly mean harem).…”

“…To investigate how factors intrinsic to the population may influence the four phenology-based harem size metrics, we calculated empirical estimates of five demographic parameters annually. As harem size and the probability of acquiring harems have been linked to the adult sex ratio (ASR; proportion of adult males in the population) in other large ungulates (Bonenfant et al, 2004;Kaseda & Khalil, 1996;Manning & McLoughlin, 2019), we calculated ASR annually as the 12-month mean of the monthly proportion of adult stallions in the entire population (males ≥3 years old/[males ≥3 years old + females ≥2 years old]) (Ancona et al, 2017;Andersson, 2004;McNamara et al, 2000). We calculated annual adult mortality as the number Because age can influence a stallion's ability to retain harems and mares within harems (Bouman, 1986), we calculated the mean age of harem holding stallions during each 12-month period.…”

Bacterial drift and dispersal drive microbiome diversity within a population of feral hindgut fermenters

“…Females in the population invariably segregate across these mixedsex social bands which are comprised of the dominant stallion, adult females (mares), and subadult (<3 years of age) offspring (Regan et al, 2019). Bands can therefore be as small as 2 (one adult male and one female), although bands as large as 16 horses have been observed (Manning & McLoughlin, 2017). Band memberships are stable across years but 67% of adult females were known to disperse to a different social band at least once during a 7-year period (Debeffe, Richard, Medill, Weisgerber, & McLoughlin, 2015).…”