Ewe are what ewe wear: bigger horns, better ewes and the potential consequence of trophy hunting on female fitness in bighorn sheep

Abstract

In polygynous species, secondary sexual traits such as weapons or elaborate ornaments have evolved through intrasexual competition for mates. In some species, these traits are present in both sexes but are underdeveloped in the sex facing lower intrasexual competition for mates. It is often assumed that these underdeveloped sexually selected traits are a vestige of strong sexual selection on the other sex. Here, we challenge this assumption and investigate whether the expression of secondary sexual traits is associated with fitness in female bighorn sheep. Analyses of 45 years of data revealed that female horn length at 2 years, while accounting for mass and environmental variables, is associated with younger age at primiparity, younger age of first offspring weaned, greater reproductive lifespan and higher lifetime reproductive success. There was no association between horn length and fecundity. These findings highlight a potential conservation issue. In this population, trophy hunting selects against males with fast-growing horns. Intersexual genetic correlations imply that intense selective hunting of large-horned males before they can reproduce can decrease female horn size. Therefore, intense trophy hunting of males based on horn size could reduce female reproductive performance through the associations identified here, and ultimately reduce population growth and viability.

Keywords: age of primiparity; cranial weaponry; fitness; lifetime reproductive success; secondary sexual trait; trophy hunting.

Figure 1.

(a) Horn length at 2 years and (b) age at primiparity for female bighorn sheep from cohorts 1973 to 2015, and (c) age at first offspring weaned, (d) fecundity, (e) reproductive lifespan and (f) LRS for female bighorn sheep from cohorts 1973 to 2012 at Ram Mountain, Alberta, Canada. Dashed vertical line represents the near cessation of trophy hunting in 1996. Smooth line was fitted using loess. Point size represents overlapping data points.

Figure 2.

Associations of three different classes of female horn length at 2 years with (a) proportion of primiparous females and (b) proportion of females that weaned their first offspring across ages. Dotted line, dashed line and solid line represent short, medium and long horn classes, respectively. Females with standardized horn lengths less than −0.5 s.d, between −0.5 and 0.5 s.d. or greater than 0.5 s.d., respectively, were assigned to the short, medium and long horn length classes, respectively. Only females that experienced primiparity or weaned at least one offspring were included: (a) n = 189 and (b) n = 182.

Figure 3.

Associations of horn length at 2 years with LRS for female bighorn sheep at Ram Mountain, Alberta, cohorts 1973–2012. Grey lines fitted with loess, for ease of interpretation.