Live Fast, Die Young: Trade-Offs Between Fitness Components and Sexually Antagonistic Selection on Weaponry in Soay Sheep

Abstract

Abstract. Males are predicted to compete for reproductive opportunities, with sexual selection driving the evolution of large body size and weaponry through the advantage they confer for access to females. Few studies have explored potential trade-offs of investment in secondary sexual traits between different components of fitness or tested for sexually antagonistic selection pressures. These factors may provide explanations for observed polymorphisms in both form and quality of secondary sexual traits. We report here an analysis of selection on horn phenotype in a feral population of Soay sheep (Ovis aries) on the island of Hirta, St. Kilda, Scotland. Soay sheep display a phenotypic polymorphism for horn type with males growing either normal or reduced (scurred) horns, and females growing either normal, scurred, or no (polled) horns; further variation in size exists within horn morphs. We show that horn phenotype and the size of the trait displayed is subject to different selection pressures in males and females, generating sexually antagonistic selection. Furthermore, there was evidence of a trade-off between breeding success and longevity in normal-horned males, with both the normal horn type and larger horn size being associated with greater annual breeding success but reduced longevity. Therefore, selection through lifetime breeding success was not found to act upon horn phenotype in males. In females, a negative association of annual breeding success within the normal-horned phenotype did not result in a significant difference in lifetime fitness when compared to scurred individuals, as no significant difference in longevity was found. However, increased horn size within this group was negatively associated with breeding success and longevity. Females without horns (polled) suffered reduced longevity and thus reduced lifetime breeding success relative the other horn morphs. Our results therefore suggest that trade-offs between different components of fitness and antagonistic selection between the sexes may maintain genetic variation for secondary sexual traits within a population.

Heterogeneity in individual quality overrides costs of reproduction in female reindeer

“…Extensive evidence for individual variation in the quality of female mammals and birds has been reported (Cobley et al 1998;Festa-Bianchet 1998;Gaillard et al 1998;Pomeroy et al 1999;Crocker et al 2001;Tavecchia et al 2005). Early reproduction has, however, been reported to be costly in several birds (Pyle et al 1997;Barbraud and Weimerskirch 2005), female squirrels (Descamps et al 2006), female reindeer breeding as calves (Reimers et al 2005), and male Soay sheep breeding as lambs (Robinson et al 2006). We also found a marked eVect of experience as the weaning success peaked for multiparous females, being lowest for Wrst-time breeders (Forslund and Pärt 1995;Weladji et al 2006).…”

Senescence in natural populations of animals: Widespread evidence and its implications for bio-gerontology

“…In birds, social monogamy makes separating the contributions of female and male age from reproductive performance traits challenging, although a few studies have found evidence of sex-differences in aging of reproductive performance traits (Lecomte et al, 2010; Reed et al, 2008; Reid et al, 2003), or interactions between male and female age influencing such traits (Auld and Charmantier, 2011). In mammals, age-dependent fecundity trajectories have been compared between sexes in wild red deer and Soay sheep; both studies suggested that age-related declines begin slightly later but progresses more rapidly in males than in females (Nussey et al, 2009; Robinson et al, 2006), although the generality of this pattern remains to be confirmed.…”

Common names of species, the curious case of Capra pyrenaica and the concomitant steps towards the ‘wild-to-domestic’ transformation of a flagship species and its vernacular names

“…Horn shape diversity is part of biological richness and seems to have evolutionary importance. Horn shape diversity has been associated with environment-dependent and sex-dependent fitness (Moorcroft et al 1996;Robinson et al 2006). Taking into account the key feature of true domestication cited above, the aim to select for horn shape in Capra pyrenaica is a clear step toward the 'semi-domestication' of the species.…”