Evidence for sex-specific reproductive senescence in monogamous cooperatively breeding red wolves

Abstract

Sex-specific senescence has been construed as a function of mating system and differential investment in parental care, with males exhibiting low parental investment predicted to have more rapid senescence due to costly reproductive behavior. In monogamous mating systems, however, where parental investment may be more evenly distributed, rates of senescence are predicted to be more equivalent between the sexes than in polygynous mating systems. While many polygynous species do appear to support this pattern, evidence from monogamous species, particularly mammals, is scarce. Wolves are an excellent system with which to test this hypothesis, as they exhibit both monogamy and cooperative breeding, where parental investment is distributed across both breeders and non-breeders of both sexes. We examined patterns of age-specific reproduction in red wolves and red wolf-coyote hybrids. We found no evidence of decline in pup production with age in male wolves; among females, the production appeared better explained by hybrid status and age at first reproduction than age per se. Remarkably, however, there was strong evidence of sex differences in pup recruitment, with males, but not females, showing a steep decline in recruitment with age. Combined with previous work on aging in captive wolves, our findings contribute not only to the current understanding of the relationship between mating system and senescence but also to the plasticity of aging and the dynamics of female mate choice in the wild. In this study, we test for signs of reproductive aging in red wolves using a 20-year dataset documenting age-specific reproduction in the wild. We find evidence that reproductive aging in males is evident for number of pups recruited, though not for number of pups born; we find no evidence for reproductive aging at either stage in wild female wolves. This sex difference stands in contrast to the general prediction that patterns of aging would be more monomorphic among monogamous species. Furthermore, this is especially of interest given that red wolves are cooperative breeders, with costs of reproduction spread across family groups. In general, this study constitutes a valuable contribution to an emerging literature geared at understanding patterns of aging in the context of diverse social and ecological variables.