Late-life costs of raising sons in bighorn sheep

Abstract

Senescence, the physiological decline associated with aging, is pervasive in nature. The age at onset and rate of senescent decline vary widely among species, among populations of the same species, and even among individuals within the same population (1). Understanding the reasons for this variability is of great importance for the field of biogerontology, as well as from an evolutionary and ecological perspective (2). In evolutionary terms, senescence can be explained by a decline in the strength of natural selection with age (3). Because not all individuals survive to the oldest ages, genes with late-acting negative effects are able to accumulate in the genome [mutation accumulation (3)], and genes that confer an advantage in early life may spread even if they are costly in later life [antagonistic pleiotropy (4)]. The disposable soma theory can be considered as a physiological manifestation of antagonistic pleiotropy and posits that optimal resource allocation between growth, maintenance, and reproduction comes with fitness consequences late in life (5). Both antagonistic pleiotropy and disposable soma theories therefore predict trade-offs in performance between early and late life (6), and high resource allocation to reproduction early in life is expected to be associated with accelerated senescence (4, 5). Douhard et al. (7) examine whether the rate of senescence in a wild population of bighorn sheep ( Ovis canadensis ) is dependent on allocation of resources to reproduction in early life, testing key predictions of evolutionary and life-history theory (Fig. 1). Fig. 1. Reproduction early in life can be measured through various traits, such as the number of offspring produced, number of sons vs. daughters produced (sex ratio), and offspring survival. Early-life reproduction may induce short-term (blue arrow) and/or long-term (orange arrow) costs on subsequent reproduction and/or survival. Early-life reproduction may thus influence patterns of age-related declines in survival and/or reproductive success. … [↵][1]1To whom correspondence may be addressed. Email: hannah.froy{at}ntnu.no. [1]: #xref-corresp-1-1