Maternal longevity and offspring sex in wild ungulates

Abstract

In species with sexual size dimorphism, the offspring of the larger sex usually have greater energy requirements and may lead to greater fitness costs for parents. The effects of offspring sex on maternal longevity, however, have only been tested in humans. Human studies produced mixed results and considerable debate mainly owing to the difficulty of distinguishing the effects of sexual dimorphism from sociocultural factors. To advance this debate, we examined how the relative number of sons influenced maternal longevity in four species of free-living ungulates (Soay sheep Ovis aries; bighorn sheep, Ovis canadensis; red deer, Cervus elaphus; mountain goat, Oreamnos americanus), with high male-biased sexual size dimorphism but without complicating sociocultural variables. We found no evidence for a higher cumulative cost of sons than of daughters on maternal longevity. For a given number of offspring, most females with many sons in all four populations lived longer than females with few sons. The higher cost of sons over daughters on maternal lifespan reported by some human studies may be the exception rather than the rule in long-lived iteroparous species.

Keywords: cost of reproduction; disposable soma; lifespan; mammals; sex ratio; trade-offs.

Figure 1.

Predicted effects of the number of sons weaned during early life (1–6 years) on lifespan (in years) for female Soay sheep that lived more than 6 years and (a) produced four to seven offspring (values within the 10–90 percentile range) during early life, (b) weaned three to six offspring (values within the 10–90 percentile range) during early life. Circles are observed lifespan in one of the imputed datasets (see Material and methods). The size of the circle is proportional to the number of females.

Figure 2.

Predicted changes in longevity (in years) with the lifetime number of sons produced (a,c,e,g) or weaned (b,d,f,h) relative to the lifetime number of offspring produced/weaned according to each species, female Soay sheep (a,b), bighorn sheep (c,d), mountain goats (e,f) and red deer (g,h). Each line represents predicted longevity for females with a different number of offspring indicated at the end of the line. Predictions are shown between the 10th and the 90th percentile of the number of offspring. Circles are observed lifespan in one of the imputed datasets (see Material and methods). The size of the circle is proportional to the number of females.