Slower senescence in a wild insect population in years with a more female-biased sex ratio

Abstract

Life-history theories of senescence are based on the existence of a trade-off in resource allocation between body maintenance and reproduction. This putative trade-off means that environmental and demographic factors affecting the costs of reproduction should be associated with changes in patterns of senescence. In many species, competition among males is a major component of male reproductive investment, and hence variation in the sex ratio is expected to affect rates of senescence. We test this prediction using nine years of demographic and behavioural data from a wild population of the annual field cricket Gryllus campestris. Over these generations, the sex ratio at adulthood varied substantially, from years with an equal number of each sex to years with twice as many females as males. Consistent with the predictions of theory, we found that in years with a greater proportion of females, both sexes experienced a slower increase in mortality rate with age. Additionally, phenotypic senescence in males was slower in years when there were more females. Sex ratio did not affect the baseline mortality rate in males, but females suffered higher age-independent mortality rates when males were in short supply.

Sex-specific patterns of senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

“…Females in the 75%-male sex-ratio condition (with the higher reproductive effort) suffered from an increased mortality compared to females in the other conditions. While we did not directly observed the mating behaviour in our experiments, frequent mating events and harassments by males may explain this accelerated mortality and is in line with previous observations from other insect species [34,[63][64][65]. Fig.…”

“…It is often assumed that most of the cost of reproduction in males involves sexual pre-copulatory competition. Thus males in the 75%-male sex-ratio condition could have been expected to engage in strong and costly intra-sexual competition for females, resulting in low reproductive success and shorter longevity compared to the other sex-ratio conditions, as previously shown in vertebrates [11,71,72] and invertebrates [34]. However, although male fertility slightly declined with age, it was not affected by the experimental sex-ratio condition, suggesting that males exhibited similar patterns of reproductive senescence, independently of their reproductive effort.…”

“…Recent correlative evidence suggests that population structure, such as sex-ratio, affects individual reproductive effort with potential sex-specific consequences on senescence [34,35]. In particular, variation in sex-ratio is predicted to modulate the cost of mating, through the strength of sexual selection in males [36], influencing the putative trade-off between reproductive effort and somatic maintenance [11].…”

Sex-specific patterns of senescence in artificial insect populations varying in sex-ratio to manipulate reproductive effort

“…Females in the 75%-male sex-ratio condition (with the higher reproductive effort) suffered from an increased mortality compared to females in the other conditions. While we did not directly observed the mating behaviour in our experiments, frequent mating events and harassments by males may explain this accelerated mortality and is in line with previous observations from other insect species [34,[63][64][65].…”

Silver‐spoon upbringing improves early‐life fitness but promotes reproductive ageing in a wild bird

“…To assess the differences among the posterior distributions of our brood size manipulation treatments we used the Kullback–Leibler divergence calibration (KLDC) (Kullback & Leibler 1951; McCulloch 1989; Karabatsos 2006; Boonekamp et al 2014; Atema et al 2016; Rodríguez‐Muñoz et al 2019), which show the mean Kullback–Leibler discrepancies (KL) between two distributions. Values closer to 0.5 imply that there is minimal difference among the distributions and values closer to 1 imply major differences (Kullback & Leibler 1951; McCulloch 1989).…”