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

Rolando Rodríguez-Muñoz,David Fisher,Tom Tregenza,Jelle J Boonekamp,Paul Hopwood

doi:10.1098/rspb.2019.0286

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.

Highlights

Alleles promote their continued existence in populations by conferring resilience on the organisms that carry them, or by facilitating the production of new individuals carrying identical copies
We should expect to see antagonistically pleiotropic alleles [1] that increase reproduction at the expense of decreasing lifespan. This is the basis for adaptive life-history theories of senescence [2,3]. These theories posit that individuals trade-off investment in maintenance of their bodies against investment in reproduction, explaining why organisms decline in physiological performance and survival probability with age
To allow us to directly compare phenotypic and actuarial senescence, we examined change in calling activity while including sex ratio as a categorical variable in the same way we did for the analysis of actuarial senescence

Summary

Introduction

Alleles promote their continued existence in populations by conferring resilience on the organisms that carry them (survival), or by facilitating the production of new individuals carrying identical copies (reproduction). Competition among males can involve direct interactions like sperm competition or fighting, or indirect interactions through sexual displays or calling to attract females for mating [4] All such interactions involve energetic costs liable to influence the trade-off between survival and reproduction. Where a higher proportion of males results in higher levels of male–male competition we would predict males should senesce faster than in generations where proportionately fewer males leads to less intense competition This prediction does not appear to have been explicitly set out in the literature, but for males it is a straightforward consequence of the well-established relationship between sex ratio and the strength of sexual selection [5], and the putative trade-off between investment in reproduction and somatic maintenance [2,3,6]. This leads to our prediction that male senescence will be more pronounced in these years

Methods

Results

Discussion