Antler size in red deer: declining selection and increasing genetic variance with age, but little senescence.

ABSTRACT

While senescence is a common occurrence in wild populations, not all traits decline with age simultaneously and some do not show any senes- cence. A lack of senescence in secondary sexual traits is thought to be due to their importance for reproductive success. However, if reproduc- tive success senesces, why would secondary sexual traits apparently not senesce? Here we explored this question in a wild population of red deer (Cervus elaphus) using antler form (number of points), a secondary sexual trait which shows little senescence, despite the occurrence of reproductive senescence. In line with expectations for traits that senesce, genetic vari- ance in antler form increased with age and selection weakened with age. Therefore, there was no indication that stronger selection on individu- als that survived to older ages was countering the dilution of selection due to fewer individuals being alive. Furthermore, the effect of selec- tive disappearance masking a slight decline in antler form in the oldest years was small. Interestingly, although genetic variance and positive se- lection of antler form were found, there was no evidence of a response to selection, supporting a genetic decoupling of antler senescence and re- productive senescence. Finally, a positive genetic covariance in antler form among age classes provides a possible explanation for the the lack of senescence. These findings suggest that antler form is under a genetic constraint that prevents it from senescing, providing an interesting evolu- tionary explanation for negligible senescence in a secondary sexual trait, and consequently, the existence of asynchrony in senescence among traits within populations.