Life-History Differences in Age-Dependent Expressions of Multiple Ornaments and Behaviors in a Lekking Bird

Age is a major factor explaining variation in life history traits among individuals with typical patterns of early life improvement, prime age and senescence in reproductive success and survival. However, age-dependent variation in the expression of sexually-selected traits has received less attention although such variation underpins differences in male competitive abilities and female preference that are central to sexual selection. Contrary to previous studies focusing on single traits, we used repeated measures of seven sexually-selected morphological and behavioral traits in male black grouse (Tetrao tetrix) to quantify the effects of age and lifespan on their expression, and quantified this variation in relation to male reproductive effort. Trait expression increased with age but long-lived males had slower increase and delayed maxima in trait values compared to short-lived males. There was evidence of terminal investment (increasing trait value during the last breeding season) in some and senescence in all traits. These trait dynamics were largely explained by the timing of male peak lekking effort. This study shows that fully understanding the variation in sexually-selected traits and fitness benefits associated with sexual selection requires accounting for the complex interaction between individual age, lifespan and the timing of individuals’ investment in reproduction