Abstract

1. Individuals' reproductive success is often strongly associated with their 18 age, with typical patterns of early life reproductive improvement and late 19 life senescence. These age-related patterns are due to the inherent trade-20 offs between life history traits competing for a limited amount of resources 21 available to the organisms. In males, such trade-offs are exacerbated by the 22 resource requirements associated with the expression of costly sexual 23
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traits, leading to dynamic changes in trait expression throughout their 24 lifespan. 25
2. Due to the age-dependency of male phenotypes, the relationship between 26 the expression of male traits and mating success can also vary with male 27 age. Hence, using longitudinal data in a lekking species with strong sexual 28 selection – the black grouse Lyrurus tetrix – we quantified the effects of 29 age, lifespan and age of first lek attendance (AFL) on male annual mating 30 success (AMS) to separate the effects of within-individual improvement 31 and senescence on AMS from selective (dis)appearance of certain 32 phenotypes. Then, we used male AMS to quantify univariate and 33 multivariate sexual selection gradients on male morphological and 34 behavioural traits with and without accounting for age and age-related 35 effects of other traits. 36
3. Male AMS increased with age and there was no significant reproductive 37 senescence. Most males never copulated and of the ones that did, the 38 majority had only one successful year. Lifespan was unrelated to AMS, but 39 early AFL tended to lead to higher AMS at ages 1 to 3. AMS was related 40 to morphological and behavioural traits when male age was ignored. 41 Accounting for age and age-specific trait effects (i.e. the interaction 42 between a trait and age) reduced the magnitude of the selection gradients 43 and revealed that behavioural traits are under consistent sexual selection, 44 while sexual selection on morphological traits is stronger in old males. 45
4. Therefore, sexual selection in black grouse operates primarily on male 46 behaviour and morphological traits may act as additional cues to 47 supplement female choice. These results demonstrate the multifaceted 48
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influence of age on both fitness and sexual traits and highlight the 49 importance of accounting for such effects when quantifying sexual 50 selection.