Abstract
Individuals' reproductive success is often strongly associated with their age, with typical patterns of early-life reproductive improvement and late-life senescence. These age-related patterns are due to the inherent trade-offs between life-history traits competing for a limited amount of resources available to the organisms. In males, such trade-offs are exacerbated by the resource requirements associated with the expression of costly sexual traits, leading to dynamic changes in trait expression throughout their life span. Due to the age dependency of male phenotypes, the relationship between the expression of male traits and mating success can also vary with male age. Hence, using longitudinal data in a lekking species with strong sexual selection - the black grouse Lyrurus tetrix - we quantified the effects of age, life span and age of first lek attendance (AFL) on male annual mating success (AMS) to separate the effects of within-individual improvement and senescence on AMS from selective (dis)appearance of certain phenotypes. Then, we used male AMS to quantify univariate and multivariate sexual selection gradients on male morphological and behavioural traits with and without accounting for age and age-related effects of other traits. Male AMS increased with age, and there was no significant reproductive senescence. Most males never copulated, and of the ones that did, the majority had only one successful year. Life span was unrelated to AMS, but early AFL tended to lead to higher AMS at ages 1-3. AMS was related to most morphological and behavioural traits when male age was ignored. Accounting for age and age-specific trait effects (i.e. the interaction between a trait and age) reduced the magnitude of the selection gradients and revealed that behavioural traits are under consistent sexual selection, while sexual selection on morphological traits is stronger in old males. Therefore, sexual selection in black grouse operates primarily on male behaviour and morphological traits may act as additional cues to supplement female choice. These results demonstrate the multifaceted influence of age on both fitness and sexual traits and highlight the importance of accounting for such effects when quantifying sexual selection.
Highlights
Sexual selection underpins the evolution of extravagant male ornaments either through female preference for higher expression of these traits or through the competitive advantage they offer during male-male competition (e.g.male weaponry, Andersson 1994)
Our results suggest that the trajectories of expression of male traits are partially independent from male mating success, and that the outcome of sexual selection cannot be directly linked to the expression of each trait separately
This study shows that the overall variation in male black grouse mating success is strongly related to male age with substantial nonlinear increase over individuals’
Summary
Sexual selection underpins the evolution of extravagant male ornaments either through female preference for higher expression of these traits (e.g. colourful feathers) or through the competitive advantage they offer during male-male competition (e.g.male weaponry, Andersson 1994). Apparent age-dependent reproductive success may arise due to selection acting against individuals in poor condition leading to an increase in the proportion of 'high quality' individuals within a cohort (selective appearance of good reproducers / selective disappearance of bad reproducers; van de Pol & Verhulst 2006), or due to the negative effects of ageing on individuals’ physiological condition, which leads to a reduction in reproductive success, i.e. senescence (Stearns 1992; Williams et al 2006; Bouwhuis et al 2010; Nussey et al 2013) All these mechanisms can explain the association between male age and their ability to mate and produce offspring. Fully understanding the variation in individuals’ fitness requires quantifying the temporal dynamics in reproductive success that emerges from life history trade-offs
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