Determinants of yearling male lekking effort and mating success in black grouse (Tetrao tetrix)

Age at first reproduction is a crucial component of individual fitness as it often determines the length of reproductive lifespan. The reproductive success of males generally varies more than that of females, but it is challenging to study because the genetic data or proper surrogate measure needed to investigate reproductive success are usually not available. In black grouse (Tetrao tetrix), a lekking species with strong male mating skew and female preference for older males, there is a strong relationship between observed matings and genetic paternity. Using this relationship, we studied the effects of morphological, and behavioral traits on probability of being territorial, mating success, and survival of yearling males. Heavier yearling males were more likely to be territorial, and higher population density increased the frequency of yearling male territoriality. Mating success was positively related to population density, lek attendance, and fighting rate, but not to morphological traits. Overwinter survival did not differ between territorial and nonterritorial yearling males. Our results show that yearling male black grouse in good condition can establish territories and have some limited mating success, especially during increasing population density. In black grouse, the direct fitness benefits gained as yearlings undoubtedly contribute substantially to individual fitness, as the high reproductive skew means few males successfully copulate during their lifetime. For other species, early reproduction may relate to individual lifetime mating success but depends both on the direction and magnitude of the relationship between age-specific mating success and survival, and, as our results also demonstrate, on extrinsic factors such as population density.