DENSITY-RELATED CHANGES IN SEXUAL SELECTION IN RED DEER

In sexually dimorphic mammals, high population density is commonly ass ociated with increased mortality of males relative to females and with female-biased adult sex ratios. This paper investigates the consequen ces of these changes on the distribution of male breeding success, the intensity of competition for females and the opportunity for sexual s election. After the red deer (Cervus elaphus L.) population of the Nor th Block of Rum (Inner Hebrides) was released from culling, female num bers rose and male numbers declined, leading to an adult sex ratio of around one male to two females. This change was the result of increase d mortality of males relative to females during the first two years of life; of increased emigration rates by young males; and of reduced im migration by males from outside the study area. The increasing bias in the adult sex ratio affected the timing of breeding as well as the di stribution of mating success in males. As the adult sex ratio became i ncreasingly biased towards females, the degree of skew in mating succe ss (calculated across all harem-holders) increased, but mature males d efended harems for shorter periods and a higher proportion of males he ld harems. In addition, a higher proportion of calves were fathered by immigrant males and the proportion fathered by males born in the stud y area declined. These results support the contention that, where high population density is associated with a female-biased adult sex ratio , competition for mates is likely to decline.