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If females suffer from increased harassment for copulation in male-biased or high male density populations (Clutton-Brock and Parker 1995; Stockley 1997), the cost of choosiness for females may increase.Females may become more likely to accept copulations indiscriminately to avoid harassment.In particular, age-dependent sensitivity to demographic parameters may affect population dynamics, thereby influencing the response of populations to natural and sexual selection (Pfister 1998).In a large number of animal species, individual performance is known to follow age-dependent patterns (Martin 1995; Gaillard et al. Typically, survival and reproductive success increase with age in younger individuals, subsequently decreasing as individuals get older (a phenomenon called senescence; Comfort 1979).
Thus they may critically affect the intensity of sexual selection in the populations.
Our results suggest that the intensity of sexual selection was weaker in male-biased populations, supporting new theoretical models.
In populations with a male-biased sex ratio, reproductive success was more equally distributed between males and, unlike female-biased populations, the choosiest females (middle-aged) did not obtain sires of higher quality than low-performance females.
Conversely, when sex ratio is female-biased or density is lower, females would be less choosy because of a low male encounter rate.
On the other hand, new theoretical works have proposed opposite predictions for demographic effects on competition for mates (Kokko and Rankin 2006).