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A population of adders (Vipera berus) in southern Sweden were studied from 1984-90 to determine whether whether there is temporal variability exists in selective forces, and whether alternative reproductive tactics exist as a consequence of variable selective forces. The results showedthat in adders, intensity of sexual selection for dominance over rival males varied considerably in different years, and depended on the number of reproducing males and females in the population. The implications for findings on the effects of sexual selection on the evolution of mating systems and sexual dimorphism based on short-term studies were discussed.

A temporal-threshold model for polygynous mating (TMPM) is presented. The model is an extension of the polygyny-threshold model and shows how polygyny costs and cyclical environments affect the temporal patterns of reproduction in polygynous animals. The mating system of the African elephant (Loxodonta africana) and the spawning behavior of the Hawaiian Potter's angelfish (Centropyge potteri) were seen to correspond to the predictions of the TMPM, which is applicable only to species where females have limited access to available males.