OPTIMAL MATING STRATEGIES IN NONTERRITORIAL UNGULATES - A GENERAL-MODEL TESTED ON MUSKOXEN

We present a marginal value model explaining intraspecific and intersp ecific variation of mating systems in nonterritorial ungulates. The mo del takes into account the simultaneous effects of spatial and tempora l distribution of females, female group size, male-male competition, f emale choice, and the operational sex ratio (i.e., the proportion of e strous females). The model predicts that higher numbers of females per group increases the average exploitation time of such groups by males . An increase in female group density, operational sex ratio, and age- specific fighting success of males are predicted to reduce the average exploitation time of female groups, leading to roving of males (i.e., moving between female groups). In contrast, an increase in the female rejection rate of males and in the time spent by males on nonmating a ctivities (i.e., foraging, lying down, ruminating, migrating) are pred icted to increase the average exploitation time of female groups and t o favor staying behavior of males (i.e., defending a female group over a longer period of time). Consequently, the model predicts that young males will tend to be ''stayers,'' whereas middle-aged and old males are expected to be ''rovers.'' Model predictions were tested with fiel d data collected on muskoxen Ovibos moschatus in a natural population in west Greenland. Observed correlations between bull exploitation tim es of cow groups and the six above-mentioned social and environmental parameters were all in the predicted direction and statistically signi ficant in five of the six cases. Overall, 69% of the observed variatio n in exploitation time of cow groups by males was explained by the mod el predictions. Stepwise regression suggested that, of the six paramet ers, variation in sex ratio (R-2 = .56) and time spent on nonmating ac tivities (R-2 = .35) had the largest effects on male exploitation time . Also, the observed age-specific variation in bull exploitation time of cow groups was as predicted.