Optimal mating strategies in nonterritorial ungulates: a general model tested on muskoxen

Mads C. Forchhammer, Jacobus J. Boomsma
1998 Behavioral Ecology  
We present a marginal value model ^plaining mtraspecific and interspecific variation of mating systems in nonterritorial ungulates. The model takes into account the simultaneous effects of spatial and temporal distribution of females, female group size, male-male competition, female choice, and the operational sex ratio (i.e., the proportion of estrous females). The model predicts that higher numbers of females per group increases the average exploitation time of such groups by males. An
more » ... 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 die female rejection rate of males and in the time spent by males on nonmating activities (Le., foraging, lying down, ruminating, migrating) are predicted to increase the average exploitation time of female groups and to favor staying behavior of males (Le., defending a female group over a longer period of time). Consequently, die 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 widi field data collected on muskoxen Ovibos moschatus in a natural population in west Greenland. Observed correlations between bull exploitation times of cow groups and the six above-mentioned social and environmental parameters were all in die predicted direction and statistically significant in five of die six cases. Overall, 69% of die observed variation in exploitation time of cow groups by males was explained by die model predictions. Stepwise regression suggested that, of die six parameters, variation in sex ratio (R 1 =» .56) and time spent on nonmating activities (iP «• .35) had the largest effects on male exploitation time. Also, die observed age-specific variation in bull exploitation time of cow groups was as predicted. Key words: female distribution, muskox, nonterritorial ungulates, operational sex ratio, optimal mating strategies, Ovibos moschatus, sexual selection. [Behav Ecol 9:136-143 (1998)] M ale reproductive strategies in mammal* are closely linked to die spatial and temporal distribution of females. These distributions are ultimately determined by variation in environmental and social conditions of local populations (Clutton-Brock, 1989; Clutton-Brock and Harvey, 1978; Davies, 1991; Emlen and Oring, 1977) so a wide range of male reproductive strategies has been reported, bodi between species and among populations within species (Clutton-Brock, 1989; Gosling, 1986). Theoretically, die interactions between die various selection forces leading to differences in ungulate mating systems are not well understood. In particular, die fitness consequences of defense and monopolization of groups of females on one hand, and die frequendy observed movements of males between female groups on die other, has been difficult to explain in quantitative terms (Gosling, 1986; Clutton-Brock, 1989). Previous evolutionary analyses of mating systems in ungulates have therefore been primarily based on classifications and verbal predictions (e.g., Clutton-Brock and Harvey, 1978; Davies, 1991; Emlen and Oring, 1977), which are difficult to test widi specific empirical data. In die only available model study, Sandell and Liberg (1992) analyzed die fitness payoffs of "roving behavior" (i.e., moving between female groups in search of estrous females) and "staying behavior" (i.e., staying and defending a series of M. C Forchhammer is now at the Division
doi:10.1093/beheco/9.2.136 fatcat:c2anc32kibfzjk24yunexvd4bi