We provide data on the timing and frequency of bellowing episodes and 261 copulations in North American bison (Bison bison) extracted from more than 8500 h of observation to examine Darwin's idea that by using breeding-season vocalizations the male “endeavours thus to charm or excite the female.” Copulatory status affected vocalization rates; after mating, the frequency of male bellowing dropped to 16.1% of precopulatory rates (p < .0001). Males bellowed at a higher rate on days when females were in estrus (p < .025) than when they were in anestrus, but females' probability of offspring production in the next year was not correlated with bellowing rate. For estrous females, bellowing was positively correlated with number of attendant males (p < .0001) and inversely related both to the number of days individual males had participated in the rut (p < .05) and to the date during the 6-week breeding season (p < .003). Body size also influenced bellowing rate: with other factors held constant, small males bellowed more than large males (p < .03). With respect to Darwin's idea that vocalizations serve as a male display to females, (1) males bellowed neither before nor after copulating when rivals were absent but (2) they bellowed both before and after copulating when rivals were present. Overall, these results suggest that bison bellows do not serve as advertisements to females but function as intrasexual displays. [Behav Ecol 1991; 2: l-6]
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Berger et al. (1991) studied this question.