Female elephants in estrus actively solicit guarding from musth males, but only rarely solicit it from nonmusth males at Amboseli. When females were in midestrus they were guarded and mated mainly by the guarding males. Over 90% of the guarding males were in musth, while 38 of 49 matings observed during midestrus were by guarding musth males. Further, nearly 90% of males that guarded females in midestrus were over 35 years old. During early and late phases of estrus, the females were usually not guarded and were frequently chased and sometimes mated by younger, nonmusth males.
Several interesting aspects of elephant reproductive behavior emerge from these observations. By soliciting guarding behavior from large, musth males, the female elephant exercises a certain degree of mate choice. The older the male, the better its success in guarding females in midestrus when conceptions are more likely. Reproductive success per unit time in males thus keeps increasing with age (except perhaps at very old age if a bull survives to its maximum potential longevity) (fig. 3.5). This is in contrast to most other mammals, for which reproductive success seems to peak during middle age and declines subsequently. Unlike other mammals, however, elephants continue to increase in body weight practically throughout their life span, certainly up to 50 or more years of age (see appendix 2). Given the positive correlation between body size and reproductive success, it is not surprising that increased reproductive success with age in male elephants is also seen.
Musth, of course, is crucial to reproductive success in male elephants. Why should this be necessary? If there are two sexually active bulls of similar size and age with different musth status, why should the musth bull have a mating advantage over the nonmusth bull? What has been the adaptive significance of musth in the course of evolution?
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