Birth timing shift with altitude and its ecological implication for Macaca thibetana at Mt Emei
I used cumulative birth rate (CBR) at the middle of the birth season to estimate birth timing in a population of Tibetan macaques in 1992, rather than the periodic survey every 2 or 3 days in 1986 on the same population. In a six day survey (beginning March 18), 11 groups at altitudes between 750-2,400m showed that the CBR significantly decreased with the decline of the groups' range elevations, verifying that infants were bom earlier in the season at higher altitudes in the subtropical-temperate transition zone. Considering seasonal changes in body weight and weight dimorphism observed in the macaque, the shift suggests that the reproductive schedule was optimized to ensure the nutrition needed by mating parents, fetuses and newborns, as well as both weaning infants and their mothers in autumn and the coming winter respectively. Accordingly, changes in temperature, photoperiod or rainfall may function as proximate factors of breeding onset to signal food production or availability in different climate zones.
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