Contrast of Estrus in Accordance with Social Contexts Between Two Troops of Wild Japanese Macaques on Yakushima.

Abstract

I observed estrus in two adjacent troops (A-troop and M-troop) of wild Yakushima macaques during the 1984 mating season. A-troop females showed short and regularly cycling estrus periods, seldom came into estrus simultaneously, and scarcely failed to mate when they were in estrus. In contrast, females of M-troop, which went through troop takeovers during the mating season, demonstrated mate competition and subsequent estrus prolongation. Early in the mating season, constantly four to seven females of M-troop were in estrus simultaneously, competed for troop males (TMs), and some of them failed to mate. Subsequently many non-troop males (NTMs) approached to the troop, estrous females mated with them frequently, and mating harassments by females decreased. Among those NTMs, one dominated all TMs, which was referred to as a troop takeover. Successively three other NTMs dominated both all TMs and the former dominant NTM, and in total, four troop takeovers occurred in M-troop during this mating season. The females continued to be in estrus during the whole episodes. All the conceived females also showed prolonged postconception estrus, and they were neither inactive nor less attractive to males. It was suggested that M-troop females increased their opportunity to mate with NTMs by estrus prolongation, at the cost of female-female mate competition, to incite active male intertroop movement and subsequent troop takeovers.