Since McClintock [Nature 229:244-255, 1971] first reported menstrual synchrony in women, a number of studies have reported similar phenomena. Many researchers have suggested that one of the proximate factors leading to synchrony is spatial proximity among females (e.g., close friends or roommates). However, most studies on menstrual synchrony have been conducted in limited spaces, and it remains to be determined whether controlled environments, such as those used in experiments, actually exist in the wild. In this study, we analyzed the relationship between proximity and estrous synchrony using data from wild female chimpanzees at Mahale, Tanzania. In the cycling females, we observed two pairs that spent a large amount of time together. We compared the estrous synchrony indices (ESIs) between these two pairs and the other females. Our results showed that the ESIs of the high-proximity pairs did not differ from those of other pairs.
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