High incidence of supernumerary nipples and twins in formosan macaques (Macaca cyclopis) at Mt. Longevity, Taiwan.

A population of Formosan macaques at Mt. Longevity exhibits an unusually high incidence of supernumerary nipples (polythelia: between 1-6 accessory nipples and/or areolae on 33% of adults), as well as a high rate of twinning (about 1% of births). The coexistence of these unusual traits suggests a connection, which is further supported by a tendency for mothers of twins to have accessory nipples and for twins to be born in troops with high incidence of polythelia.