Congenital malformations in Japanese macaques (Macaca fuscata) at Takasakiyama

From the late 1960s to the early 1970s, many congenitally malformed infants were born into provisioned Japanese macaque troops. Although the exact cause of this problem was not determined, the occurrence of malformations decreased thereafter. We examined possible factors such as total population size, number of adult females, birth rate, and volume of provisioned food. Agrichemicals attached to provisioned food are suspected as the main cause, as other factors were found to have no influence. Many more malformations were seen in males compared with females, in feet compared with hands, and in the fourth compared with other digits. We confirmed that the frequency of congenital malformation was high during the 1960s through to the mid-1970s when increased levels of provisioned food were given and that the incidence of congenital malformations was also elevated among wild macaques during this time.