Contact calls, which function to coordinate group movement and maintain contact between conspecifics, are predicted to show high levels of acoustic variability and individual distinctiveness. We investigated interindividual variation in whinnies, a contact call, between two geographically distinct communities of wild Geoffroyi's spider monkeys (Ateles geoffroyi), which were experiencing different degrees of stability in membership due to immigration. We recorded whinnies from 18 subjects, including 9 females ranging within the Otoch Ma'ax Yetal Kooh Reserve, Punta Laguna, Mexico, and 9 females ranging within the Santa Rosa Sector, Area de Conservación Guanacaste, Costa Rica. We examined 13 acoustic parameters of female whinnies using principal component analysis and discriminant function analysis. Individual acoustic variability was significantly different between the two communities. A higher percentage of the whinnies of females were assigned to the correct caller in the community with only 3 individuals immigrating within 36 mo before and during data collection than in the community with 15 immigrant individuals during the same period. We suggest that the variation in interindividual distinctiveness for each community was influenced by the stability of the vocal environment, which was quantitatively different between communities because of changes in membership. © 2013 Springer Science+Business Media New York.