International Journal of Comparative Psychology

Exaggerated sex swellings characterized by increased size of skin surrounding the anogenital region occur in female chimpanzees in response to ovarian hormone fluctuations and are associated with increased likelihood of ovulation and female receptivity. The swellings generate heightened sexual interest from males and evoke increased male competitive behavior. To determine potential attractiveness of specific visual characteristics of these signals to males, a free choice task using push-button methodology that allowed subjects to choose one stimulus in a pair by pressing a button corresponding to the stimulus choice was developed. Initially, preferences for specific food items were first determined based on animals’ selection of actual items, followed by selection of same items from photographs and subsequently selection of preferred food utilizing push-button responses to photographs. We found that when allowed to choose between photographs of sex swellings, novel objects, and other body parts, male chimpanzees preferred images of both sex swellings and other body parts over images of objects, and showed a significant preference for sex swellings over other body parts. However, chimpanzees showed no preference in sex swelling color or symmetry. Only one subject displayed a preference for normal sized swellings rather than enlarged swellings. Overall these results suggest that when considered individually, visual characteristics alone are not sufficient to provoke preference responding.