Annual physiological changes in individually housed squirrel monkeys (Saimiri sciureus)

This study investigated whether annual changes in physiology occur in indiv idually housed squirrel monkeys (Saimiri sciureus). Physiological measures were monitored for 20 months. Over the course of the study, all individuall y housed males and females exhibited clear annual changes in gonadal and ad renal hormone levels, and males exhibited species-typical changes in body w eight. Females exhibited a typical pattern of hormonal changes, with elevat ions in gonadal steroids occurring during the same months as elevations in cortisol. Males, however, exhibited an atypical pattern, as elevations in h ormone levels were not synchronized with each other; rather, elevations in testosterone occurred out of phase with changes in cortisol and body weight . The timing of annual events in individually housed subjects was compared to that in nearby social groups, in which the timing of the breeding season from year to year was determined by social group formations and was outsid e the naturally occurring breeding season. Elevations of ovarian and adreno cortical hormones in individually housed females were synchronized with ind ices of breeding in heterosexual social groups. Similarly, weight gain in m ales was associated with elevations in cortisol and, as with socially house d males, tended to precede seasonal breeding in the social groups. In contr ast, annual testosterone elevations for individually housed males were not synchronized with breeding in nearby social groups. We conclude that direct physical interaction is not required for the annual expression of breeding readiness. Synchrony of seasonality among squirrel monkeys may be accompli shed by distant social cues in females, but males may require physical inte raction for complete synchrony of annual physiological changes. Am. J. Prim atol. 47:93-103, 1999. (C) 1999 Wiley-Liss, Inc.