This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Objective: To determine how chemical communication activates the neuroendocrinology of cooperative breeding primates. Fathers of cooperatively breeding species of primates play a critical role in the successful rearing of offspring. While all family members of primate cooperative breeders carry infants, only the father provides the full range of infant care, such as assisting during the critical first days of life. Human fathers are extremely important to the well-being and increased fitness of their offspring and several studies have found that human fathers show lower levels of testosterone associated with paternal care in several different societies. The cooperative breeding male common marmoset (Callithrix jacchus) is especially responsive to social changes altering testosterone. Males show increased levels of serum testosterone in response to smelling isolated ovulatory scents of novel females and father marmosets show decreased testosterone after the birth of infants to their mate (Prudom et al., 2007). Marmosets show the same lowered testosterone response to fatherhood as human fathers, making them a unique model to study neurosensory systems and paternal behavior. Neurosensory mechanisms influence male testosterone in cooperative or monogamous primates. Recent studies in our lab have found a neurosensory mechanism occurring in father marmosets via infant olfactory cues. We have found significant differences between testosterone levels in father marmosets when smelling their own infant's scent versus smelling a vehicle control. Fathers that are experienced at rearing their own infants have significantly lower testosterone 30 minutes following smelling an isolated scent of their own three-week-old infant whereas inexperienced males, or males who have never produced and raised their own offspring show no differences between vehicle and unfamiliar three-week-old infant scent. These data suggest that a father's own infant odor (without other sensory cues) stimulates the olfactory system and ultimately stimulates the hypothalamus and pituitary to affect the release of testosterone from the gonads. The consistently lowered testosterone following birth in marmoset fathers may be due to the olfactory interactions of the male with his infants resulting in positive parenting behavior. Funding ended before this reporting period;one publication resulted.