Long-term male-to-male effect progressively activates testosterone and socio-sexual behavior in seasonally resting male goats.

The short-term buck-to-buck effect refers to the rapid stimulation of LH, testosterone, and sexual behavior in resting males after the introduction of a sexually active (SA) buck, but the long-term effects of such exposure remain unclear. This study investigated whether prolonged exposure to SA bucks stimulates endocrine activity and sexual behavior in recipient males. Recipient males were exposed either to two sexually inactive bucks undergoing sexual activation induced by a photoperiodic treatment (SA; n = 6) or to two sexually inactive (SI) bucks (n = 5). Testosterone concentrations were low and similar between groups before and during the first two weeks after exposure (P > 0.05). Thereafter, testosterone increased in recipient males exposed to SA bucks and remained higher than in those exposed to SI bucks from mid-February to mid-March (P < 0.05), before declining to comparable levels in both groups. By the end of April, testosterone concentrations were higher in males exposed to SI bucks (P < 0.05). At the time of introduction of the stimulus bucks, neither stimulus bucks nor the recipient males exhibited nudging behavior. Thereafter, from late February onward, recipient males exposed to SA bucks exhibited more frequent nudging behavior than those exposed to SI bucks (P < 0.05). These findings demonstrate that prolonged exposure to SA bucks progressively stimulates endocrine activity and socio-sexual behavior in recipient males, providing evidence for a long-term buck-to-buck effect. Notably, behavioral responses persisted even when testosterone declined, whereas delayed hormonal changes in SI-exposed males were not associated with behavioral activation.