Fear reduces sexual behavior in primates and mice

Fear has adaptive value in increasing survival against risks and dangers. However, fear poses a dilemma in that it affects fitness through sexual behavior that transfer genetic information to the next generation. How conspecific aggression and predator-induced fear affect sexual behavior and whether the deficit behavior could be restored remains to explored. With our study of wild Yunna snub-nosed monkeys (Rhinopithecus bieti), we found that bachelor males who were fearful of harem-holder males had significantly shorter mating times than harem-holder males. In our parallel study of laboratory mice, we found reduced mounting, thrusting, and ejaculating behavior in male mice with predator-induced fear; whereas the deficit behaviors were restored once the fear was removed. Using our findings, we developed a theoretical model to describe the relationship between sexual behavior and two factors: fear and relaxation. Males' sexual behavior decreased with the intensification of fear but was enhanced as fear was gradually replaced with relaxation. Ultimately, the interacting effects of these two factors establish a sexual behavior equilibrium point to maintain an appropriate strategy for survival and reproduction. These results provide us with an evolutionary framework for understanding the relationship between fear, relaxation, and sexual behavior in mammals.