Evidence for deceptive fertility in a wild primate

Animal signals typically convey reliable information, but deception can evolve when the sender and receiver have conflicting interests-especially in the context of mating. Here, we provide evidence from a Cercopithecine primate, the gelada (Theropithecus gelada), that females deceptively signal fertility when conception is unlikely, which functions as a counterstrategy in sexual conflict. In geladas, male takeovers are frequent and often lead to sexually selected infanticide, exacting high costs on lactating females. Using 14 years of demographic and hormone data from wild geladas in Ethiopia, we show that lactating females quickly resumed sexual swellings and mated with the new male following takeovers, but they took significantly longer to conceive than females resuming cycling at other times. Females that exhibited these post-takeover swellings were subsequently less likely to lose their infants to infanticide. Fecal hormone data revealed a surge in estrogens after takeovers, even among females with the youngest infants, suggesting that estrogens mediate both fertile ("true") and non-fertile ("false") swellings. These results support the idea that sexual swellings can deceptively blur fertility as an adaptive counterstrategy to infanticide.