Sex and Sexual Strategies in Deep-Diving Risso’s Dolphins

Abstract

AbstractRisso’s dolphins (Grampus griseus) in the Azores maintain a sexually stratified community with males forming stable clusters, whereas females tend to associate in temporally stable units when calving and nursing. Large relative testes size indicates a mating system based on sperm competition. Small sexual size dimorphism and long interbirth intervals may facilitate male cooperation. We describe mating tactics observed in a resident population of Risso’s dolphins based on data collected over 8 consecutive years. We distinguished groups by sex and age class and analyzed peaceful and aggressive behaviors using an ethogram. Males displayed a variety of mating tactics described in cetaceans, spanning display, contest, endurance, and scramble competition, in the assumed context of sperm competition. We observed rare behaviors including male care for neonates but also intense aggression toward newborn calves, leading to possible infanticide. Females’ mating tactics included evasive behaviors and signal discrimination. We hypothesize that male mating tactics of Risso’s dolphins are related to age class. For females, the polygynandrous mating system may serve to improve fertility, reduce sexual harassment, and reduce the risk of infanticide. Adult females seem to prefer older over younger male groups, suggesting that some level of female mate choice exists in this species.