By Anna Gionis Research Intern
Same-sex socio-sexual behaviours (SSSB) in animals are interactions between members of the same sex which are ascribed to opposite sex courtship and mating behaviours without reproduction (Bailey & Zuk 2009). These include mounting, genital contact, courting and pair bonding (Bagemihl 1999 cited by Scharf & Martin 2013). There is evidence for SSSB in more than 1500 species across the animal kingdom (Monk et al. 2019). Common bottlenose dolphins (Tursiops truncatus), for example, are known to exhibit homosexual behaviour through tactile interactions; members of the same sex, particularly in males (Silva & Spinelli, 2023), have been recorded mounting one another and stimulating the other’s genitals using a flipper, melon, fluke, or rostrum (Serres et al. 2022). Hypotheses such as social bonding, the maintenance of dominance and practice for future mating may explain this behaviour in this species (Mann, 2006). The prevalence of SSSB is puzzling: it consumes energy, decreases the opportunity for heterosexual mating and does not produce offspring (Levan et al. 2009) yet it is widespread across taxa. Considering the costs associated with it, why do bottlenose dolphins display SSSB?
Same-sex socio-sexual interactions may encourage the formation of social bonds between members of the same pod. Acosta (2015) observed that most association preferences were symmetrical homosexual interactions between males in the same pod, suggesting SSSB aids the formation of social bonds. Alliances between male bottlenose dolphins are more prevalent in calves and juveniles, and they prohibit foreign males from mating with the females of the group (Mann, 2006). Strong social bonds between young males could lead to greater group cooperation and altruistic behaviour, where recipients benefit from an individual’s behaviour at its own expense (Breed & Moore, 2015), which could indirectly increase the reproductive success of the individuals.
Establishing dominance without risk of injury may be an explanation for SSSB in bottlenose dolphins (Serres et al. 2022). Serres et al. (2019) suggested dominance hierarchies were apparent in captive cetaceans, including bottlenose dolphins, and linked to socio-sexual behaviour between males. Behaviours such as mounting, genital touching with the melon or rostrum and mouth-to-mouth contact may coincide with social dominance (Serres et al. 2019). However, in captive male bottlenose dolphins social rank did not relate to the socio-sexual behaviours they displayed (Serres et al. 2022), which contradicts this hypothesis. Conversely, in finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) these researchers found evidence for a link between SSSB and a social hierarchy, with male pairings often containing a passive male and a dominant one, who would exhibit more socio-sexual behaviour (Serres et al. 2022). This suggests that SSSB may not be a mechanism of establishing dominance in bottlenose dolphins but may be in other cetacean species.
Juvenile bottlenose dolphins may exhibit SSSB to practice for future heterosexual mating (Silva & Spinelli, 2023). Young males display higher levels of SSSB than other age classes (Silva & Spinelli, 2023), which could relate to the practice of sexual reproduction; however, the establishment of social bonds could also be the explanation of this (Acosta, 2015; Mann, 2006). Practice was rejected as a hypothesis for SSSB by both Acosta (2015), who concluded social bonding was the likely explanation for this behaviour in bottlenose dolphins, and Serres et al. (2022), who suggested SSSB may be species and context dependent. However, to my knowledge practice for sexual reproduction in bottlenose dolphins has not been directly tested as a hypothesis. Future research could test this theory in captive dolphins to determine whether males raised in a male-only pod until reproductive age have a higher reproductive success than separate males being raised in a female-only environment until reproductive age.
Social bonding as a theory for homosexual behaviour in bottlenose dolphins appears to be the most supported (Acosta, 2015; Serres et al. 2022). However, the three hypotheses are often not mutually exclusive (Bailey & Zuk, 2009) and the explanation for SSSB is likely to be species and context dependent (Serres et al. 2022). The study of socio-sexual behaviour is limited in cetaceans as they are often submerged and wide-ranging animals (Acosta, 2015). One challenge is determining whether the behaviour displayed is sexual, where reproduction takes place, or socio-sexual, where reproduction does not occur (Ham et al. 2023), and whether these interactions are with the same sex. This means behavioural interpretations can be difficult to validate (Silva & Spinelli, 2023). Bottlenose dolphins have been researched far more than other cetaceans regarding same-sex socio-sexual behaviour (Silva & Spinelli, 2023), particularly in males since they display SSSB significantly more frequently (Connor & Krützen, 2015; Mann, 2006). These interactions have been studied in other cetacean species, such as Indo-Pacific bottlenose dolphin (T. aduncus) (Mann, 2006) and some porpoises (Serres et al. 2022), though research is still limited. Future research could investigate the similarities and/or differences of same-sex socio-sexual cetacean behaviour in other marine mammal species, such as whales, to better understand its relationship with fitness. The use of underwater drones would be beneficial for future study of these interactions to increase chances of observation.
References
Acosta, NB. (2015). Same-sex Socio-sexual Interactions Among a Group of Captive Bottlenose Dolphins (Tursiops Truncatus). The University of Southern Mississippi.
Breed MD & Moore J. (2015) Animal Behaviour, second edition, p 427-428. United States of America: Academic Press.
Bailey NW & Zuk M. (2009). Same sex-sexual behaviour and evolution. Trends in Evolution and Ecology, 24, 439-446.
Connor RC & Krützen M. (2015). Male dolphin alliances in Shark Bay: changing perspectives in a 30-year study. Animal Behaviour, 103, 223–235.
Ham JR, Lilley MK, Manitzas Hill HM. (2023). Non-conceptive sexual behavior in cetaceans: comparison of form and function. In: Würsig B, Orbach DN (eds) Sex in cetaceans. Springer Nature, Cham
Levan KE, Fedina TY, Lewis SM. (2009). Testing multiple hypotheses for the maintenance of male homosexual copulatory behaviour in flour beetles. Journal of Evolutionary Biology, 22, 60–70.
Mann, J. (2006). Establishing trust: Socio-sexual behaviour and the development of male-male bonds among Indian Ocea nbottlenose dolphins. In V. Sommer & P. L. Vasey (eds.), Homosexual behaviour in animals, p 107–130. Cambridge University Press.
Monk JD, Giglio E, Kamath A, Lambert MR, McDonough CE. (2019). An alternative hypothesis for the evolution of same-sex sexual behaviour in animals. Nature Ecology & Evolution, 3, 1622-1631.
Scharf I & Martin OY. (2013). Same-sex sexual behaviour in insects and arachnids: prevalence, causes, and consequences. Behavioural Ecology and Sociobiology, 67, 1719-1730.
Serres A, Hao Y, & Wang D. (2019). Agonistic interactions and dominance relationships in three groups of captive odontocetes: Method of assessment and inter-species/group comparison. Aquatic Mammals, 45, 478–499.
Serres A, Hao Y, Wang D. (2022). Socio-sexual interactions in captive finless porpoises and bottlenose dolphins. Marine Mammal Science, 38, 812–821.
Silva VMF & Spinelli LG. (2023). Play, Sexual Display, or Just Boredom Relief? In: Würsig B, Orbach DN (eds) Sex in cetaceans. Springer Nature, Cham
