Most bearded vultures are monogamous and heterosexual, but male-male mounting occurs within polyandrous trios, likely to regulate aggression.
Most bearded vultures are monogamous and heterosexual, but male-male mounting has been recorded within polyandrous trios in the Pyrenees mountain range of Spain and France. Unpaired or free-floating males will often join a pre-existing male and female pair, creating a trio.
This homosexual behavior is not directly correlated with different forms of intrasexual competition such as sperm competition or hierarchical dominance, but the formation of polyandrous trios has led to intrasexual competition between males. These intrasexual aggressions lower the frequency of heterosexual copulations during the fertile period. It also increases population densities, which may be responsible for delayed maturity in wild bearded vultures.
As there is no correlation between dominance and mounting behavior, male bearded vultures in polyandrous trios most likely mount one another to regulate levels of aggression.
Sources: (Antor, et al., 2007; Bertran & Margalida, 2003, 2006; Ferguson-Lees & Christie, 2001; Tenenzapf, 2011)
Image: Arjan Haverkamp
