Same-sex sexual behaviour may be more common than previously believed.

A new study reveals bisexuality has evolved in a population of semi-wild rhesus macaques.

Over a span of three years, researchers from the UK closely observed macaques in Puerto Rico and discovered that same-sex behaviour was actually more frequent than different-sex behaviour. 

Surprisingly, the study also revealed that macaques engaging in same-sex behaviour had more offspring than those who did not.

The prevailing assumption that same-sex sexual behaviour is uncommon in the animal kingdom and leads to fewer offspring is now being questioned. 

Previous observations of same-sex behaviour in animals were often ad hoc and treated as exceptions rather than the norm. Consequently, the underlying factors and evolutionary implications of this behaviour have remained poorly understood.

But for a new study, led by Vincent Savolainen, researchers focused their investigation on a population of semi-wild rhesus macaques on Cayo Santiago island. 

They meticulously recorded mounting behaviours among 236 male macaques from 2017 to 2020. 

The results showed that 72 per cent of the males engaged in same-sex mounting, while only 46 per cent participated in different-sex mounting. Interestingly, those displaying same-sex behaviour were generally more sociable, often forming coalitions with other males who had mounted each other.

Contrary to expectations, the researchers discovered a positive correlation between same-sex sexual behaviour and reproduction rates. This finding challenges the notion that same-sex behaviour hinders reproductive success. 

Furthermore, the study revealed that same-sex behaviour in rhesus macaques has a heritable component, accounting for approximately 6.4 per cent of its occurrence. This indicates that same-sex behaviour is capable of evolving within this species.

While cautioning against generalising the results to other populations and species, the authors say that these findings disrupt the perception of same-sex sexual behaviour as rare or only arising from unusual environmental circumstances in non-human animals.