The De-Scent of Sexuality: Did Loss of a Pheromone Signaling Protein Permit the Evolution of Same-Sex Sexual Behavior in Primates?

Non-Reproductive Sexual Behavior in Wild White-Thighed Colobus Monkeys (Colobus vellerosus)

Rare behaviors are often missing from published papers, hampering phylogenetic analyses. Here, we report, for the first time, masturbation and same-sex sexual behavior (SSB) in both male and female black-and-white colobus monkeys. We recorded these behaviors during 32 months of observation (1573 h of focal animal sampling) on Colobus vellerosus collected at the Boabeng-Fiema Monkey Sanctuary in Ghana. Males were observed masturbating and involved in SSB more than females. Subadult males were the age-sex class that engaged in both of these behaviors most often and a third of all SSB observed in young males occurred when they were forming an all-male band (AMB), which are temporally transient social groups in this species. Our data support that masturbation in males may be a sexual outlet for individuals that do not have a current sexual partner, while in females it may function in mate attraction by advertising receptivity. SSB may occur as an evolutionary byproduct but given the temporal clustering of observed events in males prior to AMB formation, our data best support the hypothesis that these behaviors facilitate male-male bonding (i.e., act as social glue). Within AMB's, males engage in coalitionary behavior to take over social groups containing females and strong bonds are important for success and later access to females, which could have selected for SSB in C. vellerosus.

Ancestral sex-role plasticity facilitates the evolution of same-sex sexual behavior

Recent attempts to explain the evolutionary prevalence of same-sex sexual behavior (SSB) have focused on the role of indiscriminate mating. However, in many cases, SSB may be more complex than simple mistaken identity, instead involving mutual interactions and successful pairing between partners who can detect each other's sex. Behavioral plasticity is essential for the expression of SSB in such circumstances. To test behavioral plasticity's role in the evolution of SSB, we used termites to study how females and males modify their behavior in same-sex versus heterosexual pairs. Male termites follow females in paired "tandems" before mating, and movement patterns are sexually dimorphic. Previous studies observed that adaptive same-sex tandems also occur in both sexes. Here we found that stable same-sex tandems are achieved by behavioral plasticity when one partner adopts the other sex's movements, resulting in behavioral dimorphism. Simulations based on empirically obtained parameters indicated that this socially cued plasticity contributes to pair maintenance, because dimorphic movements improve reunion success upon accidental separation. A systematic literature survey and phylogenetic comparative analysis suggest that the ancestors of modern termites lack consistent sex roles during pairing, indicating that plasticity is inherited from the ancestor. Socioenvironmental induction of ancestral behavioral potential may be of widespread importance to the expression of SSB. Our findings challenge recent arguments for a prominent role of indiscriminate mating behavior in the evolutionary origin and maintenance of SSB across diverse taxa.

Interações sexuais entre indivíduos do mesmo sexo: um olhar evolucionista

Resumo Sob uma perspectiva evolutiva, as interações sexuais entre indivíduos do mesmo sexo foram por muito tempo consideradas um grande paradoxo. Isso por terem persistido no decorrer das gerações apesar de supostamente não oferecerem benefícios reprodutivos diretos, reduzindo, aparentemente, a aptidão individual. Apesar disso, são comuns em muitas espécies animais. Neste artigo, revisaremos algumas das hipóteses funcionais que tentam resolver esse quebra-cabeça evolutivo. Algumas dessas hipóteses consideram essas interações adaptativas, o que significa que trariam benefícios para os indivíduos. Outras as consideram neutras, derivadas de características realmente vantajosas. Por fim, existem as que consideram essas interações como não-adaptativas e potencialmente prejudiciais aos indivíduos. Ao final, abordaremos uma hipótese revolucionária que, de forma inédita, questiona se as interações sexuais envolvendo exclusivamente indivíduos de sexos diferentes seriam realmente o estado basal do comportamento sexual.