Vocalization can mediate male–male sexual interactions in Java sparrows

Same-sex sexual behaviors do not have direct fitness outcomes, but might be adaptive. This issue is often discussed in group-living animals, where social bonding is crucial. In our captive environment, around 33% of the Java-sparrow (Lonchura oryzivora) males kept in unisex cages have stable bonded partners. Those bonded males often perch side by side and show allopreening and joint-defensive behaviors towards others. In this study, we aimed to determine how males including such same-sex bonded individuals communicate with each other via vocalizations. We introduced same-sex pairs of Java sparrows, which were bonded or non-bonded, in sound-attenuated boxes and compared their recorded vocalizations with those from male–female pairs. For the male–male pairs, we identified two vocalizations that are usually exchanged between paired partners in intimate contexts: mating trills – which are usually emitted by females during successful copulation – and meowing calls – which are given when both members of the pair are in the nest at the same time. We found that males gave mating trills when their partner males were singing courtship songs, or performed trill duets, which sometimes also included meowing calls. Acoustic analyses revealed that there was no difference between male–male and male–female vocalizations. Meowing calls showed greater within-individual variation in duration and other acoustic parameters, but were similar in same-sex and mixed-sex contexts. Our results indicate that male–male pairs exchanged intimate vocal communications just like male–female couples.