Idiosyncratic long call-like vocalisations in two zoo-housed Bornean orang-utan females

Orang-utans are well-known for their vocal flexibility. However, the described vocal repertoire of female orang-utans is relatively limited. This anecdotal report discusses the occurrence of long call-like vocalisations in two zoo-housed Bornean orang-utan females. While Temmy demonstrated these vocalisations in response to playbacks of male long calls, Eloise proactively produced vocalisations during her ovulation before her reproductive senescence. The vocalisations produced by Temmy and Eloise seem to differ in many aspects from known orang-utan calls, although they bear some similarities to male long calls. Thus, the vocal repertoire of female orang-utans may be more extensive than previously thought. We carefully suggest that these calls might indicate proceptivity, while also discussing methodological limitations and theoretical considerations.