Aerial urination suggests undescribed sensory modality and social function in river dolphins.

Scent marking through urine spraying is known to aid mate selection, territory marking and chemical communication in terrestrial, but not in aquatic mammals. We quantify an unusual aerial urination behaviour in botos (Inia geoffrensis) and discuss its potential functions. Between 2014 and 2018, we conducted land-based behavioural surveys on wild botos in central Brazil, recording the sequence, duration and social context of aerial urination. Aerial urination starts with a boto slowly positioning itself upside down, exposing its penis above water, and ejecting a stream of urine into the air. When a 'receiver' male is present, it either approaches the urine stream with its rostrum (sometimes pursuing it) or stays where the stream contacts the water. We recorded 36 aerial urination events during 218.9 observation hours, with 67 % occurring in the presence of receivers. The events were short (11.6 ± 9.6 SD s), highly variable (range 1-41 s) and involved only males-urinators and receivers. We hypothesize that the bristles on botos' rostrums can serve a chemical sensory role (haptosense) in detecting urine streams and that aerial urination, often occurring in the presence of other males, serves social or communicative functions beyond the physiological need for waste elimination.