Evaluating rivals’ size and motivation is essential for the survival of hippopotamuses (Hippopotamus amphibius), territorial mammals that live in large social groups. Their in-air vocalizations, comprising high-pitched wheezes and low-pitched honks, are hypothesized to regulate social interactions between distant groups. However, the role of specific vocal features in conveying the emitter’s size and motivation remains experimentally untested. Here, we used playback experiments with modified wheeze-honk vocalizations to show that hippopotamuses rely on honk formant frequencies to assess the vocalizer’s size, but not on honk quantity to gauge motivation. Lower formant frequencies, simulating larger individuals, elicited stronger territorial responses compared to higher frequencies. Conversely, varying honk numbers to mimic differing motivation levels did not influence group responses. These findings clarify that hippopotamuses use formant frequencies as a key acoustic cue in territorial interactions. This study advances our understanding of hippopotamus vocal communication and informs the development of acoustic tools for mitigating human-hippopotamus conflicts and supporting conservation efforts.
Massenet et al. (Wed,) studied this question.