Bimodal distribution of inter-individual distance in free-ranging narrow-ridged finless porpoises

• Published in: Behavioural processes Vol. 222; p. 105102
• Main Authors: Terada, Tomoyoshi, Morisaka, Tadamichi, Yagi, Genfu, Kanda, Ikuko, Ogawa, Kiyohiro, Yoshioka, Motoi
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.10.2024
• Subjects: Active space; Group-living; Inter-individual distance; Social organisation; Solitary; Inter-individual distance; Solitary; Active space; IID; Group-living; Social organisation; NRFP
• DOI: 10.1016/j.beproc.2024.105102
• ISSN: 0376-6357; 1872-8308

Inter-individual distance (IID) is an important indicator of social organisation because solitary species are spatially intolerant towards conspecifics, whereas group-living species are usually gregarious with collective behaviour. Therefore, by examining the relationship between the distribution of IIDs and the active space of cues or signals, and behaviours, we can predict their social organisation. The narrow-ridged finless porpoises (NRFPs) have been described as a solitary species; however, recent studies described NRFPs tend to live in groups more than alone. To resolve the inconsistency, the present study investigated IIDs, the active spaces of sounds, and behaviours. The distribution of IIDs measured using drone was classified into three distributions. The close and intermediate distributions were significantly shorter than the distribution predicted by the angle of drone camera, whereas the far distributions were not. The far distributions were thus a random distribution within the limited angle of the camera. The close distributions were shorter than the active space, exhibiting a high proportion of collective behaviours, while intermediate distributions did not. These results suggest that NRFPs have both solitary- and group-living characteristics. Specifically, the intermediate distribution suggests a solitary aspect to maintain IIDs from others, while the close distribution indicates a group-living aspect with social interactions. •We measured inter-individual distances of narrow-ridged finless porpoises (NRFPs).•The close distributions were significantly shorter than the active space of sounds.•NRFPs frequently exhibited collective behaviours (84.2 %) within the active space.•The intermediate distributions did not differ from the active space of sounds.•NRFPs may have a mixture of solitary and group-living social organisation.