Bimodal distribution of inter-individual distance in free-ranging narrow-ridged finless porpoises—Potential use of visual and auditory cues

Abstract Animals must adjust their inter-individual distances (IIDs) by using several cues from other individuals; therefore, IIDs are expected to be restricted by the transmissive distances of such cues. In this study, the IIDs of free-ranging narrow-ridged finless porpoises (NRFPs) ( Neophocaena asiaeorientalis ) are shown to have two distributions that may correspond to the transmissive ranges or active spaces of visual and auditory cues. IIDs were measured using 2.8-h drone video data (DJI Air 2S) at a high altitude (ca. 400 m) for 12 d in two study areas in Ise Bay, Japan. The distribution of IIDs was divided into three categories based on the results of the Bayesian Information Criterion in both areas. The first (6.5 and 5.7 m, in Areas 2 and 1 on average, respectively) and second distribution (102.1 and 80.6 m in Areas 2 and 1 on average, respectively) were significantly shorter than the null model predicted by drone angle of view, whereas the third distribution (248.2 m in Area 2 and 203.2 m in Area 1 in average) did not significantly differ from the null model (Kolmogorov–Smirnov test; p < 0.01 and p = 0.48, respectively). The first distribution of IIDs corresponded to the visible range (water clarity in Ise Bay: approximately 5 m), and the second distribution corresponded to the audible range (active space of echolocation clicks in Ise Bay: median 72.15 m). Free-ranging NRFPs, which have a simple social system, may adjust their IIDs using visible and audible cues from other individuals.