Preliminary Evidence for Peak Frequency Variation Related to Body Length and Sex in Echolocation Clicks of Narrow-Ridged Finless Porpoises

Animals produce sounds that convey various types of information to conspecifics. Although narrow-ridged finless porpoises (NRFPs) are thought to rely on sounds for their underwater lives, the specific information encoded in these sounds remains unclear. In this study, we examined the effects of body length and sex on the peak frequency of echolocation clicks in 11 captive NRFPs. Presumed on-axis clicks were recorded from six females and five males housed in three aquariums. A generalized linear mixed model was applied with peak frequency as the response variable. Body length and sex were included as fixed effects, and individual identity was included as a random effect. The significance of the fixed effects was assessed using likelihood ratio tests. The results revealed that peak frequency was negatively correlated with body length and significantly lower in males than in females. Specifically, larger individuals produced lower-frequency clicks, and the intercepts of the regression lines differed between sexes. These findings suggest that NRFPs may be capable of recognizing the sex of conspecifics through click frequency, potentially facilitating the identification of reproductive partners within the active range of sounds. Published online 13 February, 2026; Print publication 30 April, 2026