Antimasking strategies of underwater vocalizations and hearing abilities of polar seals

Abstract

Detection of underwater vocalizations by polar seals is limited by their auditory sensory abilities and external masking noises from abiotic (meteorological and ice noises) and biotic (conspecific calls) sources. Attributes that present the antithesis of masking noise characteristics are thought to enhance detectability of calls. Some proposed anti-masking strategies such as call repetition/rhythm patterns are supported by evidence of lower detection thresholds (1–5 dB), while others are not (e.g., abrupt onset and offset of calls). For frequency swept calls, downsweeps have lower detection thresholds (1–5 dB) than upsweeps. The majority of frequency swept calls (greater than 0.1 oct) of bearded (Erignathus barbatus), Weddell (Leptonychotes weddellii) and harp (Pagophilus groenlandicus) seals are downsweeps (89%, 86% and 63%, respectively). Temporal and frequency separation, call lengthening, and directional clues also reduce masking effects. Diverse call repertoires and calling behaviors of polar seals (e.g., courtesy rule) appear to have evolved characteristics that reduce the effects of abiotic and biotic masking. Characteristics of many anthropogenic underwater noises differ from sounds produced in nature. To estimate masking effects of anthropogenic noise on phocid communication, it is important to determine if the man-made noises are defeating existing antimasking strategies.