Hawaiian monk seal terrestrial communication range estimates

The acoustic biology of Hawaiian monk seals (Neomonachus schauinslandi) is compelling due to their evolutionary and biogeographical separation from related species, prolonged breeding season, and endangered status. Recent auditory measurements suggest that terrestrial communication is constrained by poor hearing, but limited available information about vocal behavior precludes communication range estimates. In this study, we recorded spontaneous vocalizations from free-ranging monk seals at Kalaupapa National Historical Park (Molokaʻi, HI) to describe spectral characteristics and provide the first source levels of the in-air, low-frequency calls emitted by this species. Sound pressure levels measured over the 90% call duration recorded 4–10 m from resting seals were used to approximate source levels at 1 m. Amplitude and spectral characteristics of recorded vocalizations were combined with monk seal hearing data and representative ambient noise levels to estimate the distances over which seals can effectively communicate with conspecifics in air. Preliminary estimates suggest that high hearing thresholds and low to moderate vocal amplitudes result in restricted communication ranges < 500 m. Understanding how terrestrial communication is influenced by noise in the environment will support decision-making about anthropogenic noise exposures that may be harmful to Hawaiian monk seals. [Research facilitated by NOAA’s Hawaiian Monk Seal Research Program.]