Acoustic Analysis of Underwater Vocalizations from Crabeater Seals (<I>Lobodon carcinophagus</I>): Not So Monotonous

Abstract

Underwater vocalizations of pack-ice crabeater seals (Lobodon carcinophagus) were recorded by J. A. Thomas off the western side of the Antarctic Peninsula during the austral breeding season in November 1978. Data were collected by dropping an ANS 57 sonobuoy from the side of a ship to a depth of 15.3 m and recorded on a Nagra III reelto-reel recorder (system frequency response linear from 20 Hz to 20 kHz ± 2 dB). The acoustic properties of 315 underwater vocalizations were analyzed using Spectrogram real time software. As previous investigators documented, all crabeater seal sounds during the breeding season were of one type, a long groan. D. Cothran recorded a solitary crabeater seal of unknown age and sex foraging during the nonbreeding season in February 2007 in the same general area of the Antarctic Peninsula using a Sony TRV-900 digital underwater video camcorder. Twenty seconds of underwater acoustic data were spectrographically analyzed, and 18 vocalizations were identified and classified into four previously unreported sound types: (1) short groan, (2) whistle, (3) screech, and (4) grunt. For the first time, the acoustic characteristics of the common long groan and four previously undescribed underwater vocalizations by crabeater seals were examined spectrographically with parameters of frequency and time reported herein. The long groan showed little frequency or temporal variation and was repeated by other distant crabeater seals at about 20.0-s intervals. No long groans were recorded during the short February videotape. The four previously undocumented vocalizations were produced while a single crabeater seal foraged in shallow water. Hierarchical cluster analysis showed that the long groan was acoustically dissimilar to the four new vocalizations; however, only 18 sounds were available for analysis. Still, this videotape documents that this species does produce more than one sound type. Further research should be conducted to adequately document the underwater acoustic repertoire of the crabeater seal, especially outside the breeding season.