Underwater hearing in California sea lions (Zalophus californianus): Expansion and interpretation of existing data

Abstract

Measurements of underwater hearing sensitivity of pinnipeds have been made for 9 of 33 species, with most studies involving just one or two trained individuals. There is a need to refine and expand this limited knowledge base to improve understanding of hearing and possible anthropogenic noise effects, especially in cases where data may be extrapolated to other species (see National Research Council 2000, Southall et al. 2007). Schusterman et al. (1972) published what is still the only complete underwater audiogram for a California sea lion (Zalophus californianus). The purpose of this brief communication is to present hearing data from another trained California sea lion for comparison with the classic audiogram. Schusterman et al. (1972) originally tested the underwater hearing ability of an adult male sea lion—identified as “Sam”—using an operant conditioning method of vocal response. Sam was trained to either vocalize upon detecting a 500 ms pure tone signal or to remain silent when no signal was detected. The signals were varied in level in an adaptive, up-down fashion to obtain hearing thresholds at frequencies from 0.25 to 64 kHz. Thresholds were defined as the stimulus sound pressure level (SPL) in dB re: 1 bar corresponding to the 75% correct detection probability at each test frequency. The underwater hearing sensitivity of another captive California sea lion—an adult female identified as “Rio”—was later evaluated at relatively low frequencies (0.1–6.4 kHz) during similar auditory detection tasks conducted by Kastak and Schusterman (1998) and then again by Southall et al. (2005). The absolute threshold measurements obtained in these studies were subsequently extended to cover the full functional hearing range. The higher frequency data for Rio