The role of vocalization in the social behaviour of the northern elephant seal

The northern elephant seal breeds on islands off the coast of Mexico and California. It is a highly vocal species and during the breeding season vocalization plays an important role in the maintenance of the species' highly polygynous social organization. The importance of vocalization in this species is demonstrated by the presence in the adult male of a highly modified proboscis which appears to have no function other than the production of vocal threats.

Observation of colour-marked individuals on San Nicolas Island, California, showed that females of a harem tend to remain in a given spot. Females of low dominance are often forced to move by the aggressive behaviour of more dominant females and their pups are likely to become lost and to be bitten by other females. The voice of the pup attracts the mother and stimulates the mother to suckle the pup.

Observations in the field combined with spectrographic analysis of tape recordings reveal two main communicational and structural classes of vocalization, attraction calls and threat sounds. Attraction sounds (mother to pup) are unpulsed, of high and variable pitch, and show clear harmonic structure. Threat sounds (both female and male) have an opposite type of structure; they are highly pulsed, of low and steady pitch, are harsh, and lack clearcut harmonics. In general, pulse interval in the threat sounds tends to increase with body size. Among males, the resonance and volume of the clap-threat increases with the size and development of the proboscis.

Hissing occurs in many tetrapods and probably preceded and facilitated the evolution of vocal cords and patterned sounds; in turn the extensive use of different sounds as a means of communication probably facilitated the evolution of social life in the vertebrates. It is of interest that yearlings use the hiss as a threat and some yearling threat sounds are structurally intermediate between a hiss and the common roaring type of threat.