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Factors influencing the acoustic behaviour of male phocid seal
Abstract: A series of natural history factors were examined to identify which are likely to influence acoustic behaviour in male phocid seals. Contrary to traditional thought, the degree of polygyny did not appear to influence vocal repertoire size or the other acoustic features examined. However, degree of sexual dimorphism, stability of the pupping substrate and density, guard-ability and predictability of oestrus females, and function of the signals, all appear to be important. Species pupping in stable habitats tend to have vocalizations which are low in minimum frequency, whereas pack-ice breeders have vocalizations with high minimum frequencies. In species where oestrus females are predictably distributed and guardable (the southern elephant, Mirounga leonina, northern elephant, Mirounga angustirostris, grey, Halichoerus grypus, hooded, Cystophora cristata, and crabeater, Lobodon carcinophagus, seals) the acoustic displays of the males tend to be associated with male-male agonistic interactions and are likely to be intrasexual in function. Male vocal repertoires tend to be simple in form composed of a reduced number of primarily broadband pulsed sounds following Morton’s (1982) Motivational Structural Hypothesis. However, in species where females do not remain hauled-out with their pups until weaning it is more difflcult for males to physically guard them until oestrus. If oestrus females are inaccessible to males, either because they are moving to and from the sea or because they are widely distributed, males could use vocal displays to attract a mate. When oestrus females are widely and unpredictably dispersed long-range underwater advertisement displays are necessary (Scattergun advertizing). The acoustic displays of males of these species (leopard, Hydrurga leptonyx ; Ross, Ommatophoca rossii; bearded, Erignathus barbatus; and ribbon, Histriophoca fasciata, seals) must travel long distances. Sounds that are required to travel over long distances could change slightly in character due to the differing propagation characteristics of the varying frequency components within the vocalization. The signal also could be masked by background noise. Perhaps it is easier for a listening seal to recognise a smaller number of stereotyped vocalizations. Therefore, these seals tend to have fewer, narrowband, highly stereotyped sounds used in stylized repetitive displays which helps ensure that the signal is recognized by a receiving seal at a distance. In species where oestrus females are predictably distributed, but unguarded (Weddell, Leptonychotes weddellii; harp, Pagophilus groenlandicus; harbour, Phoca vitulina; and ringed, Phoca hispida seals) males perform shorter-range underwater advertisement displays. The signals of species advertizing to a local audience are not constrained by propagation difflculties. These species therefore adopt a large array of sound types, and include subtle variations of the same sound type, greatly increasing their overall repertoire size (Local advertizing).
Key Words: PHOCID SEALS; ACOUSTIC BEHAVIOUR; MATING SYSTEMS; PREDICTABLE GUARDABLE OESTRUS; FEMALES; SEXUAL SELECTION.
Document Type: Research article