Evolutionary ecology of the New Zealand Fur Seal (Arctocephalus forsteri) and Hooker's Sea Lion (Phocarctos hookeri)
Beentjes, Michael P
Thermoregulatory behaviour, terrestrial locomotion and postcranial skeletal morphology of the New Zealand fur seal (Arctocephalus forsteri) and Hooker's sea lion (Phocarctos hookeri) are quantified and the results compared between the two species. Differences and/or similarities are discussed in terms of the evolutionary ecology of these two sympatric otariid species. Additionally, a single chapter, describes some aspects of the behavioural ecology of P. hookeri. Quantitative and qualitative analyses of filming studies reveal that fundamental differences exist between the gaits of Arctocephalus forsteri and Phocarctos hookeri. Terrestrial locomotion of the latter species is similar to that of terrestrial vertebrates in which the limbs are moved in sequence, alternately and independently. In contrast, the gait of the New Zealand fur seal does not conform to this sequence, the hind limbs being moved in unison. The gaits of both species are defined and illustrated. The gaits are here considered to be ecological specialisations which are adaptations to the mechanical problems imposed by different habitats. Gaits of these species appear typical or representative of members of their inferred subfamilies (Arctocephalinae and Otariinae). The gaits of A. forsteri and P. hookeri are however paradoxical in light of their inferred evolutionary history since the gait of the Hooker's sea lion resembles more closely that of the putative ancestors of otariids (ursids = bears) than does the gait of the New Zealand fur seal; fur seals supposedly gave rise to the sea lions and are traditionally considered to possess retained primitive features. There were minor (statistical) quantitative differences in some of the limb bone and vertebrae linear dimensions between A. forsteri and P. hookeri but these were differences in degree rather than kind. Analysis of gait and structure of A. forsteri and P. hookeri together with several specimens of the Australian otariids (A. pusillus doriferus and Neophoca cinerea) suggest that: a) There is no correlation between gait and postcranial morphology in these otariids. b) There do not appear to be any phylogenetic differences in the linear dimensions of postcranial morphology between fur seals (Arctocephalinae) and sea lions (Otariinae). There was no statistical difference in the relative width of the humerus between A. forsteri and P. hookeri. This finding casts doubt on the utility of this character as a diagnostic indicator between fur seals (Arctocephalinae) and sea lions (Otariinae) as proposed by Repenning & Tedford (1977). While the gaits of the New Zealand fur seal and Hooker's sea lion are profoundly different, no major concomitant structural differences between the species appear to exist. Selection for the behavioural control of the gait has apparently preceded concomitant structural modifications. Thermoregulatory behaviour of the sympatric Hooker's sea lion and New Zealand fur seal in relation to air temperature and solar radiation were studied, quantified, and the results compared between the two species. Both species adjusted posture and flipper exposure so that surface area exposed to air increased as solar radiation intensified, providing quantifiable evidence that flippers are the major sites of heat exchange. The general pattern of thermoregulatory behaviour in these species, while showing minor differences in the magnitude of postural and flipper adjustments to solar radiation, does not differ in the basic sequence or type of response. Non - postural thermoregulatory behaviour was shown to be influenced by the respective habitat substrates and topographies of the two species.
Advisor: Fordyce, Ewan; Davis, Lloyd
Degree Name: Doctor of Philosophy
Degree Discipline: Zoology
Research Type: Thesis