Fur versus blubber: A comparative look at marine mammal insulation and its metabolic and behavioral consequences

This research compares the use of fur and blubber as insulation in mammals, with a focus on the otariids (fur seals and sea lions) and the transition of mammals to aquatic living. The otariids represent the only mammalian family to include both types of insulation: fur seals have dense, waterproof fur and a moderate blubber layer, while sea lions rely solely on blubber for insulation in water. To compare the effectiveness of these different mechanisms of insulation, I examined the thermal properties of the fur and blubber of various otariid species, and evaluated their role in the physiological and behavioral responses of the California sea lion (Zalophus californianus) and northern fur seal (Callorhinus ursinus) exposed to different water temperatures.;Chapter 1 presents the morphological and thermal characteristics of fur and blubber as measured on sculps (fur, skin, and blubber) from a wide variety of pinniped species. Values were then compared to those of pelts (fur and skin) from terrestrial and semi-aquatic carnivores. Morphological measurements included hair cuticle shape, hair circularity, hair length, fur density, and blubber thickness. In addition, the composition of blubber fat was determined from lipid and water content, and fatty acid profiles. The effects of hydrostatic pressure on the insulating layer were determined empirically. Measurement of the thermal conductivity of the samples allowed assessment of the overall effectiveness of each insulation type. I found consistent trends in hair morphology associated with aquatic living, which included (1) flattening and shortening of the hairs, (2) elongation of hair cuticle scales, and (3) increases in fur density for species utilizing fur for insulation in water. Such characteristics are considered critical for maintaining an insulating air layer within the fur during submersion. I also observed a secondary loss of these features in species with more developed blubber layers. Comparisons of blubber composition indicated stratification of this layer in species relying on the blubber for insulation. Lipid stratification was consistent with the use of the outer layer for thermoregulation and the inner layer for energy storage. Among otariids, blubber quality (lipid content and thermal conductivity) did not differ between fur seals and sea lions. Rather, blubber quantity (thickness) differentiated each otariid group. Overall, differences in total insulation among carnivore species, both terrestrial and aquatic, were influenced substantially by body size and habitat, and to a lesser extent by latitudinal climate.;Chapters 2 and 3 subsequently address the influence of body size, insulation type, and maturity on whole-animal metabolic responses to water temperature. Once again, California sea lions and northern fur seals were compared as representatives of two different thermal mechanisms. Lower critical temperatures (TLC ) and the metabolic consequences of at-sea behaviors were determined for both species. By comparing the TLC determined in the laboratory with routine temperatures encountered in the wild, I was able to assess a relative thermal liability for each species. With a TLC of 5°C, adult female California sea lions demonstrated thermal competence across the natural range of water temperatures encountered by this species in the wild. In contrast, juvenile California sea lions had a TLC greater than 12°C, revealing a potential thermal limitation that was mitigated by swimming activity. Northern fur seal pups, small-bodied marine mammals using only fur for insulation, had a TLC of 8°C, and thus also exhibited thermal limitations within their natural water temperature range. Based on this and the pelagic lifestyle of young fur seals, other mechanisms of thermoregulation would be required for the animals to remain in thermal balance when resting at sea. I propose that young fur seals utilize both the heat increment of feeding as well as a variety of at-sea behaviors, including grooming and a unique jughandle position, to mitigate the observed thermal limitation.;In summary, the results of these studies indicate that fur underwent considerable evolutionary modification for aquatic living. Ultimately, based on an examination of extant species, fur acts as an effective insulator in water for small-bodied endotherms. Fur is the superior insulator in terms of thermal resistance per unit thickness, as long as the air layer can be maintained among the hairs. This requires a metabolic investment by the animal through grooming. With larger body size, marine mammals can develop extraordinarily thick blubber layers, which facilitate thermal balance during swimming and diving. This form of insulation can serve additional roles such as buoyancy control, energy storage, and streamlining. Overall, I find that the most effective form of insulation for aquatic mammals depends on body size as well as habitat. Heat generated by the processing of food and through skeletal muscle thermogenesis during activity helps to mitigate thermal shortfalls in insulation, and this mechanism of thermal substitution appears to be especially important for maintaining thermal balance in the smaller aquatic species and during immature life stages.