Morphometric and Biochemical Factors Affecting Heat Loss in a Small Temperate Cetacean (Phocoena phocoena) and a Small Tropical Cetacean (Stenella attenuata)

Abstract

Harborporpoises (Phocoena phocoena) and spotted dolphins (Stenella attenuata) must cope with very different thermal situations. Harbor porpoises live in cold coastal waters where water temperatures average 10° - 15° C; spotted dolphins live in the Eastern Tropical Pacific Ocean ecosystem where water temperatures average 27° C. There are few options available to cetaceans for thermoregulation. Thepurpose of the present study was to investigate factors that affect heat loss in small Cetacea and to compare these factors in harbor porpoises and spotted dolphins. Phocoena had a smaller surface area, for a given mass, than did Stenella. This suggests possible selection for a decreased surface area/body mass ratio in harbor porpoises. Conductivity of harbor porpoise blubber (0.10 ± 0.01 W m⁻¹ °C⁻¹, n = 29) was significantly lower (P < 0.05) than that of spotted dolphins (0.20±0.02 W m⁻¹ °C⁻¹, n = 16), signifying a higher insulative quality for Phocoena. In addition to being a superior insulator, Phocoena blubber depth (1.5 ± 0.3 cm, n = 59) was double that of Stenella (0.8 ± 0.1 cm, n = 16). Blubber conductivity varied inversely with lipid content. Harbor porpoise blubber was 81.6% ± 3.6% (n = 29) lipid, whereas spotted dolphin blubber was 54.9% ± 2.8% (n = 16) lipid. Conductivity values measured for Phocoena are the lowest recorded for any cetacean species. Measures of conductivity for Stenella are similar to those of other cetaceans. The morphometric and biochemical differences found in the blubber of these two species implies that small cetaceans can use a combination of surface area and insulative quality and quantity to thermoregulate. These responses are dictated by ambient temperature but are constrained by body size and, possibly, hydrodynamic considerations.