The variability of muscle-blubber interface temperature with activity level in a captive Risso’s dolphin (Grampus griseus)

Abstract

Abstract Background: To reduce heat loss underwater, marine mammals cover their bodies with insulators and particularly cetaceans only rely on blubber for insulation. Blubber establishes a thermal gradient between the warmer body core and cooler body surface, reducing heat transfer to the environment. Measuring the temperature at the innermost part of the blubber, i.e. the temperature at the muscle-blubber interface (Tmbi), can be an important indicator for understanding thermoregulatory mechanisms in cetaceans. Although heat production through activity is another factor that plays an important role in thermoregulation, its effects on Tmbi have not been investigated in cetaceans. To assess this relationship in free-swimming cetaceans, we measured the Tmbi and activity levels in a captive Risso’s dolphin (Grampus griseus) using an implantable biologging device. Results: Tmbi and activity data were analyzed for 11 days. The average Tmbi was 35.1 ± 0.6 ºC and the temperature gradient between Tmbi and the water temperature was 13.0 ± 0.7 ºC. The Tmbi displayed distinct diurnal variation, which was higher during the daytime and lower in the early morning. The variation in Tmbi was best explained by both the activity levels and time of day. Tmbi did not simply increase with activity levels; it appeared to remain relatively constant at most activity levels. However, Tmbi appeared to decrease when the animal was inactive and increase when it was intensely active. Conclusion: Our results provide important insights into how a dolphin regulates its body temperature underwater. Heat insulation by blubber and heat production by activity were found to play important roles in thermoregulation. The whole-body heat dissipation mechanism might be used to regulate temperature increases when heat production is excessive, or the animal can reduce the thermal conductivity of the blubber by decreasing its temperature during inactive periods.