NOREN, D.P.*; WILLIAMS, T.M.: Effects of Environmental Regime and Body Condition on Resting Metabolic Rates in Northern Elephant Seal Pups: Does RMR Measured in Ambient Air Represent True RMR?

Northern elephant seal pups (Mirounga angustirostris) are weaned at body masses that are 30 – 50% lipid. Due to the wide range of body compositions at weaning, northern elephant seals provide a good model to examine how body condition influences resting metabolic rates (RMR) and thermal conductance under different temperature regimes. Percent body lipid (via tritium dilution), RMR (via open flow respirometry), and core body temperature (T_b) were determined in 17 fasting pups resting in ambient air (20.9 � 3.3 �C), cold water (3.8 � 1.7 �C), and warm water (14.1 � 1.8 �C). We found that percent lipid (range: 34.4% – 43.8%) tended to increase with body mass (range: 62.0 kg – 108.0 kg) but was not correlated with RMR. RMR was correlated with total body mass and lean body mass. RMR increased with body mass in air (R = 0.71, P = 0.002) and cold water (R = 0.66, P = 0.004), but not significantly in warm water (R = 0.45, P = 0.068). RMR (range: 293.6 mlO₂ min^-1 – 521.7 mlO₂ min^-1) differed (P = 0.050) between the three regimes, with the highest mean RMR in warm water and the lowest RMR in cold water. Furthermore, RMR tended to increase with environmental temperatures. Thermal conductance, calculated from RMR and T_b, increased with environmental temperature, more steeply in ambient air than in water. These results indicate that body condition and environmental regime have synergistic effects on RMR and thermal conductance for resting northern elephant seals. A consequence of large body size and high body lipid content is an elevation in RMR and thermal conductance, particularly when resting in ambient air, with the result that minimum RMR occurs when these mammals are in cold water.