Seasonal variation in resting metabolic rate, thyroid hormones, and cortisol in the bottlenose dolphin ( <i>Tursiops truncatus</i> )

Seasonal changes in metabolic rate have been studied to varying degrees in terrestrial mammals, but relatively little is known about this phenomenon in odontocetes. Seasonal changes in thyroid hormone levels in bottlenose dolphins suggest potential seasonal effects on resting metabolic rate (RMR). To explore this potential, the RMR of nine adult and four adolescent bottlenose dolphins were measured monthly for 13 months using open-flow respirometry. Oxygen consumption was measured while dolphins were post-absorptive and during the morning to avoid diurnal hormone fluctuations. Blood samples were collected from six of the adult dolphins immediately following oxygen consumption measurements and assessed for levels of total and free T3 (fT3), reverse T3 (rT3), total and free T4 (tT4 and fT4, respectively), and cortisol. Resting metabolic rate varied seasonally in mature dolphins; RMRs were lower in the spring and higher in the summer and fall. Suppression of RMR in the spring was followed by a progressive increase in RMR through the summer and fall. The mean mass-specific RMR of adolescent dolphins (5.49 ml O2 min-1 kg-1) was higher than that of adult dolphins (4.38 O2/min/kg), and the RMR of adults did not vary between sexes. The RMR of adult dolphins was significantly related to serum concentrations of tT3 and fT3, while rT3, fT3, and tT3 were all significantly related to photoperiod. The entrainment of seasonal changes in dolphin RMR are probably driven by photoperiod, although the signaling mechanism mediated by melatonin is likely altered in this species because it lacks a defined pineal gland.