Bottlenose dolphin ( Tursiops truncatus ) metabolic rate in relation to reduced energy acquisition

Interruptions in foraging behavior, displacement from foraging habitats, and reductions in food abundance or quality are common disruptions to energy acquisition experienced by wild animals that may be caused by naturally-occurring phenomena or anthropogenic activity. The impact of reduced energy acquisition to an organism is a function of the duration and magnitude of the energy reduction and the physiological response that mitigates the loss of endogenous nutrient reserves. Although the response to reduced energy acquisition has been studied in many terrestrial mammals, little information exists on the response in cetaceans, such as dolphins, which experience metabolic challenges characteristic of a fully-aquatic existence. To assess the response of bottlenose dolphins (Tursiops truncatus) to dietary energy restriction, five adult dolphins (3 male/2 female) were subjected to seven days of a normal diet (ND) followed by seven days of an energy-restricted diet (ERD) containing 25% less energy per day than the ND. The resting metabolic rate (RMR) was determined for each animal on days three, five, and seven of the ND and ERD periods. Blood samples were collected following each RMR measurement and processed for cortisol, free triiodothyronine (fT3), dehydroepiandrosterone, adiponectin, tumor necrosis factor-α and interleukin-6, as these hormones and cytokines are known to affect RMR, energy substrate selection, and possibly short-term catabolic signaling or metabolic adaptation. The ERD was associated with a lower RMR, which declined by an average of 5.3% from the ND treatment. The ERD was also associated with lower fT3 levels and increased adiponectin levels, and both showed continuing changes as a function of time on the ERD. Bottlenose dolphins show patterns in RMR and levels of fT3 and adiponectin under dietary energy restriction that are similar to patterns observed in terrestrial mammals. These likely help to conserve endogenous energy stores, increase reliance on lipid oxidation, and protect protein stores. The quantitative information from this study should be useful in modeling the impacts to individuals and populations that experience both natural and human-caused reductions in energy acquisition (e.g., disruptions of foraging, reduced prey quality).