A Comparative Analysis of Model Diets Consumed by Common Bottlenose Dolphins (Tursiops truncatus) May Explain Why Some Dolphins Form Ammonium Urate Nephroliths
Some common bottlenose dolphins (Tursiops truncatus) under human care form ammonium urate nephroliths, whereas free-ranging dolphins do not.1 Diet may influence urate urolith formation in dolphins as it does in other mammals.2 Both groups of dolphins consume whole fish, but free-ranging dolphins consume a variety of live inshore temperate water species, whereas dolphins under human care are fed cold water species that are frozen and thawed prior to feeding - fish bottlenose dolphins typically would not encounter in the wild. Macronutrient and total purine content were measured in fresh frozen samples of eight fish species commonly consumed by free-ranging dolphins and stored frozen samples of seven species (n = 5) commonly fed to dolphins under human care. Metabolizable energy was calculated using Atwater factors, and dietary cation-anion difference (DCAD) was calculated as (Ca2+ + Mg2+ + Na+ + K+) - (Cl- + P1.8- + S2-).3 Crude protein, fat, mineral and water contents differed among species and between species groups, and total purine content was greater in two model diets typically fed to dolphins in human care than in a model free-ranging dolphin diet (p ≤ 0.05). Mean DCAD was positive (94 mEq/Mcal) in the free-ranging model diet, and negative (-70 mEq/Mcal) in both model diets for dolphins under human care. Mean DCAD differed less among model diets if Ca, Mg and P were excluded from the calculation or absorption coefficients were included.4-6 Thus, dolphins under human care may have to excrete more anions, resulting in more acidic urine. These nutrient differences may contribute to ammonium urate nephrolith formation in dolphins managed under human care.
We are grateful for the generous financial support of the United States Navy Marine Mammal Program and SeaWorld Parks and Entertainment, Inc., without which this project would not have been possible.
* Presenting author
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