title: Comparison of metabolic substrates in alligators and several birds of prey.
abstract: On average, avian blood glucose concentrations are 1.5-2 times those of mammals of similar mass and high concentrations of insulin are required to lower blood glucose. Whereas considerable data exist for granivorous species, few data are available for plasma metabolic substrate and glucoregulatory hormone concentrations for carnivorous birds and alligators. Birds and mammals with carnivorous diets have higher metabolic rates than animals consuming diets with less protein whereas alligators have low metabolic rates. Therefore, the present study was designed to compare substrate and glucoregulatory hormone concentrations in several birds of prey and a phylogenetically close relative of birds, the alligator. The hypothesis was that the combination of carnivorous diets and high metabolic rates favored the evolution of greater protein and fatty acid utilization leading to insulin resistance and high plasma glucose concentrations in carnivorous birds. In contrast, it was hypothesized that alligators would have low substrate utilization attributable to a low metabolic rate. Fasting plasma substrate and glucoregulatory hormone concentrations were compared for bald eagles (Haliaeetus leucocephalus), great horned owls (Bubo virginianus), red-tailed hawks (Buteo jamaicensis), and American alligators (Alligator mississippiensis). Avian species had high circulating β-hydroxybutyrate (10-21 mg/dl) compared to alligators (2.81 ± 0.16 mg/dl). In mammals high concentrations of this byproduct of fatty acid utilization are correlated with insulin resistance. Fasting glucose and insulin concentrations were positively correlated in eagles whereas no relationship was found between these variables for owls, hawks or alligators. Additionally, β-hydroxybutyrate concentrations were low in alligators. Similar to carnivorous mammals, ingestion of a high protein diet may have favored the utilization of fatty acids and protein for energy thereby promoting the development of insulin resistance and gluconeogenesis-induced high plasma glucose concentrations during periods of fasting in birds of prey.
authors.affiliation: School of Nutrition and Health Promotion, Arizona State University, 411 North Central Avenue, Phoenix, AZ 85004, USA; School of Life Sciences, Arizona State University, 401 East Tyler Mall, Tempe, AZ 85287, USA. Electronic address: Karen.Sweazea@asu.edu.
authors.forename: Karen L
authors.initials: KL
authors.lastname: Sweazea
authors.affiliation: Growth Biology Laboratory, USDA, Beltsville, MD 20705, USA.
authors.forename: John P
authors.initials: JP
authors.lastname: McMurtry
authors.affiliation: Louisiana Department of Wildlife and Fisheries, Office of Wildlife, Rockefeller Wildlife Refuge, 5476 Grand Chenier Highway, Grand Chenier, LA 70643, USA.
authors.forename: Ruth M
authors.initials: RM
authors.lastname: Elsey
authors.affiliation: Raptor Center, College of Veterinary Medicine, University of Minnesota, 1920 Fitch Avenue, St. Paul, MN 55108, USA.
authors.forename: Patrick
authors.initials: P
authors.lastname: Redig
authors.affiliation: Department of Physiology, University of Arizona, 1501 North Campbell Avenue, Tucson, AZ 85724, USA.
authors.forename: Eldon J
authors.initials: EJ
authors.lastname: Braun
mesh_terms: D020155:3-Hydroxybutyric Acid / Q000097:blood; D000489:Alligators and Crocodiles / Q000378:metabolism*; D000818:Animals; D001481:Basal Metabolism / Q000502:physiology; D001717:Birds / Q000378:metabolism*; D001786:Blood Glucose / Q000032:analysis; D060435:Carnivory / Q000502:physiology*; D004032:Diet; D004044:Dietary Proteins / Q000378:metabolism; D005228:Fatty Acids, Essential / Q000378:metabolism; D007328:Insulin / Q000097:blood
chemical_list: D001786:Blood Glucose; D004044:Dietary Proteins; D005228:Fatty Acids, Essential; D007328:Insulin; D020155:3-Hydroxybutyric Acid
keywords: Carnivore; Fatty acid; Glucose; Insulin; Metabolic rate