Meeting Abstract

103.3  Sunday, Jan. 6  Gut Instinct: Digestive Capacity and the Evolution of Extreme Carnivory in Marine Mammals WILLIAMS, T.M.*; LINDBERG, D.R.; Univ. of California, Santa Cruz; Univ. of California, Berkeley williams@biology.ucsc.edu

Reinvasion of the oceans by mammalian predators 30-50 MYA required fundamental changes in physiological processes and organs originally intended for terrestrial living. Such changes appear so formidable that land-to-sea transitions by mammals seem nearly impossible. To determine how ancestral mammals might have overcome these evolutionary barriers, we examined energy demand (field and resting metabolic rates) and assimilation capacity (small intestine length) along the evolutionary paths leading to land-sea transitions in carnivorous mammals. Based on daily energetic costs of 49 extant mammalian species, we find that marine living exacts a high energetic toll on carnivores. Field metabolic rate of marine mammals averaged 1.8 times that of similarly-sized terrestrial mammals and is attributed to elevated resting rates required for counterbalancing the high thermal conductivity and heat capacity of water. A pivotal characteristic for land-to-sea transitions was exceptionally long digestive tracts that resolved conflicting physiological demands for digestion, thermoregulation and diving. Intestine length to body length ratio averaged 2.6 + 0.4 S.E. (n = 10 species) for terrestrial carnivores, 4.6 + 0.6 S.E (n = 16) for omnivores and herbivores, and 13.2 + 1.3 S.E. (n = 25) for marine carnivores. This trait explains the intriguing phylogenetic link between carnivorous marine mammals and terrestrial omnivores and herbivores, and how marine mammals evolved into comparatively voracious predators. This study also demonstrates that what was once an evolutionary springboard for mammalian radiations into colder, highly productive waters may now prove disadvantageous as their appetites exceed currently available oceanic resources.