Ecomorphology of the largest terrestrial mamalian predators and a biomechanical constraint on their body mass

Includes bibliographical references (pages [125]-134).Observations on extant mammals suggest that greater body mass confers important selective advantages on terrestrial predators of large herbivores. Yet, throughout the Cenozoic, some lineages of terrestrial mammalian predators attained greater maximal body mass than others, suggesting the existence of an evolutionary constraint on their body mass. A comparison of dental and skeletal morphology of the extinct giant short-faced bears of the genera Agriotherium and Arctodus to those of the living carnivorans and Hemicyon ursinus, another extinct ursid, suggests that the former were omnivore-scavengers, rather than carnivorous active predators as suggested by some authors, and therefore were not subject to the above constraint. In contrast, a comparison of dental and skeletal morphology of the giant bear-dogs (members of the extinct carnivoran family Amphicyonidae) of the genera Amphicyon and Ischyrocyon to those of the living carnivorans suggests that the former were carnivorous active predators as suggested by a number of authors and therefore were subject to the above constraint. On the basis of published data on the scaling of body support in the living Carnivora, the following biomechanical constraint on body mass in terrestrial mammalian predators is hypothesized. The stress set up in the humerus by the bending moment of the peak ground reaction force at a given running speed increased with increasing body mass within a lineage of terrestrial mammalian predators. This resulted in a decreasing safety factor for the humerus until a predator could no longer attain the maximal running speed of its smaller relatives. The selective disadvantage of reduced maximal running speed counterbalanced the advantages of greater body mass, preventing further increase of body mass within the lineage. This hypothesis is tested by examining the scaling of humeral dimensions and estimating maximal body masses in several lineages of terrestrial mammalian predators, including Amphicyon, Ischyrocyon , and other members of the Amphicyoninae (a subfamily of the Amphicyonidae) but excluding Agriotherium and Arctodus. The results support the proposed hypothesis but suggest that some lineages of terrestrial mammalian predators failed to reach the biomechanical limits on their body masses because of the small size of available prey.Ph.D. (Doctor of Philosophy