P. M. Gignac & G. M. Erickson (2016)
Ontogenetic bite-force modeling of Alligator mississippiensis: implications for dietary transitions in a large-bodied vertebrate and the evolution of crocodylian feeding.
Journal of Zoology (advance online publication)
DOI: 10.1111/jzo.12349View/save citation
Crocodylians undergo substantial increases in size during ontogeny. The American alligator, Alligator mississippiensis, in particular traverses nearly four orders of body mass between hatching and senescence. Accompanying such changes are modifications in rostrodental morphology and feeding capabilities that facilitate major shifts in diet. How such anatomical changes relate to ecological niche occupation across sizes is not well understood. In this study, we focused on the effects of ontogenetic changes on the force-generating mechanisms for jaw closure to assess the impacts of scaling on feeding biomechanics. We developed dissection-based, musculoskeletal models of maximum bite-force generation throughout ontogeny and compared and tested their veracity with data from an A. mississippiensis developmental series, for which bite forces were directly measured. Through examinations of the scaling patterns within the parameters of our models, we discuss how muscle pennation and positive allometry in the American alligator jaw adductor system facilitate capture strategies and oral processing of prey, and contribute to developmental niche shifts in this large-bodied taxon. On the basis of conservation of the crocodylian jaw adductor system, we argue that our findings are broadly applicable to crown Crocodylia and reflect an important, but often overlooked, aspect of the crocodylian feeding ecomorphology: littoral, sit-and-wait predation is enhanced by posteroventrally displaced, exceptionally large, and forceful ventral pterygoideus muscles, in particular. Future studies on the ontogeny and evolution of feeding in crocodylians should not neglect the functional and ecological implications of these muscles' contributions to diet.
Alejandro Gonzalez-Voyer, Manuela González-Suárez, Carles Vilà and Eloy Revilla (2016)
Larger brain size indirectly increases vulnerability to extinction in mammals.
Evolution (advance online publication)
Although previous studies have addressed the question of why large brains evolved, we have limited understanding of potential beneficial or detrimental effects of enlarged brain size in the face of current threats. Using novel phylogenetic path analysis, we evaluated how brain size directly and indirectly, via its effects on life history and ecology, influences vulnerability to extinction across 474 mammalian species. We found that larger brains, controlling for body size, indirectly increase vulnerability to extinction by extending the gestation period, increasing weaning age, and limiting litter sizes. However, we found no evidence of direct, beneficial, or detrimental effects of brain size on vulnerability to extinction, even when we explicitly considered the different types of threats that lead to vulnerability. Order-specific analyses revealed qualitatively similar patterns for Carnivora and Artiodactyla. Interestingly, for Primates, we found that larger brain size was directly (and indirectly) associated with increased vulnerability to extinction. Our results indicate that under current conditions, the constraints on life history imposed by large brains outweigh the potential benefits, undermining the resilience of the studied mammals. Contrary to the selective forces that have favored increased brain size throughout evolutionary history, at present, larger brains have become a burden for mammals.