Meeting Abstract

19.3  Sunday, Jan. 4  Brain size evolution in new and old world marsupials PETERS, J E; Univ. of Illinois peters25@illinois.edu

Consensus is that marsupial brain size is constrained compared to eutherians, and that this has reduced marsupials adaptive ability. A test case for this exists in the dichotomy between New and Old World marsupials. New World (NW) marsupials evolved alongside eutherian competitors, whereas Old World (OW) marsupials evolved in relative isolation. Given their competition with eutherians, and the apparent competitive advantage that larger brains provide, I hypothesize that NW marsupials have been under stronger selective pressure than OW marsupials to increase their brain size, and therefore exhibit larger brains. To test this hypothesis, cranial volume and mandibular length of 454 specimens belonging to 50 NW marsupial species were quantified. These variables were used to calculate encephalization quotient, combined with OW data from Ashwell (2008) and subjected to Wilcoxon or Kruskal-Wallis nonparametric statistical analyses. As predicted, NW marsupials have significantly larger brains than OW marsupials (p <0.0001). I also compartmentalized the data and tested for size trends in functional groups (e.g., within arboreal quadrupeds, terrestrial quadrupeds, etc) or within specific phylogenetic lineages (e.g., diprotodonts, didelphids etc.). Regardless of manipulation, all six orders examined independently differed (p <0.001) with NW forms (Didelphimorphia, Paucituberculata and Microbiotheria) possessing larger brain sizes than OW (Dasyuromorphia, Diprotodontia and Peramelemorphia). These results suggest that the selective pressures associated with brain size were sufficient to allow variation due to adaptive measures, suggesting that constraints play little role in relative brain size within marsupials. Comparisons between marsupials and eutherian mammals are underway to question whether developmental constraints play any role in marsupial brain evolution.