Conservatism and adaptability during squirrel radiation: what is mandible shape telling us?

PLoS One. 2013 Apr 4;8(4):e61298. doi: 10.1371/journal.pone.0061298. Print 2013.

Abstract

Both functional adaptation and phylogeny shape the morphology of taxa within clades. Herein we explore these two factors in an integrated way by analyzing shape and size variation in the mandible of extant squirrels using landmark-based geometric morphometrics in combination with a comparative phylogenetic analysis. Dietary specialization and locomotion were found to be reliable predictors of mandible shape, with the prediction by locomotion probably reflecting the underlying diet. In addition a weak but significant allometric effect could be demonstrated. Our results found a strong phylogenetic signal in the family as a whole as well as in the main clades, which is in agreement with the general notion of squirrels being a conservative group. This fact does not preclude functional explanations for mandible shape, but rather indicates that ancient adaptations kept a prominent role, with most genera having diverged little from their ancestral clade morphologies. Nevertheless, certain groups have evolved conspicuous adaptations that allow them to specialize on unique dietary resources. Such adaptations mostly occurred in the Callosciurinae and probably reflect their radiation into the numerous ecological niches of the tropical and subtropical forests of Southeastern Asia. Our dietary reconstruction for the oldest known fossil squirrels (Eocene, 36 million years ago) show a specialization on nuts and seeds, implying that the development from protrogomorphous to sciuromorphous skulls was not necessarily related to a change in diet.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptation, Biological*
  • Animals
  • Biological Evolution*
  • Feeding Behavior
  • Mandible / anatomy & histology*
  • Phylogeny
  • Principal Component Analysis
  • Sciuridae / anatomy & histology*
  • Sciuridae / classification
  • Sciuridae / physiology*

Grant support

Ministerio de Economía y Competitividad, projects (CGL2010-21672/BTE, CGL2011-27343) and research contract to ICV (JCI2010-08241), as well as Generalitat de Catalunya, project (2009 SGR 754), and the SYNTHESYS Project from the European Community Research Infrastructure (NL-TAF-4084) to ICV supported this research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.