Canids exhibit remarkable variation in feeding-related skull morphology. Considering that the skull is the main structure for obtaining and processing food in this family, they offer an opportunity to explore morphological adaptations to diet without an exhaustive study of the postcranial skeleton. This form-function relationship can be quantified through the morphospace, which has been used by several studies under both traditional measurements and coordinate – based data to infer associations between cranial morphology and diet in canids. However, many of these studies rely on analytical techniques that are inconsistent with adaptive processes, weakening the evolutionary interpretations of the morphospace considering that it has largely been inspired by the adaptive landscape metaphor. Here we use the canid skull form-function relationship to show how phylogenetic comparative methods explicitly accounting for adaptive processes offer an avenue to model the morphospace in line with adaptive landscape thinking. In particular, we use variants of multivariate Brownian motion and Ornstein-Uhlenbeck models on shape variables resulting from geometric morphometric analysis of 14 landmarks placed in the skull of most extant canids (33 species).
Results/Conclusions
The analysis of the morphospace in framework consistent with the adaptive landscape facilitates the interpretation of morphological patterns in terms of long-term selective processes. Our results, for example, suggest that omnivorous and insectivorous feeding preferences are more affected by stabilizing selection than carnivorous diets. As for the latter, prey size preference seems to play an important role in terms of the direction of change, with general carnivorous forms (which hunt small prey) showing adaptations for fast skull occlusion, and hypercarnivorous forms (which hunt large prey) maximizing cranial robustness and the mechanical advantage of biting muscles. These interpretations would be hampered by traditional approaches addressing the morphospace as a flat landscape. We present some of the main challenges faced by morphogeometric data under this framework (e.g. dimensionality and model complexity), and discuss the usefulness of simulation studies for characterizing them and proposing potential solutions. Although it is recognized that adaptation connects ecological factors (e.g. diet, habitat, climate) with morphological shape, the processes resulting in adaptations are often neglected by the analytical treatment of the morphospace. We show how merging geometric morphometrics with modern phylogenetic comparative methods can take these processes into account.