Evolutionary morphology of the rabbit skull.

Brian Kraatz,Emma Sherratt

doi:10.7717/peerj.2453

Brian Kraatz, Emma Sherratt

Open Access

https://doi.org/10.7717/peerj.2453

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Abstract

The skull of leporids (rabbits and hares) is highly transformed, typified by pronounced arching of the dorsal skull and ventral flexion of the facial region (i.e., facial tilt). Previous studies show that locomotor behavior influences aspects of cranial shape in leporids, and here we use an extensive 3D geometric morphometrics dataset to further explore what influences leporid cranial diversity. Facial tilt angle, a trait that strongly correlates with locomotor mode, significantly predicts the cranial shape variation captured by the primary axis of cranial shape space, and describes a small proportion (13.2%) of overall cranial shape variation in the clade. However, locomotor mode does not correlate with overall cranial shape variation in the clade, because there are two district morphologies of generalist species, and saltators and cursorial species have similar morphologies. Cranial shape changes due to phyletic size change (evolutionary allometry) also describes a small proportion (12.5%) of cranial shape variation in the clade, but this is largely driven by the smallest living leporid, the pygmy rabbit (Brachylagus idahoensis). By integrating phylogenetic history with our geometric morphometric data, we show that the leporid cranium exhibits weak phylogenetic signal and substantial homoplasy. Though these results make it difficult to reconstruct what the ‘ancestral’ leporid skull looked like, the fossil records suggest that dorsal arching and facial tilt could have occurred before the origin of the crown group. Lastly, our study highlights the diversity of cranial variation in crown leporids, and highlights a need for additional phylogenetic work that includes stem (fossil) leporids and includes morphological data that captures the transformed morphology of rabbits and hares.

Highlights

Though there exists a clear functional relationship between the vertebrate skeleton and locomotion, there are more limited examples of how the skull may relate to movement
The Principal Components Analysis (PCA) suggests that three primary morphogroups (I–III) exist in leporid morphospace, and they are demarcated in Fig. 3A (PC1 vs. PC2)
We found that there are significant differences in cranial shape associated with allometric size changes, this is largely driven by the smallest of living leporids, the pygmy rabbit (Brachylagus idahoensis)

Summary

Introduction

Though there exists a clear functional relationship between the vertebrate skeleton and locomotion, there are more limited examples of how the skull (cranium and mandible complex) may relate to movement. 1969; Jeffery & Cox, 2010 for further discussion) Both hominid and leporid skulls represent conditions in which the basicranial and facial regions of the cranium flex ventrally relative to one another, where the basicranium is considered the flexor in hominids, and the facial region as the flexor in leporids. While these cranial transformations have been extensively explored as they relate to locomotion in hominins, and briefly within leporids (White & Keller, 1984; Bramble, 1989), given the similarity in cranial transformations between these groups, rabbits and hares represent an ideal system to further understand the relationship between cranial form and locomotor function

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