Drs. MacLeod and Reidenberg contributed equally to this study.
Breaking symmetry: The marine environment, prey size, and the evolution of asymmetry in cetacean skulls
Version of Record online: 21 MAY 2007
Copyright © 2007 Wiley-Liss, Inc.
The Anatomical Record
Special Issue: Anatomical Adaptations of Aquatic Mammals
Volume 290, Issue 6, pages 539–545, June 2007
How to Cite
Macleod, C.D., Reidenberg, J.S., Weller, M., Santos, M.B., Herman, J., Goold, J. and Pierce, G.J. (2007), Breaking symmetry: The marine environment, prey size, and the evolution of asymmetry in cetacean skulls. Anat Rec, 290: 539–545. doi: 10.1002/ar.20539
- Issue online: 21 MAY 2007
- Version of Record online: 21 MAY 2007
- Manuscript Accepted: 6 MAR 2007
- Manuscript Received: 2 MAR 2007
- Commission of the European Communities. Grant Numbers: ERB 4001 GT 93/3630, ERB FMBI CT96 1373
- CEC DG XIV. Grant Number: Study Project 98/089. Sample collection and analysis during 2001–2003 was funded under CEC project EVK3-CT-2000-00027
- skull morphology;
- directional asymmetry;
- prey size;
- toothed whales, stomach contents, anatomy
Skulls of odontocetes (toothed whales, including dolphins and porpoises) are typified by directional asymmetry, particularly in elements associated with the airway. Generally, it is assumed this asymmetry is related to biosonar production. However, skull asymmetry may actually be a by-product of selection pressure for an asymmetrically positioned larynx. The odontocete larynx traverses the pharynx and is held permanently in place by a ring of muscle. This allows prey swallowing while remaining underwater without risking water entering the lungs and causing injury or death. However, protrusion of the larynx through the pharynx causes a restriction around which prey must pass to reach the stomach. The larynx and associated hyoid apparatus has, therefore, been shifted to the left to provide a larger right piriform sinus (lateral pharyngeal food channel) for swallowing larger prey items. This asymmetry is reflected in the skull, particularly the dorsal openings of the nares. It is hypothesized that there is a relationship between prey size and skull asymmetry. This relationship was examined in 13 species of odontocete cetaceans from the northeast Atlantic, including four narrow-gaped genera (Mesoplodon, Ziphius, Hyperoodon, and Kogia) and eight wide-gaped genera (Phocoena, Delphinus, Stenella, Lagenorhynchus, Tursiops, Grampus, Globicephala, and Orcinus). Skulls were examined from 183 specimens to assess asymmetry of the anterior choanae. Stomach contents were examined from 294 specimens to assess prey size. Results show there is a significant positive relationship between maximum relative prey size consumed and average asymmetry relative to skull size in odontocete species (wide-gape species: R2 = 0.642, P = 0.006; narrow-gape species: R2 = 0.909, P = 0.031). This finding provides support for the hypothesis that the directional asymmetry found in odontocete skulls is related to an aquatic adaptation enabling swallowing large, whole prey while maintaining respiratory tract protection. Anat Rec, 290:539–545, 2007. © 2007 Wiley-Liss, Inc.