| Summary: | Evolution of the domestic dog ( Canis familiaris ) is intimately associated with the evolution of human social behavior and domestic dogs themselves evolved from wolves. Social behavior of both domestic dogs and wolves includes facial displays as a means of communication. These displays have been described and reveal several differences between dogs and wolves. In an effort to better understand the mechanistic events in the evolution of domestic dogs and the potential morphological correlates of their facial displays, the objective of the present study was to compare gross and micro‐anatomy of facial musculature between domestic dogs and wolves. Dissections of cadaveric heads were carried out using standard dissection tools (domestic dog N=6; gray wolf N=4; red wolf N=1) and the presence/absence of facial muscles was documented along with attachments and variation in these characters. Select muscles were sampled for histochemistry and immunohistochemistry. Gross results revealed that the dog and wolf samples were similar to one another in all facial musculature except for the levator anguli occuli medialis and the retractor anguli occuli lateralis muscles. All dog specimens routinely had these muscles while the wolf specimens varied in their presence and size. Microanatomical results revealed that dogs had more robust facial vibrissae in the zygomatic region than the wolf specimens, along with more robust intrinsic vibrissa musculature. The connective tissue layer between the epidermis and facial musculature was thicker in dogs than wolves, suggesting formation of an incipient SMAS (superficial musculoaponeurotic system), a connective tissue layer found in some primates that may be associated with facial mobility. Lastly, both dog and wolf facial musculature were dominated by fast‐twitch myosin fibers but dogs had a greater percentage of slow‐twitch myosin fiber reactivity than the wolf sample, suggesting that dogs may be able to sustain facial muscle contraction longer than wolves. These results partially ...
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