Humans domesticated dogs with attention to the facial expressions that dogs produce, selecting for a suite of facial movement. Domestic dogs and humans are adept at accurately understanding one another’s facial expressions and movement around the eyes in dogs is highly valued by humans. This unique, mutual ability to accurately process facial expressions is part of the dog-human bond. An accurate understanding of how dog and wolf faces differ from one another is fundamental to understanding the processes of animal domestication, evolution of the dog, and the origins of human behavior since the Upper Paleolithic. While we know that dogs have facial musculature that wolves do not regularly possess, our understanding of evolutionary muscle physiology between these two species is incomplete. The present study was designed to assess differential distribution of slow-twitch (type I) and fast-twitch (type II) myosin fibers in select facial muscles of dogs and wolves. We tested the hypothesis that dog facial muscle physiology would be more similar to humans than to wolves in terms of myosin fiber type distribution. We used samples from the orbicularis oris muscle (OOM) and the zygomaticus major muscle (ZM) from a range of domestic dog breeds (Canis familiaris) and gray wolves (Canis lupus) that were part of previous studies. Samples were processed for myosin immunohistochemistry (IHC) and we selected 3-5 sections from each specimen stained for identification of each fiber type in both the OOM and ZM muscles. We created composite images of stained sections and assessed fiber type percentages using ImageJ. Percentage of fast-twitch fibers in dog samples ranged from 66%-95% and wolves averaged around 25%. Slow-twitch fiber percentages in dogs averaged only 10% while wolves averaged 29%. The dog range of type II fibers mirrors the range found in human facial muscles, indicating that the domestication process in dogs also involved selection for rapid facial movement, similar to the movement seen in human faces.
