Domestication not only changed the appearance and behavior of dogs but also altered the physiology of their facial muscles. A study published in October 2024 in The Anatomical Record compared the muscle fiber composition of the zygomaticus and orbicularis oris muscles in domestic dogs and gray wolves to better understand how facial expression evolved in canines.
Muscle samples from six dogs and four wolves revealed a striking difference. Domestic dogs possessed almost 100% fast-twitch (type II) fibers, allowing their facial muscles to contract quickly, producing rapid and expressive movements. In contrast, gray wolves had less than 50% fast-twitch fibers, relying instead on a higher proportion of slow-twitch (type I) fibers that support sustained contractions rather than quick, dynamic shifts.
The researchers suggested that these physiological differences reflect the evolutionary pressures of domestication. As dogs adapted to living with humans, selection may have favored facial muscles capable of rapid expression, enhancing communication and strengthening social bonds. This capacity for quick, expressive facial cues may underlie why dogs are adept at capturing human attention through gestures like eyebrow raises or subtle changes in mouth shape.
Although based on a small sample size, the study provides evidence that dog domestication involved not just morphological but also physiological divergence in facial musculature. The authors also noted that the retention of traits such as persistent barking into adulthood may have played a complementary role in the evolution of communicative signaling between dogs and humans.
Source: Burrows, A. M., Smith, L. W., Downing, S. E., Omstead, K., & Smith, T. D. The Anatomical Record, October 2024.
