Facial expressions are critical for non-verbal communication. The Canis genus epitomizes the interplay between behavior and morphology in the evolution of nonverbal communication. Recent work suggests that the levator anguli oculi medialis (LAOM) muscle is unique to dogs (Canis familiaris) within the Canis genus and evolved due to domestication. The LAOM raises the inner eyebrows, resulting in the ‘puppy dog eyes’ expression. Here, we test whether the LAOM is a derived trait in dogs by (1) examining the facial expression muscles of a closely related and ancestral wild Canis species, the coyote (C. latrans), and (2) comparing our results to other Canis and canid taxa. We discover that coyotes have a well-developed LAOM like dogs, which differs from the modified/absent LAOM in gray wolves. Our findings challenge the hypothesis that the LAOM developed due to domestication. We suggest the LAOM is a basal trait that was lost in gray wolves. Additionally, we find inter- and intraspecific variation in size of the muscles of the outer ear, forehead, lips, and rostrum, indicating potential adaptations related to sensory perception, communication, and individual-level functional variations within canids. Together, this research expands our knowledge of facial expressions, their evolution, and their role in communication.

Sample Collection. We opportunistically collected nine coyotes (5 males, 3 females, 2 unknown sex) and one dog (male, Australian cattle dog; Table 1) to examine their muscles of facial expression under Texas Parks and Wildlife scientific collections permit number SPR-1221-200. All but one of our coyote specimens (BU008) were collected in McClennan and Galveston counties, Texas. BU008 was donated from a sportsman in Fayette county, Texas.  The dog was donated from Central Texas Humane Society in Waco, Texas. All specimens were adults based on age estimates from tooth wear. Prior to the dissection, the head was separated from the body at the base of the occipital condyle. We took a small tissue sample from the tongue, ear, or temporalis for genetic analysis. All specimens were in good condition (i.e., fresh dead with little to no muscle decomposition) and kept frozen in a -20℃ freezer until dissection. 

Dissections. Specimens were dissected fresh without any chemical fixation. Prior to beginning each dissection, we took scaled photographs of the dorsal, ventral, and lateral sides of the head. For roadkill specimens, we noted any obvious fractures of the cranium or mandible that may have altered the shape of the superficial musculature. We did not include any specimens where the skull or musculature was determined to be in poor condition. We started each dissection with a rostro-caudal incision from the base of the nose pad to the occipital protuberance. The skin was separated from the superficial layer of muscle on the left lateral aspect of the head to allow for examination of the superficial muscles of facial expression.  Next, we removed the superficial musculature to reveal the deeper facial musculature. For each muscle, we took scaled photographs, described the muscle insertion, origin, fiber direction, and action, and noted any interspecific and intraspecific variation in musculature.

We documented the presence, absence, or reduction of each muscle in the coyotes and dog that we dissected. We referred to Kaminski et al. (2019) for gray wolf anatomy, and we verified our dog findings with reference to Evans and de Lahunta, (2013); this enabled comparisons among the three taxa.