Gape in mammals

The mouth opening, or gape, in mammals serves a variety of purposes: breathing, vocalisation, prehension of food, but also agonistic display. The mammalian jaw system has adapted to each species’ behaviour and diet. All mammals have in common that the frontal, prehensile part of the jaw is covered by the lips and the premolar and molar teeth (used for chopping up and chewing food) are covered by the cheeks. The cheek muscle, musculus buccinator (Fig. 1), assists the tongue in placing and keeping food where it needs to be while we chew [2].

In herbivores, the mouth opening is generally small compared to the size of the head (Fig. 2). The buccinator is usually well-developed, since tough, fibrous plant matter requires thorough chewing [3]. The cheeks are thus more spacious to temporarily store the food that is being chewed.

Carnivores have a less-developed buccinator and smaller cheeks, since they tend to slice off bite-sized chunks of meat which they swallow whole. In contrast to herbivores, the buccinator is more u-shaped in carnivores and borders the margin of the upper and lower jaw. This allows them a wider gape angle (Figs. 2, 3). As most carnivores are predators, they require a wide gape to apprehend their prey [3]. The carnivoran jaw system – unlike that of herbivores – has adapted to achieve great bite forces at wide gape angles [4].

What about humans?

The human buccinator muscle is more similar to that of herbivores (Fig. 1). Our gape angle is significantly smaller than that of carnivores (Fig. 2).

The buccinator muscles in a typical carnivore (dog), herbivore (horse), and a human.

Fig. 1:  The buccinator muscles in a typical carnivore (dog), herbivore (horse), and a human. The u-shape of the carnivores’ buccinator allows them a wider gape.

Gape angle of several carnivores, omnivores, herbivores, and humans          
Fig. 2:  Gape angle of several carnivores, omnivores, herbivores, and humans (data from [1]). Although there are some exceptions (like the hippopotamus), herbivores tend to have a smaller gape than carnivores. Humans are also on the lower end of the gape spectrum.

A yawning lioness
Fig. 3:  A yawning lioness (photograph by Ramesh Ratwatte). Go look in the mirror and open your mouth as wide as you possibly can. It’s not quite the same, is it?

REFERENCES:

[1] Herring, Susan W., and Stephen E. Herring. "The superficial masseter and gape in mammals." American Naturalist (1974): 561–576.

[2] Fehrenbach, Margaret J., and Susan W. Herring. Illustrated anatomy of the head and neck. Elsevier Health Sciences, 2013.

[3] Galton, Peter M. "The cheeks of ornithischian dinosaurs." Lethaia 6.1 (1973): 67–89.

[4] Bourke, Jason, et al. "Effects of gape and tooth position on bite force and skull stress in the dingo (Canis lupus dingo) using a 3-dimensional finite element approach." PLoS One 3.5 (2008): e2200.