Jaw motion and chewing (mastication)

Most carnivores are predators who hunt, kill, and butcher their prey [1]. These predators kill either with a single, strong penetrating bite (e.g., mustelids and felids) or several more shallow bites (e.g., canids and hyaenids). Due to the morphology of their jaws and adductors (Fig. 1), they bite with a chopping (up and down) motion. Their jaw joint is on the same level as the tooth row, which allows for a scissor-like jaw motion, but makes horizontal movement (side to side, backwards and forwards) impossible [2].

Once the prey is rendered immobile, carnivores use their sharp canine and carnassial teeth to tear and slice flesh into chunks which they swallow whole [1]. Some carnivores, like dogs, have large, robust molars, which allow them to crush bones, something cats are not capable of [1].

Although the dentition of omnivores (e.g., bears) is in general similar to that of carnivores, their carnassials are not as developed as those of true carnivores [3]. Omnivore dentition is not well adapted to process tough and fibrous plant matter [1]. Hence, their food is either swallowed in whole chunks or simply crushed, but not thoroughly ground and chewed.

Herbivores feed on a broad range of plant foods, ranging from fruits, nuts and seeds to grass, leaves, and even tree bark. Extensive chewing is necessary to process such tough plant tissue. Their jaw, adductor, and tooth morphology is adapted to their herbivorous diet. Their canine teeth are reduced or even absent. Their molars, however, are capable of crushing and grinding tough and fibrous plant matter in a mortar-and-pestle fashion [4]. The location of the jaw joint above the plane of the tooth row (Fig. 1) allows for side-to-side and back-to-front motion of the mandible (lower jaw) and, therefore, extensive chewing. (Picture a cow chewing grass.)

What about humans?

In humans – like in herbivores – the jaw joint is also located above the level of the tooth row. (Try moving your mandible from side to side and backwards and forwards.)

We also tend to chew our food quite thoroughly. (Please do not try swallowing whole chunks of food like a carnivore!)

The jaw-closing muscles (adductors), masseter and temporalis, in a carnivore (dog), a herbivore (horse), and a human.

Fig. 1:  The jaw-closing muscles (adductors), masseter and temporalis, in a carnivore (dog), a herbivore (horse), and a human. The masseter is usually larger and stronger than the temporalis in herbivores to allow side-to-side motion of the jaw for grinding fibrous plant matter. In carnivores, the temporalis is larger than the masseter to pull the lower jaw up with great speed and power. The masseter is relatively small in carnivores and mostly aids in stabilising the jaws during closing.

 

REFERENCES:

[1] Feldhamer, George A. Mammalogy: adaptation, diversity, ecology. JHU Press, 2007.

[2] Schwenk, Kurt. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000.

[3] Hillson, Simon. Teeth. Cambridge University Press, 2005.

[4] Hiiemae, Karen M. “Feeding in Mammals”. In: Schwenk, Kurt, ed. Feeding: form, function and evolution in tetrapod vertebrates. Academic Press, 2000