On The Trails: Mammalian dentition
- Mary F. Willson
- Oct 12
- 3 min read
By Mary F. Willson
Almost all mammals have teeth, whose primary function is feeding, although they sometimes have other functions such as display or weaponry.
Most of the toothed mammals produce two sets of teeth: one set early in life, later shed and replaced by a more permanent set. One term for teeth that are shed is "deciduous" (like autumn leaves). We often refer to that first set as baby teeth or milk teeth. Baby teeth are typically smaller and weaker than the permanent ones and presumably cheaper to produce.
Why do these critters produce that temporary set of teeth? Infants of all mammals feed chiefly on their mother’s milk. But most of them start to feed on other things quite early in life and even small teeth might be useful for that. Perhaps the small size reduces the risk of damage to the mother’s tissues, although those baby teeth can be very sharp. And most references note the important role the first set plays in making and holding a place for the adult teeth as the jaw grows. In humans, the replacement process is typically complete by age 12 or 13, in horses by age five, and in dogs by age seven months.
There are exceptions to this common, general pattern, of course, and some odd variations.
In a few species, the teeth are continually replaced throughout the animal’s life. As the anterior molars* get worn, they are replaced by other molars pushing forward in a continuous process. Examples include some that have milk teeth, such as elephants, and others that lack milk teeth (pygmy rock wallaby, manatees); in the silvery mole-rat, the incisors* are included in the replacement process. Although the mechanisms of continuous replacement differ among these examples, it serves to maintain a battery of functional teeth as the older ones wear away from dealing with tough materials.
In some kangaroos, a premolar is shed before the permanent premolars come in, and then continuous molar replacement goes on, moving forward. Aardvarks, which feed mostly on ants and termites, are born with front teeth (incisors and canines) that soon fall out, leaving rootless, enamel-less, peg-like molars that wear out quickly and are continually replaced.
In most cases, the permanent teeth reach their mature size and stop growing. However, in many rodents, hares, and mole-rats the incisors can grow continuously. In horses, the molars do so too.
There are some mammals that produce just one set of teeth that are never replaced. Rodents typically do, and those teeth grow continually. And the toothed whales also have just one set per lifetime; the teeth are used in prey capture and the prey is generally swallowed whole. Elephant tusks (incisors) are never shed but grow continuously.
Some mammals have no teeth at all. Adult baleen whales have no teeth, capturing food with their baleen plates, but…surprise!...their fetuses do have rudimentary teeth that are resorbed very early in development. Platypuses feed chiefly on small aquatic invertebrates, grinding them up in the bill. Anteaters, pangolins, and echidnas eat mostly ants and other insects. They grind up their food by smashing it against the roof of the mouth with the tongue or it is ground up in the stomach.
And then there are some species that have no front teeth and small, weak molars, but very different diets from all those ant-eating species. Armadillos feed on invertebrates and small vertebrates, and sloths feed on leaves. Hmmmmm….
I could pepper all these variations with many questions, but this time I’ll leave it to readers to consider many Hows and Whys….
*Footnote: In case a reminder of the names of teeth is useful: here is the human dental formula, starting at the front (for one side, either up or down): two incisors, one canine, two premolars, three molars for a total of 32 teeth.
• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.
