What animals have molars that grow continuously ?
No mammals that I or my colleagues are aware of, only some few whose
incisors grow continuously.
Most vertebrates are "polyphyodonts" meaning that they replace teeth
continuously through out their lives. All the teeth aren't replaced at
once, but in waves so that the animals always have functional teeth around
those that are lost.
Most mammals are "diphyodonts", which means that they have only 2 sets of
teeth: baby teeth and adult teeth.
The teeth of herbivore mammals, those which eat grasses, seem to grow
throughout their lives. But really, the teeth are very long and extend far
down into the jaws. They gradually move up in the jaw toward the surface
over time, with the area beneath them filling in with bone.
Elephants are especially interesting because they have six molars, or
grinding teeth. Only the first 2 come in and are chewed with until they
wear out and fall out. Then the next pair and then the last pair. So the
elephants' teeth can last the ~70 years of their life span.
Horses often have to have their teeth "floated" because the lower molars
are narrower than the upper molars. Sharp points form as the teeth wear on
the outside edges of the upper teeth and inside edges of the lower teeth.
These are filed off by veterinarians so that the points don't cause pain
and interfer with eating.
Some rodents, rabbits and a few other animals have specialized
"open-rooted" teeth that can continue to grow through-out their lives.
This can include their cheek teeth and incisors. If the front teeth are
misaligned, this continual growth can cause the teeth to grow across into
the opposite jaw. For pets, veterinarians can trim these front teeth when
they start to get too long.
Thanks for Asking NEWTON!
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Update: June 2012