The rate of wear can be quite rapid, as in the case of rodent incisors, or slow, as in the case of human molars. The rate depends partly on the conformation of the tooth's occlusal surface, partly on the abrasiveness of the animal's diet, partly on how the tooth is used, and partly on whether it is rooted or unrooted. It is a fairly common practise of mammalogists to estimate an animal's age by examining the wear on its molars. There are a few exceptions to the pattern of diphydont replacement of teeth. In pinnipeds and many rodents, both sets of teeth are present but replacement takes place in utero, that is, before the young is born.
Elephants eat a very abrasive diet, and the teeth wear strongly as they move forward.
By the time the tooth is lost, it is largely worn away. Generally only two teeth are active at any time. Manatees, which also feed on abrasive vegetation, have a similar pattern, but their dentition includes up to 20 teeth per quadrant, six to eight functional at one time. Kangaroos also have a similar pattern of tooth replacement. In young kangaroos, two blade-like premolars erupt first on each side.