Whereas rodents have gnawing incisors that grow throughout life , the teeth of shrews wear down throughout life , a problem made more extreme because they lose their milk teeth before birth , so have only one set of teeth throughout their lifetimes .
Unlike rodents , which have gnawing incisors that grow throughout life , the teeth of shrews wear down throughout life , and they lose their milk teeth before birth . Therefore they have only one set of teeth for their lifetime .