Some animals do clean their teeth, they just don’t need a toothbrush to do it. For example, cows keep their teeth healthy by chewing grass, while elephants brush their tusks (which are actually modified incisors) by using them to chisel tree bark. However, most animals don’t have much of a problem with dental hygiene because of their diet. Humans suffer from tooth decay because we eat foods that are rich in refined sugars, but animals don’t come across these in the wild.
Some pet foods do contain sugar, so a regular tooth clean may be required for your pets, but the high pH value of canine and feline saliva means they are less susceptible to cavities. Often the relatively short lifespan of animals mean that they don’t live long enough to suffer from dental damage. Or, in the case of sharks and crocodiles, their teeth don’t last long enough as they regularly replace them throughout their lives.