It is not accurate to say that all mammals lack gizzards, but the vast majority do not have them, while earthworms and birds do.
* Earthworms: As invertebrates, they lack a backbone and specialized vertebrate organs. Their gizzard is a muscular organ that uses grit, such as small stones and particles, to grind up plant matter. This is crucial because they lack teeth.
* Birds: Most birds have gizzards, though some, like doves, do not. The gizzard is a powerful muscular organ that grinds food into smaller particles, helping them digest seeds, insects, and other tough materials. Since birds typically have beaks rather than teeth, they cannot chew their food in the usual way.
* Mammals: Most mammals have teeth adapted for chewing and breaking down food, making a gizzard unnecessary. There are exceptions, however:
* Monotremes: Egg-laying mammals such as the platypus and echidna have a digestive structure similar to a gizzard, called the gastrolith chamber, which also uses grit to break down food.
In summary: The presence or absence of a gizzard relates to an animal's diet and how it processes food. Earthworms and many birds need gizzards because they lack teeth and must break down tough materials. Most mammals, equipped with teeth, use a different digestive system.