Here's why:
* Vertebrates are animals with a backbone, or spinal column. This is a defining characteristic that sets them apart from invertebrates like insects and worms.
* Mammals are a group of vertebrates that share several additional characteristics, including:
* Hair or fur: This provides insulation and helps with thermoregulation.
* Mammary glands: These produce milk to nourish their young.
* Three middle ear bones: These are involved in hearing.
* Endothermy: They are warm-blooded, meaning they can regulate their own body temperature.
* Live birth: Most mammals give birth to live young.
In short: All mammals are vertebrates because they have a backbone, but there are many other vertebrates (like fish, amphibians, reptiles, and birds) that are not mammals.