* Biological Classification: Mammals are a class of animals. To have a true "opposite," you'd need another class.
* Key Mammal Characteristics: Mammals are defined by things like having fur, giving birth to live young, and producing milk. There's no one animal group that completely lacks all of these traits.
Some possible ways to think about an opposite:
* Within the Animal Kingdom: You could consider non-mammalian vertebrates (fish, amphibians, reptiles, birds) or invertebrates (animals without backbones, like insects, worms, etc.).
* Based on Key Features:
* No Fur: Reptiles, amphibians, birds, fish
* Lay Eggs: Birds, reptiles, amphibians, fish, insects, etc.
* Don't Produce Milk: All animals besides mammals
The best "opposite" depends on what aspect of "mammal" you are contrasting.