That’s a pretty big question. I guess I would put my answer into 2 main categories: Physiology (changes inside their bodies) and behavior (what they do).

For example, when it gets colder, some animals might put on more fat, or grow thicker coats. All of those are physiological responses to low temperature in the environment. Animals can also respond with behavior, like finding a warm rock to lie on or migrating to a warmer climate.

Think about all the many things an animal might have to respond to in its environment. We looked at weather, there’s also the need for food and water (if they live on land), predators, competitors, potential mates, maybe offspring. What else can you think of?

Different animals respond to different things in different ways. Most people think of the word “animal” as meaning only mammals. In fact, animals could be sponges, jellyfish, worms, insects, amphibians, birds, even corals. If it moves at some time in its life, eats, and is made of more than one cell, it’s an animal. Do you think a clam needs to respond to the same things that a seagull does? How do you think their responses might be different?

You might want to study animal ecology.