Through the science of archiology. They dig up the bones and other remains of animals, reassemble them kinda like a 3-D jigsaw puzzle, and study the skeletal sructure of the animal. This gives a lot of general information. They then study individual parts of the skeleton to learn some more detailed things. The teeth for instance will tell if they were an herbavore (with flat grinding teeth), a carnavor (with sharp cutting teeth), or an omnivor, with a combination of types of teeth (like humans have)
they learn behavioral patterns of the animals.
They don't, they guess.
Their bone structure from fossils and their morphology
They learn about diseases Take a chimp for example scientists use chimps so that they can discover about diseases that can spread to humans.
The more scientists study animal behavior, the more they learn that animals were smarter than they thought. Now we know that even animals like octopus and alligators can use tools. Apes can learn sign language. Dolphins have names for themselves. That's just the beginning.
Aside from fossils we can learn about organisms and how they have changed over the years by looking at the plants and animals we have today.
Scientists learn a few different things. Scientists learn new things very day.
Scientists that learn about the past are called historians because they learn about history/past.
Zoologists are like scientists but who study animals. They observe animals both in their natural habitats and in the laboratory in order to learn as much as possible about animal life.
Scientists use observation and experimentation to learn about the natural world.
It is not impossible to study plants and animals that are still alive. Scientists frequently observe plants and animals and even run tests and experiments on them in order to learn more about them.
Scientists that study zootomy specialize in the comparative anatomy of animals. In order to learn the anatomy and physiology of the animals, most research is conducted through dissection.