Sam and Ben have discovered some strange animals living in the woods behind their house. Ben is excited because he believes they are marsupials, an animal he's never observed in the wild before. His brother is sure it is not. What feature should the boys consider in deciding whether the animals are marsupials?

They can observe if the animal has a marsupium, or a pouch at the abdominal area, with a baby inside

Explanation:

The marsupials have their name because they have a marsupium, or a pouch in the abdominal area and they carry their babies there, because when they born they are not fully formed and they finish their development there, and inside that marsupium are also the mamarial glands, so the baby can eat without going outside.

The feature the boys should consider in deciding whether the animals are marsupials is if the animal has a "pouch".

Marsupials carry their young, which are born not fully formed, in a pouch near the stomach area.