Caring for the young
There are seventy six species of opossum (Family Didelphidae) in America, with the smallest (Marmosa murina) being mouse sized and not possessing a pouch (the young simply cling to the teats between their mother's hind legs. The largest is the water opossum or Yapok (Chironectes minimus) and is almost the size of a small otter, and possesses webbed feet for swimming. Its young are saved from drowning in the pouch when their mother goes, a sphincter (ring-shaped muscle) which closes the entrance the entrance of the pouch. The young inside are able to endure several minutes of submergence and breathe air within the pouch that has a higher concentration of carbon dioxide than most mammals could survive.