Monday, May 23, 2011

Marsupials and Monotremes

In case being a mammal hasn’t made you an expert, today’s post covers two of this Class’ underrepresented members: marsupials and monotremes.

In general, mammals are air-breathing vertebrates characterized by three traits: hair, mammary glands in mothers, and three middle ear bones for hearing. After these universal traits, birth method distinguishes the three different groups of mammals.

Placental mammals are by far the biggest group and include humans (along with almost every other mammal that comes to mind). During fetal development, this Group employs a placenta – an organ that connects a gestating fetus to its mother, to supply nutrients and oxygen and eliminate waste.

Marsupials – the second biggest group – don’t use a placenta. After developing 4-5 weeks, the marsupial fetus climbs from its mother’s womb into a warm, protective pouch on her body. To make this journey, all newborn marsupials – called joeys – possess overdeveloped front limbs (basically, a gummy bear with claws). Unable to regulate their temperature or fend for themselves, joeys remain inside the pouch sometimes up to a year, nursing on their mother’s milk.

Almost all marsupials are native to Australia and it’s neighboring islands, including bandicoots, wombats, koalas, sugar gliders, an extinct breed of wolf and – of course – kangaroos, wallabies, and wallaroos. The only marsupials not from Australia are the Americas’ opossums. Note that only the “o” variety of opossum is known for playing dead, whereas possums – another species of marsupial native to Australia – are in a completely different Order of animals. Because all marsupials must have grasping claws to travel from womb to pouch, they are unlikely to develop flippers, hooves, wings or other appendages common to placental mammals.

To address a popular myth, male marsupials actually do have two-pronged penises. Fittingly, female marsupials possess two lateral vaginas and a third birth canal that share one external opening  (contrary to popular use, “vagina” actually refers to an internal canal, not an external opening).

Lastly, we have the monotremes, the smallest group. Monotremes are probably the most recent common ancestor between mammals and birds and reptiles. Like birds, amphibians, and reptiles, monotremes have a cloaca – a single orifice for reproduction and waste disposal.  Using their cloaca, monotremes lay leathery eggs. That’s right: a mammal that lays eggs. After they hatch, monotremes nurse their young like any other mammal; although they don’t have nipples, mothers secrete milk through openings in the skin.

Also native to Australia and New Guinea, monotremes only include platypus and echidnas. Most people are familiar with the platypus’ jumbled physiology: beaver-like tail, duck-like bill, otter-like feet. Echidnas are similarly confusing creatures with spiny backs and beak-like mouths. Additionally, male monotreme hind ankles each sport a spur. Although the spur is nonfunctional in echidnas, platypus spurs contain venom that causes excruciating pain. Not so cute now, huh?

Hopefully, this post shed some light on what it really means to be a mammal. And remember: being a part of a group doesn’t mean you know everything about it.

Sources: 123