Millions,
Last night I got caught up in a heated discussion about wombats. Okay, that’s a lie. But I was talking about wombats over dinner. Which got me thinking: don’t all marsupials live on the continent of Australia? I mean, they’ve got the kangaroo, the koala, the bandicoot—God knows what else. My question: Why? Is there a reason I can get that isn’t just “they all evolved there”? All I’ve got right now is wild conjecture.
There are some marsupials that live in the Americas, e.g., the Virginia opossum. However, the reason why they are mainly found in Australia and New Guinea is that that area got separated from the rest of the world a long time ago, making it difficult for placental mammals to get there. However, there are some bats and rodents native to Australia: the bats’ ancestors presumably flew there between islands of the East Indies, while the rodents’ ancestors may have floated across. It would only take one pregnant female rodent on a floating piece of timber for a species to cross into Australia; however, larger animals would have found it harder to cross until humans built boats, and (for example) introduced the dingo into Australia.
Aintcha never herd of 'possum?
It’s fat–but pretty good eatin’, 'f there’s sweet taters on the table!
EZ
Yes…yes…I seem to have forgotten the opossum. I have another post in GD relating to the bags under my eyes, so I’m going to plead drowsiness combined with haste. There any other non-Aussie marsupials I’m also forgetting?
No, all marsupials don’t live on Australia.
There is one familiar marsupial species in North America, the opossum. South America has several, New Guinea has several.
New Guinea’s marsupial population can be explained because during the ice ages Australia and New Guinea were connected and part of a single land mass. North America’s opposum migrated from South America to North America when the isthmus of Panama formed. Before the isthmus of Panama, South America was an island continent like Australia and home to many marsupial species, most of which are now extinct.
So why would marsupials be more prevalent on island continents? Marsupials and placental mammals evolved back in the time of dinosaurs, 65 million years ago. On most other continents the marsupials all went extinct, but South America and Australia were isolated from the other continents, and marsupials survived there. When South America was reconnected a lot of new placental species invaded, and most of the marsupials went extinct. Australia hasn’t had a land connection to another continent since the time of dinosaurs, so their marsupials haven’t faced competition from cosmopolitan placental species. However, nowadays there are lots of human introduced placentals that are displacing native marsupials. Dingos displaced thylacines already, cats are displacing the small marsupial carnivores, and rabbits, mice and rats are displacing the small herbivores.
Of course, in 20 million years Australia is due to crash into southeast Asia anyway and will then have a permanent land bridge with the rest of the continents.
Yes http://en.wikipedia.org/wiki/Marsupial
There are between 260 and 280 species of marsupials, almost 200 of them native
to Australia. This leaves at least 60-80 non-Aussie marsupials.
This is a list of most of the American ones: almost all are called opossums however. http://en.wikipedia.org/wiki/Didelphimorphia
http://en.wikipedia.org/wiki/Paucituberculata
The Shrew Opossum doesn’t look like a Virginia Opossum at all.
Jim
Lemur: The point about marsupials in island habitats interests me. Question, though. Why would/do placental animals tend to drive marsupial species extinct or, at the very least, displaced? What’s the reason there?
Well, that’s a very contentious question.
The short answer is that island species of all kinds, not just marsupials, are frequently outcompeted by cosmopolitan species. This isn’t always the case, of course…the oppossum, armadillo and porcupine from the island continent of South America do just fine competing against the big bad cosmopolitan species of North America. So are island species wimps? It’s true that cosmopolitan species tend to have experienced more competition in their evolutionary history, this is especially true of island bird species on small islands with no land predators. And so you have cases like the dodo, which didn’t recognize humans and dogs as potential dangers.
But this isn’t likely to be the case for island continents. The most plausible explanation is just that there are lot more cosmopolitan species than there are island species. So if you take an island with 10 species, and a mainland with 100 species and connect them, even if island and mainland species are on average equally well adapted, just by the odds you’re going to see a lot more mainland species invading and displacing island species than you are going to see island species invading and displacing mainland species. There’s just not as much diversity available.
The placentals had a better union.
South American Tertiary and present fauna is among the most fascinating topics to get into. But suffice it to say that “opossum” covers a multitude of animals in a variety of niches there, plus the other surviving group.
There used to be a hypothesis that the placentals dominated because of an inherent advantage to placental reproduction over marsupial, but the actual case seems to be that the placentals, faced with more competition in the larger area, tended to evolve into forms better adapted to the niches that converged with those of the marsupials. The dingo, for example, is no better physiologically at predation than a thylacine, but it’s a smarter animal. Ruminants can do a better job of grass-processing for nutrients than marsupial herbivores. And so on. So no single factor accounts for the replacement, but in general and for widely varying reasons the placental generally has a leg up on the marsupial in such competition.
The opossum survived because it’s the consummate generalist, omnivorous and able to adapt to a wide range of temperate and subtropical econiches. In seven years of living a few hundred feet from a wooded area, I have seen: a wide assortment of birds, a black rat snake, a turtle, feral cats, cottontail rabbits ad lib., one aquatic mammal briefly (looked very much like an otter from what I could see, but might have been a muskrat), and a good dozen opossums. They’ve apparently dominated the small carnivore/omnivore niche around here. (It’s also worth noting that Family Didelphidae is the only mammalian family to survive from before the K-T event down to the present.)
Apparently marsupials mommies have two vaginas. This is confusing to poor birdmonster this morning. My God! Males have two dicks! Lesser evolved, my ass!
Pandas live in China.
True…but pandas are mammals, not marsupials.
I had a very young Opossum run into my Apartment one morning. It was chased by one of the dozens of Feral cats living there. Thankfully a combination of my Cat and me with a pillow chased it back out.
Strangest excuse I ever had for being late to work.
Jim
In fact from what I have read, they are more closely related to Racoons than Bears.
Bifurcated, think a tuning fork.
A guy can dream, can’t he? Dream of being reincarnated as a well-compensated kangaroo?
Pandas and marsupials are mammals. Marsupials are just a weird version of mammals.
I think there were some placental mammals in Australia but they died out. Tim Flannery’s The People Eaters is a great read and clears alot of this up. He used to work in the Australia Museum in Sydney with my old neighbour! (note to self: claims to fame are useless unless the people listening know who the ‘famous’ people are).
Not as wierd as monotremes (which are only found in Australia and New Guinea).
As I said above, there are many different kinds of rodents and bats in Australia, which have been there for a long time and show no signs of dying out.
