Why were marsupials (with the obvious exception of the opossum) out-competed by placental mammals everywhere but austrailia? The ease of giving birth they seem to enjoy looks like a rather large advantage.
But I think Steven Jay Gould addressed this in one of his essay compilations.
Source: http://www.australianwildlife.com.au/features/marsupials.htm
Perhaps the later isolation of the Australian continent, coupled with a lack of predators found everywhere else, caused marsupials to become dominant.
One way to look at it may be to ask the question whether, at the point in their development when their weight is 1% of an adult’s, offspring fare better in a uterus or in a pouch. Nature seems to have given the nod to the former.
Perhaps someone knows a site containing it? I would be ever so appreciative.
Also …
The question itself is somewhat flawed. Placentals haven’t dominated everywhere but Australia.
Marsupials and placental diverged from a common ancestor. By the time they were ready to take over the world, the continents were diverging into several major blocks. In one of the continents (Australia) marsupials outcompeted the placentals, in another (Euramerica) placentals outdid the marsupials. In a third (South America) neither side gained the upper hand. In a game of survival these are the only three options. Given that we have a neat 3 way split it seems like neither group has an inherent advantage. Whatever allowed primarily marsupials to hang on in Gondwana and placentals in Euramerica and Africa/India may have been climatic or it may have been essentially chance. We can’t really know.
The scenario was played out again millions of year later when South and North America collided. However this was complicated by the fact that it wasn’t just marsupials vs placentals. It was native marsupials and placentals vs the exotic hordes. On that continent the marsuoials and placentals had co-existed in a stalemate for many millions of years, and there was a diverse placental and marsupial fauna.
When the Panama land bridge formed either the locals or the exotics could have triumphed… As it turns out the exotics dominated. At least in part this was due to the sheer amount of evolutionary pressure the placentals that arrived in South Am had faced. Having been selected in the competition from the best lifeforms that 4 four continents had to offer. These creatures were survivors.
In addition to being the pick of the crop from 4 major landmasses, they were also weeds. By that I mean that they had undoubtedly been culled back numerous times through contact with other species, as well as major climate changes including ice ages. These processes must have selected for species that were generalist, as well as being able to respond rapidly to favourable conditions.
In contrast the South Am mammals had been largely isolated since they began their radiation. This had led to a number of highly specialised groups of animals. These sorts of creatures have a large advantage in the environment in which the evolved. However the invasion of South Am by the North Am placentals occurred at a time of climate change. In addition the sheer number of new species of flora and fauna would have produced an changed ecosystem. Specialists don’t fare well in the conditions.
Had all things remained constant the new placentals would probably never have got a foothold. Primates for example had arrived in South Am after the mammal fauna had pretty much settled down. Although their adaptability gave them the potential to spread into numerous niches As they have in Africa, they radiated very little from their original form and lifestyle. Presumably this was because they arrived at a point in time when the system was not under any pressure and they couldn’t get a significant foothold against the old established species.
The fact that huge numbers of native mammals vanished in the face of exotic competition in South America is very telling. The marsupials weren’t inherently less fit. Rather it was the whole suite of local species that was unfit.
In contrast thousands of species of placental mammal have been introduced to Australia, many of them with considerable effort. Despite this only a couple of dozen havestablished, and the loss of endemic mammal life has been fairly trivial in the greater scheme of things. This despite system altered radically by humans in the last 40, 000 years or so. It seems that in Australia placentals have lost the battle.
Marsupials don’t have any inherent disadvantages. Small marsupials breed somewhat slower than comparable placentals, but in contrast large marsupials breed notably quicker than placentals. A female with a juvenile in a pouch has a slight mobility advantage over a placental. A placental mammal has a better system for gearing up the neonate’s immune system. While a juvenile may have a higher risk of injury on the journey to the pouch the small size and underdeveloped placenta also mean that the chance of the mother dying during pregnancy or delivery is almost nil.
In general there is no particular reason why placentals won out in most areas. With two groups that are so similar morphologically and behaviourally competition is inevitable. One tactic or the other had to dominate. As with so many of these things once one group began to dominate then it was going to continue doing so simply because it could draw on the variety of such a huge part of the planet. The initial division of the world gave the placentals dominance over the greater land surface area. You could almost call it a random throw of the dice. That may have been all that was required to ensure greater diversity and competition amongst the placentals. And that’s all that was required for dominance in South America.
Wow, good answer.