Why do placentals win?
May 5, 2009 7:20 PM Subscribe
Why is it that placentals outcompete marsupials when they're in the same ecosystem?
Umpteen million years ago, South America was dominated by marsupials and North America by placentals. Then plate tectonics created a land bridge between them, and critters from both started crossing it.
North America got possums out of the deal. And placentals from North America moved south and replaced virtually all the marsupials who had lived in South America.
The only significant ecosystem in the world where marsupials dominate is Australia. Wikipedia says that the marsupials did defeat the placentals there, but it was apparently much longer ago than the N/S America war.
On the face of it, it doesn't seem obvious that placentals are superior to the marsupials evolved for the same niches. A kangaroo is the marsupial equivalent of a deer, for instance; and I see no reason why deer should have better survival rates. And though their reproductive systems are drastically different, it isn't obvious to me that one is notably superior to the other. Each has its tradeoffs, and there are ways in which the marsupial system is superior. (For instance, female kangaroos are always pregnant. In case of the marsupial equivalent of a miscarriage, the kangaroo has an embryo in reserve and can give birth to it in days.)
Why is it that marsupials lose out when forced to compete directly against placentals, with rare exceptions like possums?
Umpteen million years ago, South America was dominated by marsupials and North America by placentals. Then plate tectonics created a land bridge between them, and critters from both started crossing it.
North America got possums out of the deal. And placentals from North America moved south and replaced virtually all the marsupials who had lived in South America.
The only significant ecosystem in the world where marsupials dominate is Australia. Wikipedia says that the marsupials did defeat the placentals there, but it was apparently much longer ago than the N/S America war.
On the face of it, it doesn't seem obvious that placentals are superior to the marsupials evolved for the same niches. A kangaroo is the marsupial equivalent of a deer, for instance; and I see no reason why deer should have better survival rates. And though their reproductive systems are drastically different, it isn't obvious to me that one is notably superior to the other. Each has its tradeoffs, and there are ways in which the marsupial system is superior. (For instance, female kangaroos are always pregnant. In case of the marsupial equivalent of a miscarriage, the kangaroo has an embryo in reserve and can give birth to it in days.)
Why is it that marsupials lose out when forced to compete directly against placentals, with rare exceptions like possums?
It might not be just one reason since there are numerous differences, only placentals have a corpus callosum for example. You know various monotremes still exist too.
posted by jeffburdges at 7:41 PM on May 5, 2009
posted by jeffburdges at 7:41 PM on May 5, 2009
I remember reading that marsupials have much much smaller brains.
That this is usually a distinct disadvantage, but in the australian deserts where food can be scare, it can become an advantage to not need to sustain a large energy hungry brain.
Perhaps there are also subtle metabolic differences?
posted by compound eye at 7:43 PM on May 5, 2009
That this is usually a distinct disadvantage, but in the australian deserts where food can be scare, it can become an advantage to not need to sustain a large energy hungry brain.
Perhaps there are also subtle metabolic differences?
posted by compound eye at 7:43 PM on May 5, 2009
Best answer: Every time bioenergy is converted to edible matter and then eaten and digested, there is a huge energy loss. For a given mass of plant matter- by the time it winds up as blood glucose in a first order consumer - perhaps 80% of the energy the plant had used is gone. When that herbivore winds up in the blood of its predator, there has been a similar energy loss.
The production of marsupial milk is bioenergetically costly, as is the digestion of it within the fetus' belly. Conversely, a placental mother wastes no energy passing nutrients to baby. All other things being exactly equal, placentals will displace marsupials in short order as has been demonstrated by all placental imports to Australia conducted by people.
YIFIAABE (yes, in fact I am a behavioral ecologist).
posted by fydfyd at 8:06 PM on May 5, 2009 [13 favorites]
The production of marsupial milk is bioenergetically costly, as is the digestion of it within the fetus' belly. Conversely, a placental mother wastes no energy passing nutrients to baby. All other things being exactly equal, placentals will displace marsupials in short order as has been demonstrated by all placental imports to Australia conducted by people.
YIFIAABE (yes, in fact I am a behavioral ecologist).
posted by fydfyd at 8:06 PM on May 5, 2009 [13 favorites]
There is no such thing as a superior species. Nor are animals from either group currently competing in many niches. We do not know what would happen if large numbers of animals from each group were to compete with one another.
What we can say is that millions of years ago, large numbers of marsupials existed in South America and few placental mammals. Then the numbers then flipped. Why? We do not know why. We can infer that adaptive radiatation occured and placentals may have taken over niches formerly dominated by placentals.
posted by Ironmouth at 8:28 PM on May 5, 2009
What we can say is that millions of years ago, large numbers of marsupials existed in South America and few placental mammals. Then the numbers then flipped. Why? We do not know why. We can infer that adaptive radiatation occured and placentals may have taken over niches formerly dominated by placentals.
posted by Ironmouth at 8:28 PM on May 5, 2009
Response by poster: But I thought that displacement happened with the formation of the Panama bridge.
posted by Chocolate Pickle at 8:32 PM on May 5, 2009
posted by Chocolate Pickle at 8:32 PM on May 5, 2009
Best answer: Ironmouth, while Chocolate Pickle did indeed use the term "superior", the original phrasing was "outcompete" - and it would be hard to argue that placental mammals didn't outcompete marsupials on 6/7 continents.
We do not know what would happen if large numbers of animals from each group were to compete with one another.
We do. It was during the Cenozoic/Pleistocene changeover. Marsupials were displaced from almost all ecological niches by placentals, everywhere they coexisted.
posted by IAmBroom at 9:18 PM on May 5, 2009
We do not know what would happen if large numbers of animals from each group were to compete with one another.
We do. It was during the Cenozoic/Pleistocene changeover. Marsupials were displaced from almost all ecological niches by placentals, everywhere they coexisted.
posted by IAmBroom at 9:18 PM on May 5, 2009
There is no such thing as a superior species.
What?
Then what is the point of evolution?
posted by The Light Fantastic at 10:20 PM on May 5, 2009
What?
Then what is the point of evolution?
posted by The Light Fantastic at 10:20 PM on May 5, 2009
Response by poster: Evolution doesn't have any point; it just happens.
I used the word "superior" in the restricted sense of "an evolutionary winner" and I meant nothing else by it. A "superior" species is one which can replace an "inferior" species if they compete in the same ecology. "Superior" species are winners. (And of course, a million years from now a new gun may come riding into town who's an even faster draw.)
Don't make too much of my usage of that word, please.
posted by Chocolate Pickle at 11:36 PM on May 5, 2009
I used the word "superior" in the restricted sense of "an evolutionary winner" and I meant nothing else by it. A "superior" species is one which can replace an "inferior" species if they compete in the same ecology. "Superior" species are winners. (And of course, a million years from now a new gun may come riding into town who's an even faster draw.)
Don't make too much of my usage of that word, please.
posted by Chocolate Pickle at 11:36 PM on May 5, 2009
There are no evolutionary winners either. Every species becomes extinct. I think what is trying to be said here is that why did placental mammals largely displace marsupials in many niches on many continents?
posted by Ironmouth at 11:40 PM on December 10, 2009
posted by Ironmouth at 11:40 PM on December 10, 2009
This thread is closed to new comments.
One possibility that suggests itself to me is that the use of milk isn't as efficient as a placenta at nourishing the baby, so placental babies can go to full birthsize faster than equivalent marsupials. Is that true, and is it enough of a difference to explain the competitive advantage?
posted by Chocolate Pickle at 7:26 PM on May 5, 2009