Quite a few issues ago, I speculated on what it would have taken to produce a marsupial-dominated world. In that essay, I said that the main factor in why marsupials lost out in our world was not necessarily some innate inferiority, but simply the size of the continents that they dominated when mammals gained an opportunity to become a major factor in the animal equation. I’m going to expand on that idea a little by showing why larger continents tend to generate more successful animals. This has some implications in terms of what is possible in populating products of alternate geography, as well as implications for some types of alternate history.

Why do animals from larger continents tend to be more competitive?

1) Relatively niche-independent structural advantages.  A group of animals may develop structures or behaviors that give them an advantage in many environments--better eyesight, more efficient gait, better predator or anti-predator behavior.  Those structures or behaviors then allow them to colonize a variety of niches.  For example, the sheep/goat/deer group of species have structures in their legs that make them very efficient runners.  That gives them an advantage in a wide range of habitats.  How does that relate to smaller versus larger continents?  Other things being roughly equal, the larger continent is more likely to produce more of those niche-independent structural advantages.  Oversimplifying a lot, it's like two people playing poker, but having one person getting to choose which of two or three hands to play.  The person with more hands to play won't win all the time, but they'll win more than they'll lose.

2) Niche or environment-specific advantages: Let's say that an animal is well-adapted to a relatively minor environment or niche on a continent.  That environment or niche then spreads to cover most of a continent.  New groups usually do colonize the expanding environment, but the ones that have been in that environment for a longer time will probably tend to expand with it.  For example, let's say that dry open grassland environments existed in Australia in the early Miocene but  were a minor part of the environment.  They then expanded to cover most of the continent.  The animals that were originally adapted to that minor part of the Australian environment expand with that environment.  Maybe dasyurids and certain types of kangaroos become a much more important part of the environment.  How does that relate to smaller versus larger continents?  The larger continent is more likely to already have a significant area of whatever environment emerges to become important, and its animals have a head start on adapting to that environment. 

For example, parts of northern Australia and New Guinea are now tropical rainforests.  How long have there been tropical as opposed to temperate rainforests in the Australia/New Guinea area?  I don't know for sure, but I would guess that tropical rainforests didn't develop in northern Australia and New Guinea until the late Miocene or early Pliocene.   How long has there been tropical rainforests in southern Asia?  I'm guessing that it stretches back to the Cretaceous or very close to it.

If New Guinea/Australia connected to Southeast Asia in the geologically near future, chances are very good that Southeast Asia's monkeys, lorises, and tarsiers would simply overrun the tree kangaroos, ringtails, and cuscuses of New Guinea and northern Australia.  They had roughly a 50 million year head start in adapting to that environment.

That brings up an interesting issue, by the way: why is the niche for a frugivore/omnivore like a capuchin monkey or a rhesus apparently empty in New Guinea and northern Australia?  The ringtails, cuscuses, and tree kangaroos are apparently all primarily leaf-eaters.  Have any of them developed in a frugivorous monkey-like direction?  Not that I can tell from a fairly intensive amount of reading on their ecology.  Are there extinct species that developed in that direction?  Based on the example of Madagascar, diurnal frugivores appear to be more vulnerable to extinction when humans arrive than nocturnal leaf-eaters.  A close look at fossil possums might detect some that were headed in the diurnal frugivore direction.  On the other hand, the niche may have covered by something else--birds or bats.  There might not have been time for any major changes along those lines to develop.

3) Predator-related advantages: Fair warning: this is going to involve some major over-simplifications. I’ll point them out later.

There should theoretically be two predator-related advantages for a larger land-mass over a smaller one. First, the maximum size of predators on larger landmasses should be higher. Large predators require huge territories. They also require a minimum population size in order to maintain a viable population. That minimum population has to be large enough to maintain genetic diversity even after a series of climate disruptions. Obviously this assumes that the continents are similar in productivity per amount of land area.

For example, to drastically oversimplify the situation, let’s use some educated wild guesses to get a general idea of how this would work. Let’s say that a predator of about 100 pounds needs a home range of around 100 square miles on average. Australia has an area of roughly 3 million square miles. If a predator could operate in approximately two-thirds of that territory, that predator could on average have a population of 20,000 animals. An ecologically equivalent predator in Eurasia might be able to have a population as high as 140,000.

What is the minimum population necessary to maintain a genetically healthy population? I would guess that if a population went through a bottleneck where the population was much smaller than a thousand animals, that population would suffer some problems, most importantly with susceptibility to disease until genetic diversity was reestablished. A lot depends on how long the population stayed at that low level. I don’t have any empirical evidence that a thousand is the magic number. It could be as low as a couple of hundred or as high as five thousand.

How large of a population would it take to avoid bottlenecks of that nature? Assuming a minimum population of a thousand, then I would guess that the carrying capacity of an environment would have to be on average at least ten times that high, and the figure is more likely to be 20 times that. In order to survive in the long term, a species needs to be able to survive a once-in-several-million-years combination of unfavorable events with their genetic diversity reasonably intact. They have to be able to weather the worst possible combination of the first onset of one or more new diseases, drought, volcano-induced temperature fluctuations, die-offs in prey species, large-scale fires, and flooding. They have to do all of that without losing too much genetic diversity.

If those educated wild guesses are reasonably correct, a one-hundred pound predator should be able to survive over a long period in Australia. A population of twenty thousand animals is enough that even if 95% of the population died out there should still be sufficient diversity to allow the species to continue. Would a 200 or 300 pound animal be able to survive long-term? If all of the assumptions in my little analysis were true, then they probably couldn’t.

Obviously, none of this should be taken too seriously as an absolute limit to size of a mammal predator in Australia. There is a lot of uncertainty in all of those numbers. I would be very surprised if they are off by a factor of less than two. Also, pound for pound a larger predator will use somewhat less energy than a smaller one, so for example a 200 pound predator would probably not require twice the range of a 100 pound one. The point is that there is an upper limit to a predator’s size based on the size of the continent. That limit may be 100 pounds. It may be 350 pounds, but there is a limit.

There are two other implications of that analysis. First, a smaller continent is more likely to lose all of its predators at or above 100 pounds than a larger continent is. That means that the larger predators on the smaller continent are likely to have spent less time at their current size range than those of the larger continent. That means that they are likely to have had less time to develop the techniques specific to being a predator of that size compared to a predator from a larger continent. Also, there is room for fewer types of predators at a given size range on the smaller continent. Eurasia might be able to support four or five predators in a given size range for every one that Australia can support, and have a larger population of each of them than the one Australian species.

What does all of this mean in terms of which species wins when a Eurasian and Australian predator competes? Well, the larger Australian predators will be competing for prey and for carcasses with predators larger than themselves. Those predators will be more specialized, because their larger continent allows them to be. Australian predators can’t specialize to the same extent because they have to cover a larger niche in order to maintain a large enough population. The predators from Eurasia will have adaptations for competing for carcasses that the Australian predators have not had to develop. In Eurasia and Africa, animals like lions and hyenas compete fiercely for carcasses. A Thylacoleo (Marsupial lion) might have to compete with another member of its species for carcasses, but unlike Eurasian predators Thylacoleo might not be adapted to competing with other species in its size range for possession of kills.

The Australian predator would also face another problem. As Eurasian predators moved into an area they would probably depress prey numbers. That is one of the implications of their ability to be more specialized. Each of them is going to be better at a subset of predatory techniques than the Australian competition. The Australian environment simply doesn’t have room for those specialized predators in the long run. As prey gets scarce, competition over kills gets fiercer. That reinforces the importance of adaptations for competing over kills.

Having faced smaller and less specialized predators has implications for prey species too.  For example, there is a point at which an animal is too large to be attacked as an adult under normal circumstances. That point would be lower in Australia than in Eurasia because of Australia’s smaller predators. For example, adults of the larger Diprotodonts might be too large for any Australian predator to attack. They probably wouldn’t be too large for some of the larger tiger species or predatory bears to tackle. If a Diprotodont had to deal with a tiger, it would probably find that standing and fighting the tiger wouldn’t work. At the same time, it wouldn’t be adapted to running away from the tiger because it hasn’t had to run away as an adult. The species would have to adapt quickly or die off.

More specialized predators would also be a problem for their prey. A kangaroo species that usually is able to outrun its predators suddenly faces a predator like a dingo that has been able to specialize in running down fast-moving prey far more than any Australian predator could afford to. Kangaroos get scarce until they develop mechanisms to defeat the new predators. That depresses the number of Australian predators far more than it does those of the Eurasian ones.

That’s all kind of disappointing, isn’t it? You can’t just put a little island some place and expect it to have sabertooth tigers surviving on it—not for long anyway. At the same time it is possible to have some pretty large, capable predators on a continent the size of Australia. That’s becoming obvious as the fossil record for Australia gets better.

For a long time there were only a few scattered marsupial fossils representing the entire time before the Pleistocene. In the last ten to fifteen year Australian paleontologists have uncovered a pretty good fossil record extending back to the Miocene and to some extent the Oligocene. That includes a lot of predators, including several big ones and a lot of medium-sized ones. Stephen Wroe writes in the Australian Journal of Zoology that fossils of 16 Miocene species of marsupial carnivores 2.5 kg or over (roughly cat-sized and larger) have been found so far. The largest Australian marsupial predator found so far is the Marsupial Lion, which may have weighed as much as 160 kg (around 350 pounds).

I said at the beginning of this section that what I was going to present was an oversimplification. It is for several reasons.

• All square miles aren’t created equal. A square mile of sand dunes isn’t going to support the same mass of prey that a rich savannah is. Surprisingly, heavily forested areas don’t support large masses of mammals. Too much of the potential food is locked away in inedible tree trunks and roots. If you want large predators in a relatively small area, make that area mostly savannah.
• All predators aren’t equal. A relatively small difference in metabolic rate or locomotor efficiency can make a major difference in the carrying capacity of an area. Komodo Dragons can live on small islands partly because they have a metabolic rate about one-tenth that of a mammal in their size range.
• All ecologies don’t function the same way. There is always some degree of potential competition between animals of different sizes. Does the man eat the sunflower seeds or does the flock of birds? Different ecologies have different relationships between large and small animals. An ecology where large plant-eaters are more dominant than normal should be able to support more large carnivores.
• Some continents are more isolated than others. That has a couple of impacts on carnivore size. On the one hand, an isolated continent can’t replenish its supply of carnivores the next time a land bridge opens up. On the other hand, all other things being equal the more isolated continent would suffer fewer periods of extreme ecological instability. Times when continents exchange animals should be times of extreme instability, with animals forced to deal with new predators and a wide range of new diseases. That could force the maximum size of predators in occasionally interconnected continents down to some extent.
• Omnivores can get bigger than strict carnivores, but they can still be fierce predators. Bears come to mind. If you have to have something big and fierce on a relatively small land area, make it an omnivore.

4) Smaller continents are more vulnerable to catastrophes. It is becoming increasingly clear that the world can be a very dangerous place for its animals even without human intervention. Large volcano blow-ups have at times covered hundreds of thousands of miles and devastated even more. We keep finding out about more and more devastating asteroid strikes, though none of them during the age of mammals has been devastating enough to essentially empty continents.

How does that relate to the issue of larger versus smaller continents? Well, let’s say that an asteroid hits just off the western coast of India. It devastates half a million square miles almost completely. It has very nasty effects on a million and a half more square miles and lesser effects on a larger area radiating out from the center of the strike. It probably also has some world-wide effects. How much difference would that make to the long-term development of Eurasian mammals? Probably very little. A lot of individual animals would die, and probably a few species. Animals from the rest of Eurasia would flood in and replace the dead animals and the extinct species. Eurasian animals would probably continue to develop without much long-term change of direction.

Now put the same asteroid strike off the east coast of Australia. Whole eco-systems would be wiped out, along with whole categories of animals.

5) Structural cul-de-sacs: Sometimes a structure can initially be an advantage for a species or group of species, then eventually make it difficult for that group to compete. In Eurasia, Africa, and North America there have been several waves of animals in the carnivore slots. Creodonts were initially dominant, then gave way to a succession of dominant predators more closely related to modern mammal carnivores. The point is that the larger continents had a wide range of competing groups. If one group found itself at a dead end, there were plenty of other groups of mammal carnivores to replace them. Australia could not support that kind of diversity. To a lesser extent, neither could South America.

What kind of cul-de-sac might the Australian carnivorous marsupials have gone down? This is extremely speculative, but they may have found themselves inhibited by their lack of an equivalent to our corpus callosum(sp)—the main pathway between the hemispheres of the brain. Placental mammals have one. Diprotodont marsupials have an analogous structure. Marsupial carnivores apparently have no equivalent structure. That could explain why at least two groups from the primarily plant-eating diprotodonts were able to take to a carnivorous life-style. Again, I want to emphasize that this is extremely speculative, but it does illustrate the kind s of problems that might arise due to the limited range of candidates for an ecological niche in a smaller continent.

How important are these factors? Well, they don’t mean that animals from the smaller continents are incapable of competing in the larger world. Usually some are quite competitive. When North and South America met a lot of animals headed south, but a few headed north too. Ground Sloths got as far north as Alaska before they died off along with most of the rest of the large mammals of the New World. Opossums seem quite capable of dealing with the competition, as do armadillos. If I had to pick winners from Australia I’d probably point to some of the kangaroos and possibly some of the smaller “marsupial mice”. They seem quite competitive.

Which is the most important of these factors? We have an interesting natural laboratory to see which of the first two of those factors is most important. When North and South America joined, North America had the edge in the niche-independent structural advantages category. It is a larger continent and it had exchanged animals with the even larger continents of Eurasia and Africa. On the other hand, South America had one of the largest rainforest areas of the world, while North America had a relatively small rainforest area in Central America, and that area may not have remained rainforest throughout the age of mammals. That would give South American rainforest mammals a major edge in the niche or environment-specific advantages area.

When North and South American mammals compete in rain forest environments, which set of mammals wins? In many cases that is still to be determined, but in general it looks like the North American animals have an edge so far. For example, marmosets and tropical squirrels have very similar niches. They seem to be competing rather vigorously, with individual trees being divided—one side with marmosets, the other with squirrels. The competition is taking place in South America though, with a number of squirrel species colonizing South America but no marmoset species pushing into Central America. Owl monkeys and kinkajous (a distant relative of raccoons with a prehensile tail) share a very similar niche, and even respond to each other’s territorial calls. Kinkajous are widespread in South America. Owl monkeys have not moved into Central America.

There don’t appear to be any North American mammals directly competing with the more arboreal opossums, and opossums have spread north into Central America and even North America. There also don’t appear to be any North American ecological equivalents to the typical South American monkeys, though coatis (another raccoon relative) sometimes appear to want to compete in that niche. Capuchin monkeys tend to chase coatis out of trees when they find them there. Spider monkeys and to some extent capuchin monkeys have spread to Central America.

Overall, it looks like the initial advantage goes to the North American mammals and to the niche-independent structural advantages. On the other hand the two groups of animals have only been competing for what in geological terms is a very short time. The South American mammals are probably still adapting to the new set of predators from North America. They may move north more effectively once they’ve adapted more thoroughly.

A side note: I’ve often wondered if the absence of ground-living New World Monkeys might be partly due to the influx of North America predators when the two continents were joined. A new and apparently more effective set of predators might well have killed off some species of ground-living monkeys and forced others to retreat to the trees. On the other hand, there is limited evidence of mammal predators attacking New World monkeys.

To sum all of this up: marsupial and placental mammals were closely related animals at approximately the same level of development when the dinosaurs disappeared. Marsupials dominated some continents. Placental mammals dominated others. The gap in competitiveness that we currently see between the two groups is explainable by differences in the size of the continents initially dominated by the two groups. There may or may not be inherent differences in the capabilities of the two groups, but there is no need to posit such differences to explain the current competitive balance. This all means that you have to be careful when you populate alternate geography continents and islands with their animals.