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This feature is not unique to mammals, because it also evolved in dinosaurs, and in some prehistoric crocodiles - which, rather alarmingly, could run rapidly across dry land. Today, of course, it is also found in birds. Nonetheless, it is a key feature of mammals, and the starting point for a whole series of adaptations in mammalian limb structure. From this beginning, mammalian limbs have taken on a wide range of different forms, adapted to all kinds of different lifestyles.
One example would be the development of hooves. The stance of hoofed animals, standing on the very tips of their toes, is thought to give the limb greater flexibility and allow them to run faster. Hooves have therefore evolved in herbivores, to, at least in part, allow them to escape from predators. Given the speed at which, for example, a cheetah moves, hooves clearly aren't essential for rapid running, but they do seem to help, and one might wonder why predators - as keen to catch prey as the prey is not to be caught - never evolved them. The answer, most likely, is that claws are just too useful to carnivores, which use them as weapons to bring down their prey; something that wouldn't work with a hoof.
With the great variation of mammals that exist, its quite obvious that the proportions and shapes of limbs vary considerably between them. Still, there are some limits to this variation if you want to be able to walk or run effectively, and a couple of years ago, Manuela Schmidt and Martin Fischer of Jena University conducted a study comparing the limbs of a wide range of mammalian species to look at those limitations. One of their conclusions was that the hind limbs of mammals vary far less than the fore limbs.
This, they suggest, is because the hind limbs are the main source of propulsion, important for pushing off from the ground as the animal walks or runs. Fore limbs, on the other hand, can be modified for a range of different functions, such as digging, grasping objects, and so on. In some mammals, such as humans, the fore limbs aren't used for walking at all (at least after the first year), but they still need the hind limbs to do that. Bats are another extreme example, with their arms turned into wings, but still needing their hind limbs as... well, legs.
No great surprise there, then; front limbs vary more among mammals than hind limbs do. Sure, hind limbs do vary - consider rabbits, gazelles, badgers, and monkeys, for example - but not as much as the fore limbs. But it seems that the story may not be quite the same when it comes to marsupials.
Now, marsupials only represent around 6% of mammal species, but they have had a whole continent to themselves (not to mention sharing the Americas), and have evolved to fit a wide range of lifestyles and habitats within it. Many of them closely resemble placentals in similar environments. Even the most distinctive marsupials, the kangaroos, aren't a million miles away from a rabbit in shape, if not in size. But there are some body forms they just haven't evolved.
For one thing, there are no marsupial dolphins, which rules out what are arguably the most extreme limb adaptations of any mammals. Given that the pouch would fill with water, that's scarcely surprising. Indeed, while there are some semi-aquatic marsupials, water in general is something they've not been too effective at exploiting. But it's more than just that; there are no marsupial antelope, either, or bats. Some marsupials can glide, but all of the flying mammals in Australia are placental bats that made the trip across from south-east Asia. Did they just never get the chance to evolve wings, or is there more to it than that?
Kelly McKenna and Karen Sears of the University of Illinois thought that there might be, so they conducted a new survey of limb shapes among mammals. Like the German study, they examined the variation in the width and length of the various segments of the limbs - shoulders/hips, upper limbs, and lower limbs - but, this time, they specifically compared marsupials to placental mammals. Their first finding was that placental limbs are, indeed, more variable than those of marsupials. Given that they were including dolphins, seals, and bats in the survey, that's not exactly surprising news.
What's more interesting is that they also compared marsupials and placentals that shared a similar method of locomotion. Animals that walk on all fours through the trees, for example, such as squirrels and possums, might be expected to be fairly similar between the two groups. Yet even here, in most cases, the marsupials varied less than the mammals. The one exception was when it came to animals that hop along on their hind legs, where kangaroos and wallabies do vary more than, say, rabbits and jumping mice. But even then, while the placentals they looked at did not vary very much, they were more specialised. That is, while their limb shapes were all pretty much the same, they were less like those of the "average" mammal than those of kangaroos and wallabies.
More than that, it seemed that when marsupials did have more specialised limbs, they were more likely to be the hind limbs than the front ones. Again, think of kangaroos. That's the opposite of what happens in other mammals, and it didn't show up in the German study for the simple reason that the great majority of mammals aren't marsupials. It's not just that marsupials never have wings, in general, their fore limbs are less varied than those of placentals... but, if their fore limbs are just as free from the need to be the main source of propulsion as they are in placentals, why is that?
The answer, as one might expect, is almost certainly to do with reproduction. A marsupial is born as a tiny, undeveloped baby, little more than an embryo, and must crawl through its mother's fur to reach the pouch (or at least, the teats) where it can finish its development. To do that, it uses its front limbs to pull itself along, so it is they, at this crucial stage of life, that are the important ones. That means that the baby has to be born with front limbs in, more or less, the same shape, which limits what the can eventually change into as it grows. That is why marsupials never have wings; it would be harder for a winged infant to crawl into the pouch than for one with grasping claws. Hooves, presumably, would be even worse.
It surely wouldn't be easy to fly about with a baby in your pouch, but if a winged marsupial couldn't even be born in the first place, you'd never get the chance to try.
[Picture from Wikimedia Commons]