From insect mouthparts to to human/ape hands to bat/bird/pterosaur wings, the anterior limb appendages of various critters have rather frequently been diverted from locomotive purposes to other adaptations.
My question is, have there been any instances where formerly locomotory hindlimbs have been diverted to other uses while limbs further frontwards remain locomotory appendages?
I’m adding that clause to exclude things like boa/python claspers where the animal has other means of locomotion than limbs and retains the otherwise vestigial humerus for a specialized function. I’ll also rule out abdominal gills in arthropods on the basis that that’s not an adaptation but pure specialization of a previously multipurpose appendage, arthropod limbs apparently having been originally biramial, with both locomotory and gill functions on different branches. So a gill-leg combo specializing as either gill or leg is not the sort of “new” adaptation I’m looking for.
With those exclusions, are there any examples of rear-limb specialization anyone can come up with?
Insect wings are the first example that springs to mind. The wings were formerly legs, that became gills and are now wings, so the objection to the use of crustacean gills isn’t valid since the wings no longer fulfil either of the original roles. And before someone argues that wings have a locomotory function, in tens of thousands of insect species they do not, having become specialised as armour, display signals and a plethora of other uses.
The other obvious, and less contentious, example are the gonopods of millipedes. These are the 7th leg pair, and have become modified for sperm transfer during sex. They are never used for locomotion in most species, and many species actually keep them tucked up under the carapace to prevent damage. So that’s a clear case of a rear appendage being converted while the 6 anterior appendages are still used for walking.
Octopodes and squid often have a leg adapted for mating, but I’d say that doesn’t really count, since defining anterior/posterior legs is problematic, and the legs usually aren’t the primary mode of locomotion anyway.
Some skunks stand on their forelimbs so they can swing their rear end up over their head and spray in a forward direction. The rear legs are still used for locomotion though. In the early days of studying dinosaur fossils, some theorized that dinosaurs walked on their front legs because of the characteristic fossil configuration of neck bones and skull turned backward due to rigor mortis. More recent theories about future evolution contended that bats that evolved for terresterial life would walk on their forelimbs (most bats legs are useless for locomotion) with their hind limbs and clawed feet turned forward over their heads for weapons. All of that is fanciful though.
Zsophia has the best answer so far. The bats forelimbs have become wings, and the hind legs serve little purpose except for hanging upside down.
If this qualifies, then hummingbirds and swifts also fit the bill, since the hind limbs are too small and weak to function in locomotion. They are used only for perching and clinging. Neither swifts nor hummingbirds can walk using their hind limbs.
We had a cat once who could not only stand on her forelimbs, she could walk on them, with her butt balanced up in the air, and that wasn’t even an evolutionary change, just learned. But her rear legs she couldn’t use for much of anything at all (she had no bones in them, due to a birth defect), so she’s not an example for this thread.
There’s also the kangaroos, which can stand on their tails so as to fight with their hind limbs, but their hind limbs are still used for locomotion as well.
What about the Blue Crab, Callinectes sapidus? The hindmost pair of limbs have been modified for a different means of locomotion, changing from crawling claws to swimming paddles. As a bonus, the frontmost pair of legs are also highly modified into non-locomotive tools.
Pretty cool cat! I can’t recall the specifics, but I think there other animals that use the hand stand technique for various reasons. Besides the kangaroos use of their tails as an additional limb, there is one of the most remarkable adaptations of mammals, the prehensile tail of New World monkeys. Although some other mammals and marsupials have prehensile, or partially prehensile tails, I can’t think of another one that can use their tail as a fully functional limb the way some of the monkeys do. In the rest of the animal world, it seems only snakes have developed that level of control and utility.
Humming birds have managed to change their point of balance as well. Most maintain a vertical body position that concentrates the CG, enabling their ability to hover. This is similar to single screw helicopter design as opposed to the airplane-like design of most birds. While some birds can manage brief hovers without wind, only the hummingbird shows fine control and can even fly backwards.
Note: I forgot about this - geese are reputed to fly backwards over Pittston Pa.
Way off the OP, you could consider sponges and the like which have repurposed locomotory flagella into pumps for moving water through a structure.
In vertebrates, one might consider birds of prey (including owls): while the hind legs are indeed used for support and even moving very short distances, their most important use is grasping prey.
Since that list is so short, I’ll add vertebrates that use hind limbs for both locomotion and other uses: Horses, bulls, ostriches, (and kangaroos?) etc. use their hind limbs as defensive weapons. Cats (both domestic and many large) and sickle-clawed dinosaurs use(d) their hind limbs to disembowel prey.
Sort of, except Spunky had a more vertical posture when she walked: Her butt wasn’t back, like that sheep’s, but up. And her forelimbs were probably better-muscled than mine.
I dunno, prehensile tails are nice and all, but I’m still more impressed by the elephants and their prehensile noses. I mean, most mammals have at least a decent range of motion in their tails (enough to swat flies with them, or signal emotions), but how many can do anything at all worth mentioning with their nose?
Well, aside from Samantha, Tabitha, and some guy I saw on Letterman, the elephant’s nose is pretty remarkable. There are still things I’d rather do with a tail than my nose though.