Two questions on evolution

  1. What was the evolutionary process that created the sloth? What benefit does the thing have within the food chain/ecology? Does it have any natural enemies or are they, for some reason, slower than it? It can hide in a tree and as an armor of fur. How in the world did this creature come about and why is it still around today?

  2. How come no creatures ever developed with both gills and lungs? I’ve heard of some fish that can survive on land for a few hours at a time and there are other creatures that don’t necessarily die when submerged. But I’ve never heard about any creature that has both sets of breathing aparatuses and I just think this would be a natural evolution to at least one, if not more, species. First person to mention Aquaman gets slapped across the face with a large trout.

Well here are a couple of musings:

Sloths: A lot of animals don’t rely on speed for protection. Apparently it can be sufficient just to stay up in trees and lie low, so to speak. Not too many large predators can get way up into a tree.

Gills and lungs. Generally speaking body parts have to evolve from something else, they don’t just appear. I’m sure birds would think that it would be convenient to have wings and arms, but it doesn’t work that way. Lungs presumably evolved from gills in some fashion.

  1. Sloths are arboreal, which means they can usually stay out of harm’s way. The main predators of the three-toed sloth are jaguars, anacondas and Harpy Eagles; however, because of their camouflage and periods of activity, they can usually manage to remain undetected, or at the least, unmolested. If pressed, they can take a swipe with their claws, which can leave a deep, nasty wound if the attacker isn’t careful. Moving slowly has its advantages as well, since many predators are attracted to motion.
    As for why the sloth came to be at all…well, evolutionarily speaking, it seemed like a good idea at the time - which is to say, a niche existed, and sloths filled it. The fact that they are still around shows that whatever we may think of them, they have a viable survival strategy.

  2. There are, in fact, more than 20 genera of bony fish which are considered “habitual air breathers”, many of which retain gills to rid the body of CO[sub]2[/sub]. Such animals tend to live in warm, swampy areas, where it is often impossible to meet oxygen requirements through gills alone (cold, moving water retains oxygen better than warm or stagnant water). The lungfish Neoceratodus forsteri is but one example.
    [after preview notes]
    Cher3 - it is generally thought that lungs evolved from the gut, rather than from gills.

Thanks, Darwin’s Finch. (I actually typed Darwin’s Fish first). That’s interesting.

Just to expand slightly on Darwin’s Finch’s comments:

Lungs appear in the late Silurian and Devonian periods , when geological evidence suggests that the earth’s climate was similar to what you find in some tropical countries today, i.e. well-defined alternating dry and wet seasons. Hence the selection for “air-breathing” fish as DF mentioned above. Structures such as the swim bladders found in many ( most ) bony fish almost certainly derived originally from lungs.

Re: developmental origin of lungs - “In…lungfish and amphibians, the lungs develop embryonically as a ventral evagination from the floor of the pharynx, just caudal to the last pair of pharyngeal pouches…The primordia of lungs appear to resemble a pair of displaced pharyngeal pouches in some amphibian larvae, and this has led some researchers to suggest a homology.

Re:Sloths - Kind of restating DF, but - Slow, cryptic animals are not uncommon in nature ( though obviously less so among mammals ). If you’re already good at hiding and don’t need speed to hunt down prey, why bother with the energy demands of being active more than you need to be to get along? Also, specialized muscle groups that are dandy for climbing and clinging in one spot for hours at a time, may not lend themselves to rapid movement. So you end up with a trade-off.

  • Tamerlane

Re: sloths. Another reiteration I guess, but I remember nature program on sloths which said that they moved slowly precisely because the harpy eagle relies on movement to detect predators.

It was a nasty but amazing show - showed the eagle picking up a sloth (who hadn’t moved slowly enough, presumably) by it’s head and flying away with it.

Gills and lungs are both complex structures and could only caome baout if their was a demand fro both. The first vertebrates had functional gills, which was all they needed for life in the water. The advantage of being able to utilise a vacant niche on the land ‘allowed’ for the evolution of lungs. There would have been numerous intervening stages that had sets of both with various degrees of functionality. IIRC some salamanders still retain vestigial gills when adults and I believe the axotl has partially functional lungs as well as gills.
Once this process was over land creatures then had functional lungs. Gills had little advantage and so the ‘use it or lose it’ principal came into play.
Land creatures that have returned to the water have functional lungs and the efficient repiratory/circualtory system necessary to cope with life on the land and don’t have a pressing need for gills to exploit their niche. Despite this marine turtles and terrapins have developed a fully functional gill substitute. They move water into and out of their highly vascularised cloaca, and by so doing can remain underwater for indefinite periods of time. So there’s one creature that genuinely does have lungs and gills. FWIW many fish, including eels, can remain out of water for more than a few hours. Provided they are kept moist enough for them to breathe through their skin and don’t overheat or get sunburn they can apparently breathe air indefinitely. Of course has has already been noted numerous fish gulp air and can live happily in completely de-oxygenated water.

Some reptiles breathe through their cloaca? That is WAY more information than I needed to know. Way more. Yuck.

More fun with lungs - And of course, there is the fascinating salamander family, the Plethodontidae, or <i>Lungless Salamaders</i>, that have secondarily lost their lungs. This, despite the fact they are mostly terrestrial. They’re certainly all direct developers, with no aquatic larval stage as in some other groups. It’s also the single largest family of salamanders, with a center of distribution in the Southeastern U.S. . They breath through their moist, highly vascularized skin.

  • Tamerlane

Correction - I meant to say, “…center of diversity…”

  • Tamerlane