Submitted by [deleted] t3_yy12a8 in askscience
Before people mention this, I know that crocs and gators are quite distantly related despite the physical similarities. But still, one would imagine that given the major physical similarities (due to similar pressures, I assume), and the tail being equally important to both crocs and gators, the pressures that caused the alligator to develop/retain its tail regrowth trait would also apply to crocodiles. So why can't they do it?
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Given that the discovery that alligators can do this is relatively recent, I wouldn't expect there to necessarily be any confident answers, but we can obviously make guesses.
It's apparent why alligators might need to develop some kind of defense against predators. They're born small and vulnerable, and while the adults have no natural predators, the eggs and young have several. Several survival traits are shared between alligators, caimans, and crocodiles, such as large clutch sizes and protection from the parent for the first year or two.
But, a specific adaptation would depend on what the animals chief predators are and a good amount of chance for what mutations happened to occur in the population. Since alligators and crocodiles largely live in different parts of the world, it's conceivable that alligators had predators that were more likely to damage or destroy only the tail, so it was advantageous to be able to regrow it (I don't know enough to speak to what difference in predators they might have).
That's an interesting possibility, but the only issue is we don't have any concrete evidence on how or why predatory pressures are different on young alligators compared to young crocodiles (though I realise everything we are discussing here is conjecture anyway). According to what we know today, both alligators and crocodiles are vulnerable to predators as juveniles.
I have my own hypothesis, which I've added in a comment under this post just now. Lmk if you have any thoughts.
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OP here attempting to answer my own question, lmk what you think if you want to.
It is my understanding that alligatoridae (or at least the direct ancestors of the two extant species of alligators that survive today) evolved, on average, in considerably colder climates than the ancestors of crocodilidae (the oldest alligatorid fossil currently is from Alberta, Canada). It's a known fact that alligators can take significantly colder temperatures than crocodiles, including but not restricted to the fact that alligators are able to survive short periods of living in frozen swamps in the peak of winter, by sticking their snouts above the ice and going into a sort of inactivity. Crocodiles can't do anything close to that. Since "over-wintering" is a thing for alligators, where they go into longish periods of inactivity/brumation, I would assume that the tail would provide a vital reserve of fat etc to burn through while they're in that state. For a larger, robust animal, it might not make enough of a difference to persist evolutionarily, but for a juvenile with a small body, that extra reserve of fat might be (or might have been in the recent past) the make or break factor that decides whether it survives a period of inactivity or not.
Of course, regrowing a tail even partially is resource-exhaustive in itself, but if they can time it right (I mean reptile metabolism slows down in the cold so if they're regrowing it they're probably doing the bulk of it in warmer weather anyway), there are probably some benefits to be had that outweigh the losses.
The fact that the regrowth is only partial and imperfect might point to the fact the benefits were considerably more relevant at some point in the recent past (ice ages?) than they are now, whereas those same benefits probably haven't been relevant at all for a long time for crocodiles, which have mostly persisted in warmer climates throughout the recent past.
Would love to hear feedback/thoughts on this, or if anyone has anything to add.
Unfortunately, the regrown tail portion is all collagen and cartilage. There's no fat or muscle stores for it to provide that kind of value.
- A robust, adaptive immune system is seen as an impediment to regeneration, which could be why mammals and birds are both so rubbish at regrowing lost body parts compared to reptiles and amphbibians. In salamanders, limb loss provokes only a weak immune response and regeneration is fairly robust. In the clawed frog Xenopus, regenerative abilities are robust in juveniles but reduce as the immune system matures. Adult crocodilians have robust immune systems like birds and mammals do, and as their only tail regeneration was seen in very juvenile alligators, its possible that their underdeveloped immune systems allows them to (partially) regenerate in a way that wouldn't be possible once the full power of their adult immune system is established.
- We have fossil evidence of partial tail regeneration in the Jurassic marine crocodile Steneosaurus bollensis (unfortunately, the papers the authors link are both in German and I don't understand them). The authors (briefly) speculate that tail regeneration was something all archosaurs inherited from their reptile ancestors, but that this ability was subsequently lost in the dinosaur/bird line (again, possibly and partially because they evolved more adaptive immune systems). So rather than being a new trait that evolved in alligators, it might be a partially-inherited ability that was potentially diminished/lost in some other surviving crocodilians.
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Little bit of context for the question that i had to look up: alligators and crocodiles are in the same order but different families, and their most recent common ancestor was in the late cretaceous (65+ million years ago). So that's not exactly "distantly related", but certainly less close than I'd expect given how similar they are.
Last common ancestor lived around 80 million years ago so "distant" compared to how close people expect them to be. "Distant" and "close" are generally relative terms. So alligators and crocodiles split off just a little bit of time (in evolutionary terms) after placental mammals and marsupials split from each other, which are generally considered to be distantly related to each other (relatively speaking, and especially useful when explaining to someone that the flying squirrel and the sugar glider are not close).
The earliest known whale ancestor that wasn't itself a cetacean is known from around 50 million years ago. It looked like a dog/raccoon and lived on land. Alligators and crocodiles split off more than 1.5x further back in time compared to then.
Finally, and this may blow your mind, at the time alligators and crocodiles last shared a common ancestors, humans and pigs also shared a common ancestor. So yes, I would reiterate that crocodiles and alligators are distantly related.
If I understand your question correctly, you’re basically asking why the evolution of one species went different than the evolution of another species. I don’t think anyone can answer this other than some generic answer.
Don't agree with that. Since no one was around to physically witness the evolution of any species, that would mean that our ideas on why any two species that shared a common ancestor evolved differently over time would just be generic statements. But in fact there are many such cases where one (by that I mean scientists, academicians) can make strong, educated guesses based on multiple factors like the fossil record, climatic differences, predators etc.
For example, the orangutans in Sumatra (especially the recently classified Tapanuli orangutan) are almost entirely arboreal whereas the Borneo orangutan is comparatively more comfortable on the ground. The educated understanding of the scientific community is that this likely happened because Sumatran orangutans evolved in close proximity to Sumatran tigers, unlike the Borneo species. That's not a generic answer and is certainly an interesting thing to ponder on.
That's the kind of thing I was looking for (to arrive at a probable/possible answer based on current evidence, if there is any). Not "tell me what the exact reason is, I want the exact reason!" I know you can't often find objective answers like that when it's evolutionary biology you're talking about.
That makes somewhat sense. Maybe I read it in a too much of a “give me the specific answer” rather than “give me your best guess.”
Now it would be great to know if the first Alligators could do it, or wether it was a trait they obtained later on.
If the first one applies, it could be the case that they had it from their “pre-alligator ancestor?” While crocodiles could have a different family line/background?
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djublonskopf t1_iwucusk wrote
Crocodiles actually can also regrow their tails to a limited degree. Both crocodiles and alligators have limited tail regenerative ability though...it seems that crocodilian (including alligator) tail regrowth is limited to a rod of cartilage wrapped in mostly collagen, without even any bone or muscle tissue. Alligator regrowth seems limited to only a few inches, and only when young, and is very slow (something like 15-18 months). Crocodile tail regrowth is not nearly so well studied (although, to be fair, the alligator study was only published 2 years ago and was something of a surprise to researchers at the time.)
Regeneration in general is metabolically expensive, which is probably why young alligators take so long to regrow anything at all, and even then don't bother regrowing the muscle or bones. It seems like the tip of the tail is very vulnerable to damage in crocodilians, and also important enough to propulsion in water that it's worth regrowing something to make up a little of the lost surface area, but that's about the extent of it.