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FillRevolutionary900 t1_iwuhkba wrote

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.

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djublonskopf t1_iwupdne wrote

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.

The alligator-tail paper authors point out a couple of additional pieces of information you might find interesting:

- 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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