Basking in amphibians

 (writing in progress) 

To understand amphibian ecology and biogeography, particularly the north/south difference in salamanders, it’s essential to bear in mind certain basic facts about amphibians in contrast to reptiles.

Reptiles, particularly lizards, bask./However, amphibians do not bask to the same extent. This is consistent with the moistness of the amphibian skin and the implied evaporative cooling.

Salamanders in particular do not bask. Frogs do bask in some cases but the function of basking differs from that in lizards.

Basking in frogs occurs mainly in two groups, viz

  • toads, and
  • tree- and reedfrogs.

In neither case can one assume that the function of basking is similar to that in lizards.

In toads, basking is possible because the skin is relatively dry. Certain Bufonidae of extremely high altitudes in the tropical Andes are extremely dark, which is presumably to enhance absorption of solar radiation. However, a basic difference between toads and lizards seems to be that after basking toads become less active, whereas after basking lizards become more active.

An implication emerges here: that even those amphibians that do bask do not use basking to boost muscle power or endurance.

The second group of frogs that bask is some of the various perching frogs (Hyperoliidae, Hylidae, presumably also Rhacophoridae), which sit still on branches or stems during the day. These frogs often expose themselves to full sunlight for many hours at a time, and rub waxy coatings on their skins to reduced evaporation so that they do not dehydrate in the process.

But here the odd thing is that their skins do not darken, as seen in lizards, to enhance the absorption of solar energy. Although climbing frogs are remarkably able to change colour (along the lines of chameleons), I have not heard of them darkening up (e.g. chameleons are pale by night and darker by day) while basking. If anything the opposite: some treefrogs are ghostly white by day, even to the degree of spoiling their crypsis/disruptive colouration and making them more conspicuous.

And again the climbing frogs do not use basking to boost muscular activity. Instead, they just keep sitting immobile until the sun goes down and the temperature falls. They are active in the cool conditions of dusk or night.

I am unsure why tree- and reedfrogs bask, but I suspect that it has to do with staying inaccessible to snakes. Although they make themselves somewhat obvious to birds by this self-exposure by day, they tend to be toxic enough to protect themselves from birds even if they are not aposematic.

(A frog is less likely to be found on a perch than on the ground, if only because for a snake to explore the branches is complicated and time-consuming, relative to the exploring the ground. Exploring the ground requires dealing with only two dimensions, so that odour trails can be easily followed. Exploring the branches means many backtrackings before a frog is discovered, whether by its odour trail or just by observation. And I do also think that birds in general would be less resistant to toxins, owing to the slow metabolism of snakes relative to birds. It’s hard to say whether birds would find it easier than snakes to discover tree frogs hiding among the branches and foliage.)

So although I have yet to see this stated directly in the literature, I suggest that amphibians do not bask in the same sense as reptiles, i.e. to boost muscular power and endurance. Amphibians do not seem to thermoregulate as such; instead they are passive reflections of environmental temperatures. And because they tend to be active at lower body temperatures than those seen in lizards, this makes sense overall in conjunction with the amphibian affinity for water and moist skin.

Tadpoles can stand body temperatures up to 34 degrees C for periods as long as they can cool down at night. It has been found in laboratories that, if tadpoles are kept at 34 degrees C (>3 degrees less than normal body temperature for humans), they die. However, if they are allowed to cool down for part of the diel cycle, they live (research by Amanda Niehaus).
 
An implication is that, whatever the salamander’s tail is used for, it is not basking.
 
So, w.r.t. the north/south question in salamander distribution:
Perhaps a useful perspective is that salamanders are essentially non-basking lizards, which are active mainly at night. I suspect that salamanders differ from geckos, which do not bask by day, by avoiding even the indirect basking used by geckos under cover. Geckos find warm places by day, e.g. under bark and roof tiles, to raise their body temperates. Although they are not active by day, they are the more ready to flee if encountered by predators breaking open their hiding places. I suspect that salamanders do not covertly bask in this way – but this needs to be checked in the literature.
 
Based on this framework, can readers see an explanation for the north/south discrepancy in the world distribution of salamanders?

Also see https://books.google.com.au/books?id=oaS-OpEjPtUC&pg=PA215&lpg=PA215&dq=basking+in+salamanders&source=bl&ots=N9hV9-SKYA&sig=2tHqu1CUK8Q8NX8xzgWlgvN9KKo&hl=en&sa=X&ei=6tYEVd27CYnN8gWAjoKAAQ&ved=0CC0Q6AEwAg#v=onepage&q=basking%20in%20salamanders&f=false
 
http://www.jstor.org/discover/10.2307/1931248?sid=21105648206581&uid=2&uid=4

 (writing in progress) 

Publicado el 21 de junio de 2022 por milewski milewski

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