Frogs flee from friction (amphibians locomote to avoid bruising)

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Of the three orders of living amphibians, frogs (https://en.wikipedia.org/wiki/Frog) are the most widespread on Earth, with salamanders (https://en.wikipedia.org/wiki/Salamander and https://parcplace.org/wp-content/uploads/2018/03/State_of_the_Salamander.pdf) being largely restricted to one hemisphere, and caecilians (https://en.wikipedia.org/wiki/Caecilian) being patchily distributed in the tropics.

Their extensive distribution and radically different body form suggest that frogs – characterised by long hindlimbs, short thorax, broad head, and taillessness – have achieved their success by virtue of the most economical of designs in energetic terms.

However, the common theme of body form in frogs requires an adaptive explanation beyond energetic efficiency. This is because:

• frogs have greatly diversified - and specialised - into swimming, hopping, climbing, walking, gliding, digging, and subterranean forms,
• salamanders and caecilians are not as widespread as lizards and snakes, despite having body forms and gaits convergent with these reptiles, and
• no reptile, living or extinct, has emulated frogs in losing the external tail, despite the fact that reptiles radiated from ancestral amphibians into the greatest variety of body forms achieved by any class of vertebrates.

Thinking laterally:
Please consider the vulnerability of the amphibian skin to pathogens, and the particular risk of infection caused by bruising.

Crucially, amphibians are the class of vertebrates most vulnerable to abrasion and fungal attack. This is mainly because they consistently retain an epidermis thin and moist enough to breathe through. However, it is also because - unlike most fishes and all freshwater fishes - they intermittently expose their skin to fungal spores in the air.

So, what if fundamental aspects of amphibian biology can be explained as ways of minimising the friction that tends to make skin abrade and become infected?

Please consider the following adaptations of body form and modes of locomotion.

• Tailless leaping, hopping and reaching reduce contact with plants and other obstacles during locomotion.
• Many frogs burrow, typically digging backwards with one small flexible tubercle on each hind foot while the body rotates (https://www.sciencephoto.com/media/820782/view/eastern-spadefoot-toad-s-holbrookii-foot and http://www.californiaherps.com/frogs/images/scouchiispadeaz05.jpg and http://www.californiaherps.com/frogs/images/scouchiispades.jpg and http://www.californiaherps.com/frogs/pages/s.couchii.html). This reduces friction, partly because the partial extension of the hindlimbs increases the excavated diameter within which the body can sink into the earth.
• Salamanders, unlike frogs, cannot use special gaits to avoid abrasion. Instead, they keep their body temperatures lower than those of frogs, ensuring that they move slowly enough to avoid obstacles, and that fungi are retarded in growth.
• Caecilians minimise bruising by maintaining lubricated burrows and possessing special protection in the form of segmented annulations (partly analogous with those of earthworms) that dissipate frictional energy.

Further aspects of the above, worth considering:

• A peculiarity of digging in most frogs is that the wear is concentrated on small trowel-like surfaces with complex articulation relative to the leg as a whole. These are not as large, hard or numerous as claws, but efficient enough.
• Salamanders generally have body temperatures lower than those of frogs in similar climates, partly because salamanders do not bask as some frogs do.
• Both frogs and salamanders tend to have chemical defences against predators. However, warning colouration – signifying toxic skin – is particularly common among salamanders (https://www.zsl.org/science/news/salamander-eating-fungus-found-to-be-widespread-in-european-private-amphibian-trade), in which chemical defence compensates for a reluctance, or inability, to sustain rapid flight.
• The skin of caecilians differs from that of other amphibians in possessing scales. However, these scales differ from those of reptiles in being buried in the dermis.

In summary:

Frogs are more successful than lizard-like or serpentine amphibians.

This is possibly because frogs - whether leaping, hopping, digging or climbing – seem to have effectively swopped the external tail for extended - but foldable - hindlimbs, thereby reducing the risk of bruising

This substitution has not necessarily saved energy during movement.

However, a combination of long hindlimbs, short thorax, broad head, and taillessness may be particularly versatile in compensating - in concealment and fleeing alike - for the special proneness of amphibian skin to infections.

Posted on June 14, 2022 12:32 AM by milewski