Gecko skins itself to escape predators

Some of you may find this gross, but it’s still a remarkable achievement of natural selection, and one of those weird things that abound in nature but most of which are yet to be described.  It’s the discovery of a “fish-scale” gecko that easily sheds its scales when caught, revealing a bizarre, naked reptile that has to regrow its scales but which has escaped predation. That’s a worthwhile tradeoff!

The animal was described in a new paper in PeerJ by Mark Scherz et al. (reference below; free download), and has been publicized widely on Twitter by astonished biologists, as well as in a short piece in The New York Times. 

Geckos are lizards in the family Gekkota. The genus Geckolepis, which contains about ten species endemic to Madagascar and the Comoros Islands, are known as “fish-scale geckos” because they have large scales that are shed easily. Presumably, they lose many of their scales, including a portion of their skin, when seized by predators, allowing them to escape.

The new paper tries to resolve the number of species (or “operational taxonomic units”) in the group, but the cool part is the description of a new species, Geckolepis megalepis—the first new species described in this genus in 75 years. Most of the paper is full of arcane details of its anatomy and morphology (needed to describe a new species and distinguish it from existing ones), but we needn’t be concerned with most of those. Here’s what the new species looks like:

Geckolepis megalepis, newly discovered in Madagascar, has the largest scales of any fish-scale gecko. Credit Frank Glaw

What is novel about G. megalepis is that its scales are relatively larger than those of any other fish-scale gecko: the authors say that if we were covered with scales of the same relative size, they’d be the size of our hands. These scales, which are highly mineralized, are shed easily when the animal is grabbed; in fact, the researchers had to catch the animals with wads of cotton to try to preserve them intact.  Below are some quotes from the paper:

Geckolepis megalepis was observed active at night both in the rainy and dry seasons, on trees and tsingy limestone rock. When captured, these geckos showed a strong tendency to autotomize [shed] large parts of their scales, leading to partly ‘naked’ geckos without any visible (bloody) lesions. In a subjective comparison this tendency appeared to be even more developed than in other Geckolepis species.

And this is what one looks like after it’s been grabbed and released; one of the authors describes the scale-denuded beast as looking like a “naked chicken breast.”:

For those of you who want more gory detail, read on (I’ve divided one long paragraph into three), for it’s not just the scales that are shed (my emphasis):

Many reptiles have evolved the ability to shed some part of their body in response to predator attack. The most widespread form is caudal autotomy, the shedding of all or part of the tail, which is widespread among Lepidosauria, from amphisbaenians to rhynchocephalians, even being found in some snakes (Arnold, 1984; Bateman & Fleming, 2009).

Geckolepis species are also able to shed their tails, and indeed few specimens survive to adulthood with their original tails intact (see for instance Figs. 3A and 3B). In addition, these geckos have evolved an even more extreme adaptation, i.e. the autotomy of virtually their entire integument when seized or even touched. Earlier studies have shown that the autotomized layers include epidermis, underlying connective tissue, and subcutaneous fat tissue, and that a layer between the integument and the underlying tissue represents a pre-formed splitting zone (Schubert & Christophers, 1985).

The shedding process is most likely achieved by contraction of the network of myofibroblasts in the preformed splitting zone, with vasoconstriction in the most superficial vasculature of the dermis to avoid bleeding (Schubert & Christophers, 1985). This process is thus completely different from the normal skin shedding of squamate reptiles, which leads to a loss of keratinized epidermis only (Schubert & Christophers, 1985) The scarless regeneration of the whole integument occurs within a few weeks, apparently starting from stem cells of the deeper layers of the connecting tissue and is considered as unique among vertebrates (Schubert, Steffen & Christophers, 1990). Superficially, no differences are apparent between regenerated and original scales, due to the irregularity of scalation patterns and some variability in scale size. The same is true for regenerated tails; indeed, it is often hard to be certain that a Geckolepis tail has been regenerated without X-ray images showing that the vertebrae are absent.

But does this presumed defense against predators really work? Well, predation events are hard to see, but at the end of the paper the authors describe one observation of a related species grabbed by a larger gecko, and in that case “the Geckolepis individual [not the new species] slipped from the mouth of the Blaesodactylus [big predatory gecko] ca. 30 seconds after being captured, and escaped denuded, thereby providing the first direct evidence of successful escape by skin shedding.”

Well, it would be good to have more observations, but I suspect that the loose scales and integument have indeed been molded by natural selection to resist predators. These animals pay a big cost in having to regenerate their scales, which must be a big metabolic expense, and they’re also denuded for much of that time, making them more vulnerable. But that’s a smaller price to pay than being ingested by a predator!

h/t: Nicole Reggia

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Scherz, M. D., J. D. Daza, J. Köhler, M. Vences and F. Glaw. 2017.  Off the scale: a new species of fish-scale gecko (Squamata: Gekkonidae: Geckolepis) with exceptionally large scales. PeerJ 5:e2955; DOI 10.7717/peerj.2955