Skin-ditching gecko inexplicably leaves body armor behind when threatened

Skin-ditching gecko inexplicably leaves body armor behind when threatened
Fish-scale geckos shed their skin when threatened. But CT scans reveal bony deposits -- essentially body armor -- in their scales, raising the question of why the geckos seem to have conflicting defense strategies. Credit: by Paluh et al. in the African Journal of Herpetology

When trouble looms, the fish-scale geckos of Madagascar resort to what might seem like an extreme form of self-defense—tearing out of their own skin.

Their unusually large, overlapping scales flake off so easily that one biologist in the late 1800s tried collecting the with cotton, but even with careful handling, few fish-scale gecko specimens have been preserved with all scales intact.

Now, new research published in the African Journal of Herpetology shows the geckos' fragile skin contains a hidden strength. Inside the scales are bony deposits known as osteoderms, the same material that makes up the tough scales and plates of crocodilians and armadillos. But the presence of osteoderms in fish-scale geckos raises a herpetological mystery: If they have armor, why do they discard it?

"The big question is why there are these conflicting defense strategies," said Daniel Paluh, the study's lead author and a doctoral student at the Florida Museum of Natural History. "This gecko can actually drop its skin as a defense mechanism, but it also has these mineralizations—usually thought of as body armor—that it's just leaving behind."

As part of his research as a master's student at Villanova University, Paluh studied hundreds of geckos using computed tomography (CT) technology, which uses thousands of X-rays to create high-resolution, multilayered 3-D images of specimens. The CT scan of Geckolepis maculata, a species in the fish-scale genus, revealed dense bony material inside the skin, a feature Paluh had not noticed in most other geckos.

Skin-ditching gecko inexplicably leaves body armor behind when threatened
The delicate scales of fish-scale geckos easily shed when trouble looms but contain osteoderms, tough bony deposits. Credit: Frank Glaw

"I thought, 'Wow, this is really strange,' " he said. "We started diving deeper to verify that what we were seeing in the CT scan were actually these mineralized elements."

Osteoderms are found in some lizards, but they are rare in geckos, a group that includes more than 1,600 species. Prior to Paluh's study, only the genus Tarentola, or wall geckos, and Gekko gecko, the tokay gecko, were known to have this protective outer armor.

Most geckos have thin skin covered in tiny, granular scales and tend to rely on camouflage and the cover of night to hide from predators, Paluh said.

Some groups, such as Geckolepis, have also evolved weak skin as a form of defense, said Aaron Bauer, the Gerald M. Lemole Endowed Chair in Integrative Biology at Villanova University and co-author of the study. When a predator strikes, these geckos can rip out of their skin to escape, "like the tear-away football jerseys of the 1970s," he said.

Skin-ditching gecko inexplicably leaves body armor behind when threatened
The inset, left, shows the tightly interlocking osteoderms in a Geckolepis maculata scale magnified 100 times. On the right is Schmidt’s 1911 illustration of the osteoderms he observed in a G. polyepis scale. Credit: Paluh et al. in the African Journal of Herpetology

The apparent paradox of "sheddable armor" contributed to the widespread questioning of a paper by biologist W.J. Schmidt, who in 1911 published his observations of osteoderms in the scales of Geckolepis polyepis. His findings were met with skepticism for decades until Paluh's CT scan proved Schmidt was right.

"Schmidt had to illustrate what he saw, which could have contributed to his work being questioned," Paluh said. "It was unclear whether he was replicating the histology accurately. The advantage we have today is that we can combine newer, innovative tools with traditional methods than have been used for hundreds of years. It provides a new perspective to some of the classical anatomical observations."

To verify the presence of osteoderms in G. maculata, Paluh used techniques similar to Schmidt's. He cleared and stained excised patches of skin containing multiple scales to determine if the tissue was mineralized. Removing the revealed tiny, interlocking osteoderms similar to those Schmidt described and illustrated more than a century earlier.

"Schmidt was a great anatomist, and I'm sure that he was confident in what he saw," Bauer said. "Indeed, anatomists of his time, working with much less technically advanced equipment than we have today, were pretty good about getting animal anatomy right."

Skin-ditching gecko inexplicably leaves body armor behind when threatened
CT scans produced these reconstructions of the skulls and osteoderms of Gekko gecko, from left, Tarentola mauritanica and Geckolepis maculata, showing the relative thickness and density of their osteoderms. Credit: Paluh et al. in the African Journal of Herpetology

Paluh said osteoderms might not necessarily serve as a defensive shield. They could contribute calcium for egg development in female geckos or help regulate body temperature.

The researchers hypothesize that osteoderms likely evolved independently in Geckolepis, Tarentola and Gekko gecko. The geckos are not close relatives, and CT scans showed osteoderm structure and density vary among the three. G. gecko and Tarentola mauritanica have plate-like and granular osteoderms, while in G. maculata, the deposits resemble the small irregular pieces of a mosaic.

Further research is needed to determine how these bony deposits develop in Geckolepis and whether they can be regenerated after scales have torn away, Paluh said.

"There are plenty of interesting questions left to answer," he said. "Clearly, our understanding of gecko anatomy isn't yet complete."


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More information: Daniel J. Paluh et al, Sheddable armour: identification of osteoderms in the integument of Geckolepis maculata (Gekkota), African Journal of Herpetology (2017). DOI: 10.1080/21564574.2017.1281172
Citation: Skin-ditching gecko inexplicably leaves body armor behind when threatened (2017, August 4) retrieved 17 September 2019 from https://phys.org/news/2017-08-skin-ditching-gecko-inexplicably-body-armor.html
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Aug 04, 2017
Imagine you are a gecko accosted by a cat. It brings down its paw on you. The claws are blocked by the scales initially but the pressure of them on your pack could still pin you down unless your scales detach and basically act as little sleds that allow the claw (tooth, etc.) to ride the sled off your body. You of course have fewer scales after wriggling free, so this suggests that your attacker generally only gets one shot at you. Perhaps it is an ambush predator that has a quick strike but can't 'run' very fast (snakes come to mind) or perhaps it's a bird, and once you wriggle free from its beak or talons, you quickly drop into the undergrowth and can't be found again.

Aug 05, 2017
And, the bird/snake/cat is left with a partial mouthful of bony, nasty scales that are probably uncomfortable if not actually cutting and likely taste bad. I wonder though, if this makes them more succeptible to predation during the healing/regrowth period?

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