A new study published in the journal Scientific Reports has revealed that a lizard native to the Greek archipelago has evolved back camouflage that precisely matches the various surface rocks of a host of very different islands. This remarkable ability to remain so inconspicuous helps the reptiles defend themselves from the clutches of a range of predatory birds.
Reptiles have been evolving for nearly 320 million years, so it's no surprise that they have developed various ingenious camouflage mechanisms over this epic time span. The Aegean wall lizard, Podarcis erhardii, is no exception to this. This particular species has made its home in the Peloponnese peninsula in Greece, living up to its name by clinging to the walls of people’s houses. Scrambling around outside, however, leaves them vulnerable to predators, such as a swift falcon or crow.
Although the lizards have evolved camouflage, allowing them to blend in with their environment, just how effective is this mechanism against predating birds? Kate Marshall, a graduate researcher from the University of Cambridge’s Department of Zoology, decided to investigate just how well P. erhardii is able to hide itself in the region.
Trekking across a selection of islands, Marshall noted that each had very different biogeographical characteristics. Some had far less avian predators, and others had entirely different bird species altogether. The varying geology of the islands, from the highly volcanic Santorini to the metamorphic rock-covered Syros, was also clear to see.
The P. erhardii lizards have been separated out on to these very different islands over time, and they’ve been present on the island chain for many hundreds of years. Therefore, they were expected to have evolved slightly different camouflage mechanisms from each other.
Peekaboo! Those on new volcanic islets within Santorini have rapidly evolved to become a similar dark hue to the black volcanic rocks at the surface. Marshall et al./Scientific Reports
As expected, Marshall found that the lizards' back camouflage on Santorini matched the volcanic rock types there far more convincingly than it would do if it were placed on Syros. In fact, each separate P. erhardii population had evolved to visually match the surface rock types for each of their own islands, giving them the best chance to avoid predation from birds.
This was confirmed using visual modeling, which looked at the lizards’ back camouflage in different wavelengths of light, including ultraviolet, in order to visualize what the predatory birds would see. Once again, the camouflage was near-perfect for each individual island’s populations.
However, despite each population having distinctly different back camouflage patterns, they haven’t evolved into other, unique species yet. “In 2009, a study did find that there was genetic variation between lizards on some of the islands,” Marshall explained to IFLScience. This means there are at least some subspecies of lizard present, but true species divergence has yet to occur.
Intriguingly, specific rocks on each island would provide the very best camouflage, and the lizards appeared to know which ones to sit on – but how? “They don’t have the same visual systems as the birds hunting them down, so that’s the first problem,” Marshall said. “Secondly, they also can’t really see their own backs, certainly not as well as birds can.
“So one possibility is that they learn in early life which rocks to sit on, either through copying other lizards, or from experience” – essentially, by not getting eaten while hiding on those rocks.