Geckos are superb wall-crawlers. These lizards can scuttle up sheer surfaces and cling to ceilings with effortless grace, thanks to toes that are covered in microscopic hairs. Each of these hairs, known as setae, finishes in hundreds of even finer spatula-shaped split-ends. These ends make intimate contact with the microscopic bumps and troughs of a given surface, and stick using the same forces that bind individual molecules together. These forces are weak, but summed up over millions of hairs, they’re enough to latch a lizard to a wall.
Many geckos have these super-toes, but not all of them. There are around 1,450 species of geckos, and around 40 per cent have non-stick feet. A small number are legless, and have no feet at all. Initially, scientists assumed that the sticky toes evolved once in the common ancestor of all the wall-crawling species. That’s a reasonable assumption given that the toes look superficially similar. It’s also wrong.
Tony Gamble from the University of Minnesota has traced the evolutionary relationships of almost all gecko groups, and shown that these lizards have evolved their wall-crawling acumen many times over. In the gecko family tree, eleven branches evolved sticky toes independently of each other, while nine branches lost these innovations.
Thanks to this study, we now know that the geckos are a superb example of convergent evolution, where different groups of animals independently hit upon the same traits as solutions to the same challenges. Faced with sheer surfaces, they have evolved subtle variants on the same adaptation, time and again. And time and again, they have lost those adaptations when the need for them disappeared.
The geckos first emerged during the earliest days of the dinosaurs, between 180 and 225 million years ago. Since then, they have spread across the world, and many of them have climbed up it. By the Cretaceous period, when the dinosaur reign was closing, the geckos had already evolved their adhesive feet, as we know from a beautifully preserved fossil entombed in amber.
Gamble has now documented how the gecko dynasty expanded and diversified, by building the most comprehensive look at the family’s relationships yet. He pieced the genealogy together with Eli Greenbaum from Villanova University, by analysing six genes from 244 gecko species. Together, these represent 107 of the 118 genera (groups of closely related species). It was extensive but important work, since earlier assumptions about a single origin for sticky-footed geckos were based on a survey of just a few species.
Gamble showed that geckos probably acquired their adhesive toes in at least eleven separate events, and lost them again on nine occasions. The numbers may not be exact, but they certainly paint a picture of repeated gains and losses.
The sticky setae evolved from tiny hair-like growths called spinules, which cover the body of all geckos and are thought to help them shed their skin. This means that the entire group already has the basis of a wall-crawling grip in their skins. To transform spinules into setae, it’s a simple matter of lengthening them and flaring their tips out into split ends. Other simple adaptations provided an even better grip, including flattened toe pads, and tweaked tendons to control how the foot lifts off the surface.
Many gecko groups lost these adaptations when they changed their lifestyles, from wall-scuttling to, say, burrowing in sand. There, sticky feet would have been a hindrance, and they were lost. Of course, this isn’t a case of rolling back to a more primitive state – evolution doesn’t work that way. It forges ahead, rather than retreading old ground. For example, the giant ground gecko from Namibia isn’t a climber but it still has setae (albeit grossly reduced ones) and its feet still have the specialised muscles and tendons that gave its ancestors a better grip.
Gamble thinks that his family tree will provide seeds for a lot of fruitful future research. For a start, it gives a better idea of the relationships between the geckos. Previously, many scientists tried to classify these lizards according to the similarities of their toes. Now we know that such traits may have arisen independently.
The study may also be useful to the many people who are trying to duplicate the geckos’ powers, by mimicking their feet using man-made materials. If those feet evolved many times over, that could tell us about the factors that are necessary for a good grip, and those that make for a particularly strong one.
If we took several different makes of cars, we could easily separate the most important features – wheels, engines, brakes, and so on – from the dispensable ones like sunroofs or radios. Likewise, comparing gecko feet that evolved independently will tell us about the core features of a wall-crawling toe – the ones that absolutely must be used in a robot with the same abilities.
Reference: Gamble, Greenbaum, Jackman, Russell & Bauer. 2012. Repeated Origin and Loss of Adhesive Toepads in Geckos. PLoS ONE http://dx.doi.org/10.1371/journal.pone.0039429