Anthony P. Russell and Michel-Jean Delaugerre
The ability of geckos to scale smooth surfaces fascinates both scientists and the public at large – holidays taken in warm climates are often enlivened by geckos roaming in the evenings on the walls of hotel rooms, apartments and restaurants. The toe pads of these adept climbers are composed of plate-like scales, which bear microscopic hair-like outgrowths creating reversible molecular bonds with the surfaces on which they move. The form of these toe pads has long been used by scientists to distinguish between different types of geckos; the names of the genera often reflect this (for example, Hemidactylus [half-toe], Lepidodactylus [scaly-toe], Phyllodactylus [leaf-toe]).
One of us (A.P.R.) has been studying gecko evolution and the form and the function of their adhesive apparatus for over 40 years, and has always been aware of two basic “designs” of this system – either a series of broad, overlapping sheets, like the blades of a Venetian blind, along almost the whole of the underside of the toe, or as one or two pairs of squarish, leaf-like plates at the very tips of the toes. The reason for the existence of these two different patterns has, however, defied explanation, although it has been proposed that the leaf-like pattern appears to be particularly associated with rocky surfaces (although the alternative pattern is also encountered among species occupying this type of habitat). What was needed to bring us closer to an answer was a more in-depth ecological study in an environment that would reveal whether there were potential functional differences between these two patterns.
The other one of us (M-J. D.) noted that on Giraglia, a very small, uninhabited island off the north Corsican coast, an otherwise very aggressive invasive gecko species, the Moorish gecko (Tarentola mauritanica), had colonized it in about 1950, when a concrete building was erected to house the power-plant for the newly-automated lighthouse. The Moorish gecko, which has the familiar pattern of gecko adhesive pads, with the plates extending along much of the length of each toe, likely arrived with the construction materials and established itself on the new building. In the subsequent 65 years, however, the species has failed to make inroads on the rest of the island. Already present, and native to the island, was the European leaf-toed gecko (Euleptes europaea), which elsewhere has not been able to withstand invasion by the Moorish gecko. The leaf-toed gecko has not colonized the newly-erected concrete building, but is widespread on the rocky outcrops of the island, and on the two previously-existing buildings (the 19th century lighthouse and a 16th century lookout tower), both of which are constructed from rock excavated on the island. That rock is prasinite, a metamorphic schist that is crumbly and coated with “dust” derived from the microscopic particles that weather from its surface.
If the Moorish gecko is placed onto the prasinite rock, it has great difficulty moving and clinging, because its large toe pads become fouled with the released dust. By contrast, on such dusty surfaces the toe pads of the leaf-toed gecko do not become fouled because they can be curled away from the surface, leaving the claws to provide grip. The Moorish gecko, because of the highly specialized movements of its toes that bring about attachment and detachment of its large adhesive pads, cannot use its claws independently of the pads and thus cannot negotiate, and is therefore unable to colonise, the dusty prasinite surface.
The fortuitous combination of circumstances on Giraglia (two species of gecko with different adhesive pad structure and a dusty rocky surface) has provided a window into the functional differences between the two adhesive pad configurations. Our work has provided the foundation for more detailed observations of how geckos with these two toe pad types might subdivide surfaces on which they move in different parts of the world.