Gonatodes humeralis on a tree trunk in French Guiana. Photo taken by Tim Higham.
The ability for some lizards to adhere to smooth surfaces has attracted considerable attention from scientists, engineers, and the public for quite some time. Anoles can exhibit considerable amounts of adhesion, although they lack the fancy specializations that most pad-bearing geckos have, such as the upward curling of the digit tips (to detach the adhesive system) before the foot is lifted from the surface. This might be related to the higher adhesive forces exhibited by geckos in comparison to anoles. Unlike anoles, the gecko adhesive system has appeared and disappeared several times. The simplification of the system appears linked to the transition from a climbing to terrestrial lifestyle. However, it has been unclear how this innovation might arise and how the early stages might appear.
The evolution of digit form in Gonatodes
That’s where the genus Gonatodes comes into play. Gonatodes is a reasonably diverse (27 species are currently recognized) and ancestrally padless clade of mostly diurnal sphaerodactylines that is sister to Lepidoblepharis. After examining the microscopic anatomy of a number of species from the genus Gonatodes, it was clear that one species (G. humeralis) was a bit different. Upon investigation of the subdigital micro-ornamentation, we found that the spinules in the vicinity of the digital inflection are longer than in other species of Gonatodes and are expressed as branched, spatulate-tipped setae on the free distal margin of these scales. In other words, it looked like this species of gecko was showing signs of incipient adhesion without actually having any toepads. Now that the morphological differences were identified, we wanted to know how/if this translates into functional and ecological differences.
Gonatodes humeralis in French Guiana. Photo by Tim Higham.
The origin of frictional adhesion in geckos
In collaboration with Anthony Russell and Tony Gamble, We sought to understand how this incipient adhesive system works in nature, whether G. humeralis can generate adhesive force, and what it permits these lizards to do on smooth surfaces in the lab. I traveled to French Guiana with Clint Collins, a Ph.D. student in my lab, in order to collect G. humeralis and examine its adhesive force. After that, Anthony Russell and I traveled to Trinidad & Tobago to collect a number of other species, in addition to G. humeralis, to see how they used their habitat and whether G. humeralis could out-perform the other species in the lab. To our surprise, G. humeralis was found on smooth bamboo stalks, whereas other species lived on the ground or on rough tree trunks. In the lab, G. humeralis could exhibit considerable adhesive force (for its size), exceeding that of skinks, but falling short of anoles and other pad-bearing geckos. That’s quite impressive for a gecko that lacks all of the bells and whistles of a typical pad-bearing gecko! Importantly, no other species of Gonatodes that we collected could generate any measurable force, agreeing with our previous morphological analyses! Now to the locomotor tests. Pad-bearing geckos are renowned for their ability to ascend vertical smooth surfaces, so we decided to test the ability of different species to climb different inclined smooth acrylic surfaces. A closely related species, G. vittatus, was unable to ascend any incline greater than 40 degrees. However, G. humeralis could climb up a vertical surface, as shown above.
T. Higham looking for geckos in Trinidad
What does all of this mean?
Although major transformations in vertebrate evolution are common, and often very complex, their origins are often elusive. We offer a glimpse into the early development of the complex adhesive system of geckos. However, the setae of G. humeralis are effective without all of the muscle, tendon, and vascular modifications that are often associated with gecko adhesion. Much like the anoles, the relatively simple setae of G. humeralis provide a dramatic advantage in areas of the habitat typified by leaves or other smooth surfaces (e.g., bamboo stalks). As noted in our paper, our discovery of a functionally intermediate form in the transition to frictional adhesion in a lineage of geckos highlights a statement by Ernst Mayr back in 1960: “Perhaps most astonishing is the relative slightness of reconstruction that seems to be necessary for successful adaptation to rather drastic shifts of adaptive zones.” The relatively simple morphological modification in G. humeralis has permitted a dramatic shift in biomechanics and likely habitat use.
The paper:
