Lamellae, scansor pads, setae and adhesion... and the secondary loss of all of these things (gekkotans part IV)

Back to gekkotans: time to look at digits.

Geckos are well known for the ability of many species to cling to vertical surfaces, and even to ceilings. In fact, this is usually the one thing about geckos that everyone knows. The powers of gecko adhesion are such that geckos can support their entire weight while hanging from a single digit, they can remain stuck to leaves while exposed to 98 km/h winds, and they can also remain stuck to a surface when dead (Russell 1976). Expanded digital pads - called scansors or scansor pads - are lined on their undersides with rows of lamellae, and these are covered with tiny structures termed setae. The setae have spatulate or branched tips and are microscopic: there are about a million of them on the fingers and toes of a Tokay. Anoles (part of Iguania, and hence only very distantly related to gekkotans) and some skinks have convergently evolved very similar lamellae and setae.

While it has been suggested that static electricity or 'molecular glue' might explain how the setae allow geckos to cling to surfaces, it seems that intermolecular attractive forces - van der Waal's forces - provide the answer (Autumn et al. 2002). A good question about these microscopic setae is how they avoid getting fouled and clumped together. It seems that they're self-cleaning, and are both hydrophobic and able to shed dirt particles during normal function (Hansen & Autumn 2005). And, yes, the domestic or industrial applications for technology incorporating this knowledge has been investigated [weird right foot of Calodactylodes aureus, from Russell & Bauer (1989a). Scale bar = 5 mm].

Setae might have evolved from the so-called Oberhäutchen spines that cover the gecko epidermis (again, anoles have similar spines). Species of the gekkonid Cyrtodactylus exhibit what appears to be a morphological series - it's illustrated below, from Russell (1976) - whereby the setae on the subdigital scales are short and Oberhäutchen spine-like in 'primitive' species, but longer and straighter in more 'advanced' ones; in the species with the longest setae, the setae have spatulate or branched tips (Russell 1976). The other components of the digits seem to have become elaborate in step with the setae, and the digits as a whole are relatively straight in the species with the shortest setae, but strongly arched with hyperextended joints in the species with the longest setae.

Gecko phalanges (= the bones that make up the digits) are broad and dorsoventrally flattened compared to the more cylindrical digital bones of other squamates, and the interphalangeal joints are especially mobile.

Symmetrical hands and feet, and the reversion to asymmetry

What also makes gecko digits unusual is that they're arranged around the wrists and ankles in a more symmetrical fashion that is typical for lizards (note that manual digit I has been lost in some gekkonids, and pedal digit I is strongly reduced in some as well). These symmetrical hands and feet presumably evolved because digits that radiate widely, like spokes on a wheel, function better at providing purchase on smooth surfaces. Anoles that have scansor pads and setae have strongly asymmetrical feet like most other lizards, but it might be that they haven't been climbing on vertical surfaces for as long as geckos have. They definitely aren't as good at it.

Complicating things is the fact that some gekkonids have apparently reversed the development of a symmetrical foot (Russell et al. 1997). In Aeluroscalabotes and Carphodactylus the foot is zygodactyl: in the former, digits I and II oppose III-V in chameleon-fashion, while in the latter I-III oppose IV-V. Incipient zygodactyly is also present in Ailuronyx [skeleton of left hindfoot shown here, from Russell & Bauer (1989b)]. Rhoptropus - a diurnal gecko that inhabits rocky deserts and frequently leaps from rock to rock - has a foot more like that of a typical lizard, with long toes and an especially long digit III. Phylogenies indicate that Rhoptropus is nested within symmetrical-footed gekkonids, so its possession of a 'typical lizard foot' is secondary.

Several terrestrial gekkonid taxa (like Colopus and Geckonia) have strongly reduced or absent scansor pads. The position of these taxa within phylogeny - they're surrounded by climbing taxa that have well-developed scansor pads and other climbing structures - shows that their pared-down digital anatomy must also be secondary, and "they show certain osteological and myological features which indicate their derivation from an ancestral climbing stock" (Russell 1976, p. 232) [image below, from Russell (1976), shows how the main components of gekkonine gecko digits have apparently been assembled in piecemeal fashion (digit shown is fourth pedal). From top to bottom the image shows a straight, 'primitive' digit, a kinked digit (as in Cyrtodactylus), a kinked digit with one absent phalanx (as in Ancylodactylus), a 'modified kinked' digit with paired scansor pads (as in Phyllodactylus), and a 'modified kinked' digit with multiple scansor pads (as in Ptyodactylus)... See below for more elaborate complexity!].

The internal anatomy of the digits is extremely complex. Cartilaginous elements termed paraphalanges extend in parallel to the phalanges and are connected to a series of tendons and muscles.

A complex of blood vessels arranged around these structures are connected to a large, blood-filled sinus. By controlling the blood flow in the vessels and sinus, geckos can control the size and shape of the scansor pad. In fact, to deploy the lamellae and setae, a gecko has to pressurise the sinuses in its digits by filling them with blood; when it wants to pull any given digit off from a surface, it has to depressurise the sinuses by forcing blood to move up and away from the scansor pad. "All of this happens with every single step the gecko takes!" (Bauer 2000, p. 589) [image below, from Russell (1976), shows more of the crazy digital complexity present in various gekkonine geckos. All the taxa shown have so-called multiscansorial digits; note that some (like Thecadactylus) have hyperextended interphalangeal joints (this is where the distal-most phalanges are raised up and off the substrate), others (like Hemidactylus) have many tendons, with each attached to a different scansor segment].

The scansor pads themselves are complicated, lobed, leaf-like structures in some taxa (see the Calodactylodes foot shown near the top of the article). While claws have an important function in many geckos, they don't in others and have been strongly reduced or lost in a few lineages (Russell & Bauer 1989): the Mascarene and Madagascan Ebenavia species are sometimes called clawless geckos. Some gekkonid species hold their claws up and off the substrate thanks to hyperextendable joints between their phalanges: in this respect they resemble cats and even some predatory dinosaurs.

The diversity of gecko digits really is pretty staggering, and there's so much more I could have said. Note that we only looked at gekkonid gekkotans here, not at all gekkotans. Next: leaf-tailed geckos and parachuting geckos.

For previous Tet Zoo articles on gekkotans see...

• The Tet Zoo guide to Gekkota, part I
• Gekkota part II: loud voices, hard eggshells and giant calcium-filled neck pouches
• Squirting sticky fluid, having a sensitive knob, etc. (gekkotans part III)

For previous Tet Zoo articles on neat squamates see...

• Mosasaurs might have used the same microscopic streamlining tricks as sharks and dolphins
• Tongues, venom glands, and the changing face of Goronyosaurus
• Dinosaurs come out to play (so do turtles, and crocodilians, and Komodo dragons)
• Tell me something new about basilisks, puh-lease
• 'Cryptic intermediates' and the evolution of chameleons
• The Great Goswell Copse Zootoca
• Of giant plated lizards and rough-necked monitors
• Ermentrude the liolaemine
• Evolutionary intermediates among the girdled lizards
• Hell yes: Komodo dragons!!!
• Amazing social life of the Green iguana
• Arboreal alligator lizards - yes, really
• Pompey and Steepo, the world-record-holding champion slow-worms

Refs - -

Autumn, K, Sitti, M., Liang, Y. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T. W., Fearing, R., Israelachvili, J. N. & Full, R. Y. 2002. Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences 99, 12252-12256.

Bauer, A. M. 2000. Lizards. In Cogger, H. G., Gould, E., Forshaw, J., McKay, G. & Zweifel, R. G. (consultant eds) Encyclopedia of Animals: Mammals, Birds, Reptiles, Amphibians. Fog City Press (San Francisco), pp. 564-611.

Hansen WR, & Autumn K (2005). Evidence for self-cleaning in gecko setae. Proceedings of the National Academy of Sciences of the United States of America, 102 (2), 385-9 PMID: 15630086

Russell, A. P. 1976. Some comments concerning interrelationships amongst gekkonine geckos. In Bellairs, A. d'A. and Cox, C. B. (eds) Morphology and Biology of Reptiles. Academic Press (London), pp. 217-244.

- . & Bauer, A. M. 1989a. The morphology of the digits of the golden gecko, Calodactylodes aureus and its implications for the occupation of rupicolous habitats. Amphibia-Reptilia 10, 125-140.

- . & Bauer, A. M. 1989b. Ungual asymmetry in the context of pedal symmetry in Ailuronyx (Reptilia: Gekkonidae): modification for an opposable grip. Journal of Zoology 218, 1-9.

- ., Bauer, A. M. & Laroiya, R. 1997. Morphological correlates of the secondarily symmetrical pes of gekkotan lizards. Journal of Zoology 241, 767-790.