Squirting sticky fluid, having a sensitive knob, etc. (gekkotans part III)

More on gekkotans, and this time were going to look at various details of gekkotan anatomy. Gekkotans are, being lizards, lizard-shaped (though with the near-limbless pygopodids being snake-like). But what makes them really special is that certain parts of their bodies - in particular, their hands and feet, and often their tails - are highly specialised and mightily weird [images here - from wikipedia - show the fat-tailed eublepharid Hemitheconyx caudicinctus and the gekkonine Ptychozoon kuhli with its tessellated tail].

We'll start by looking at tails. Many gekkotans use the tail for fat storage, and in some individuals of some species it can be grotesquely swollen and broader one-third along its length than it is at its base. A fat tail on a Tiger gecko Pachydactylus tigrinus or Golden-spotted gecko P. oculatus, for example, can be shaped like a parsnip. Agile geckos are able to use the tail as a prop when climbing, and they can also use it dynamically to help control their trajectory when falling or leaping (Jusufi et al. 2008) [a leaping Flat-tailed house gecko Cosymbotus platyurus is shown right at the bottom of the article]. Pygopodids use the tail to help them leap: more on that later! Species that rely on crypsis may have flat, leaf-shaped tails, or possess flanges or flaps of skin that help the geckos camouflage themselves against rock or bark. As we'll also see later (I mean, in a later article), flaps like this seem to have been co-opted for parachuting behaviour in at least one gecko lineage.

Some geckos (species of the gekkonine Lygodactylus and diplodactyline Bavayia) possess adhesive organs on the underside of the tail tip that are virtually identical to the digit-tip structures that geckos use to climb with (more on digits and digital structures a little later). The tail thus functions very much as a 'fifth limb' in these species. Contrary to what you might predict, these geckos are able to lose the tail as an anti-predator function. More on that subject in a minute.

Geckos with sensitive knobs (on their tails)

Extremely short tails are present in some diplodactylines, most famously in the Australian knob-tailed geckos (Nephrurus) [N. sheai* shown below: photo by Geckoadz, taken from the Geckos Unlimited message board. Finding good, useable photos of Nephrurus is just about impossible. Yeah, just like it is for most other obscure herps the world over].

* N. sheai was only named in 1994. Most other Nephrurus species have also been named relatively recently (as in, since the 1950s): witness N. laevissimus Mertens, 1958, N. cinctus Storrs, 1963, N. occidentalis (Storrs, 1963), N. pilbarensis (Storrs, 1963), N. vertebralis Storr, 1963, N. stellatus Storr, 1968, N. deleani Harvey, 1983 and N. amyae Couper & Gregson, 1994 (not all of these species are accepted by all authors... N. occidentalis and N. pilbarensis are regarded as subspecies of N. levis by some authors. Yeah, I'm avoiding the whole Wells & Wellington issue for now).

The tail isn't just short in these geckos; it's weird, terminating in a hard, rounded knob [tail of N. asper shown here in dorsal view, from Russell & Bauer (1987). The 'p' stands for peduncle and the scale bar = 5 mm]. Set within the polygonal scales covering the surface of the knob are tiny sensory organs (or sensilla), so it seems that knob-tailed geckos use the tail tip to collect information about their environment (Russell & Bauer 1987). The tail tip is also highly vascularised and might serve a thermoregulatory role. There's some suggestion that these geckos sit in their burrows with the tail tip directed toward the entrance. Most knob-tailed geckos are capable of autotomy, but the rates of tail loss and regeneration observed within populations are really low. Pianka & Pianka (1976) reported an autotomy frequency of just 0.6% in a wild population of N. laevissimus, while N. asper is apparently unique among gekkonids in being totally unable to autotomise its tail.

Spines are present on the tails of other diplodactylines (like some Diplodactylus species), and these geckos are also able to shoot a sticky fluid out of glands contained within the tail. They can apparently shoot the fluid for a distance of about a metre, and have good aim. This fluid is either black or pale yellow and has been described as having a musky odour, or, more colourfully, "an unpleasant odour resembling that of crushed legume seeds" (Richardson & Hinchcliffe 1983) [tail secretion appearing on a specimen of a Spiny-tailed gecko D. ciliaris shown here (the arrows point to small beads of fluid that are just starting to ooze out). Photo from Rosenberg et al. (1984)]. The fluid isn't toxic, but it may discourage attacks from predators, particularly from the big arthropods that prey on these geckos (Bauer 2000). On a vaguely related matter, long-time readers might recall the mention here - long, long ago - of similar squirting tail organs in some plethodontid salamanders (I'd taken this alleged fact from Mark Carwardine's Guinness Book of Animal Records). I asked Mark Carwardine's agent about this (Mark himself was not available), but never got a response. I agree with those who thought that this was erroneous.

At least some diplodactyline geckos - I'm thinking of the Fat-tailed diplodactylus D. conspicillatus - have a broad, depressed tail protected on its dorsal surface by broad, plate-like scales. This kind of tail is supposedly used by these lizards to block burrow-entrances and hence provide protection from predators. Rock geckos (Pristurus) curl their tails over their backs when engaging socially with each other, and are sometimes called scorpion geckos for this reason. In fact, quite a few gekkotans hold their tails up and over their backs, often while walking with stiff legs and a high gait, and all might be engaging in scorpion mimicry.

One final point perhaps worth mentioning on gecko tails: the weird structures just discussed (sensitive, knob-shaped tail tips, fluid-squirting glands, spines, adhesive tail tips, plate-like scales etc.) are not reflected in the osteology, so if we only knew these lizards from their skeletons, we'd have no idea about any of this stuff. That's a fun point to consider if you're a palaeontologist.

Shedding tails and.... skin!

Many geckos can autotomise their tails (autotomy is the ability to shed or drop any body part as an anti-predatory response), and the tail is in fact so fragile in some species that it drops off as soon as the animal is touched (note, however, that there is some degree of control over this, so it isn't as if the animal is poorly put together or anything). Some geckos (most famously, the Australian marbled gecko Christinus marmoratus [shown here, from wikipedia]) grow back two or even three tails in replace of the original - why? Is this just a developmental quirk of some sort?

It has been hypothesised that a reliance on caudal autotomy means that gekkotans have been unable to evolve bipedalism (in contrast to many iguanian and anguimorph clades). Bipedalism in squamates requires enlarged caudofemoral muscles, while caudal autotomy requires reduced caudofemoral muscles (Russell & Bauer 1992). Or... is it the other way round? That they have small caudofemoral muscles, and hence have been able to rely on caudal autotomy as an anti-predator response? Whatever, squamates can either be good bipeds, or good tail-shedders; they can't be good at both. No gekkotans are definitely bipedal, though - having said this - bipedal behaviour has been reported in Sakalava's velvet gecko (aka Grandidier's velvet gecko) Blaesodactylus sakalava (Rösler 1984) [B. sakalava shown here; image by Rob Dufek, from Geckos Unlimited]. Come on, animals: read the literature and be consistent!

Far weirder anti-predator behaviours than tail-shedding have evolved in other geckos. Members of several gecko groups are able to shed huge quantities of epidermis when grabbed by a predator: the skin literally peels off in great chunks, and the gecko escapes, with huge, pink expanses of musculature now on show. Writing of the Namaqua gecko Pachydactylus namaquensis, Branch (1988) wrote "[W]hen gripped, they twist violently, tearing off large areas of the skin, and often slipping away. The flayed gecko is a frightening sight but the skin is quickly regenerated" (p. 205). The same trick is practised by the skin-sloughing geckos (Ailuronyx) and fish-scale geckos (Geckolepis): in the latter, virtually the entire skin can be shed, leaving the animal looking pink, naked and totally grotesque (a photo of a naked G. maculata is included in Glaw & Vences (2007)). House geckos (Hemidactylus) can also do this, so it seems to be quite widespread in phylogenetic terms [the image below shows leaping Flat-tailed house gecko Cosymbotus platyurus using its tail to control its trajectory. Image: Robert Full/UC Berkeley, copyright PNAS/NAS 2008].

A few other details of gekkotan anatomy are unusual compared to those of other squamates. The vertebrae of gekkotans, for example, are peculiar in being amphicoelous (this is where both ends of the vertebral centrum are concave): those of other squamates are procoelous (where the anterior end of the centrum is concave and the posterior end is convex). It's been proposed that neoteny is the explanation for this condition, and indeed many gekkotans do seem 'under-developed' (in terms of skeletal growth) compared to most other squamates. Their skull bones are typically weakly ossified, for example, and their scales are often thin.

The anatomical features that gekkotans are best known for are, however, found of course on their hands and feet, and that's what we're going to look at next.

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

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 - -

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.

Branch, B. 1988. Field Guide to the Snakes and Other Reptiles of Southern Africa. New Holland (London).

Glaw, F. & Vences, M. 2007. A Field Guide to the Amphibians and Reptiles of Madagascar. Vences & Glaw Verlags (Cologne).

Jusufi, A., Goldman, D. I., Revzen, s. & Full, R. J. 2008. Active tails enhance arboreal acrobatics in geckos. Proceedings of the National Academy of Sciences 105, 4215-4219.

Pianka, E. R. & Pianka, H. D. 1976. Comparative ecology of twelve species of nocturnal lizards (Gekkonidae) in the Western Australian desert. Copeia 1976, 125-142.

Richardson, K. C. & Hinchcliffe, P. M. 1983. Caudal glands and their secretions in the western spiny-tailed gecko, Diplodactylus spinigerus. Copeia 1983, 161-169.

Rosenberg, H. I., Russell, A. P. & Kapoor, M. 1984. Preliminary characterization of the defensive secretion of Diplodactylus (Reptilia: Gekkonidae). Copeia 1984, 1025-1028.

Rösler, H. 1984. First observation of bipedal locomotion in a gecko, Homopholis (Blaesodactylus) sakalava (Reptilia: Sauria: Gekkonidae). Journal of the Herpetological Association of Africa 30, 13-14.

Russell, A. P. & Bauer, A. M. 1987. Caudal morphology of the knob-tailed geckos, genus Nephrurus (Reptilia: Gekkonidae) with special reference to the tail tip. Australian Journal of Zoology 35, 541-551.

- . & Bauer, A. M. 1992. The m. caudifemoralis longus and its relationship to caudal autotomy and locomotion in lizards (Reptilia: Sauria). Journal of Zoology 227, 127-143.