In the previous few gekkotan articles we looked at the seriously weird and highly distinctive leaf-tailed geckos of Madagascar. There’s another group of especially unusual, highly notable gekkonid gekkotans I want to write about: the flying, gliding or parachute geckos (Ptychozoon) of south-east Asia and India. These geckos are weird: the adjacent pic (widely available online, but only at frustratingly small size; it’s credited to Tim Macmillan/John Downer) makes them look like tiny screaming dragons…
Parachute geckos are cryptic, forest-dwelling lizards. Patterned in greys or browns and possessing blotches and wavy dark lines on their bodies, they are good at concealing themselves on trunks and branches. Like the majority of the 1500-2000 gekkotan species (remember: this is a big, important group), they’re fairly small, with SVLs* of between 58 and 108 mm.
* snout to vent lengths (i.e., not the same as total length, as it excludes the tail).
As is the case in a few other geckos (like the leaf-tailed geckos), flaps of skin grow from the sides of their heads, flanks, limbs and tails. Despite having known of these lizards for, certainly, the whole of my adult life I still think that the idea of lizards with lateral winglets, skin fringes and massively webbed digits is pretty freaky: look at the photo of the P. intermedium holotype here (from Brown et al. 1997), and hopefully you’ll see what I mean (incidentally, this specimen was destroyed during WWII).
Rounded flaps are present on both sides of the head, there are continuous fringes running along the sides of the body, flaps on the rear surfaces of the thighs, and semi-circular or subtriangular fringes project from the sides of the tail. The fringes are not supported internally by muscles or skeletal structures, and they scale allometrically with the size of the lizard (Russell et al. 2001). Flaps also extend along the leading and rear margins of the forelimbs in some species, and in one species* (P. lionotum, sometimes called the Smooth-backed parachute gecko) a notch – termed the predigital notch – separates the webbing round the thumb from that on the inside of the lower arm [notch presence (in P. lionotum) and absence (in P. trinotaterra) is shown in the diagram below: from Brown (1999)]. I wonder if this serves some aerodynamic function (could it work as a leading-edge slot?), though why it’s seen in P. lionotum alone and not the others is an interesting question.
* Yup, there’s more than one species. We’ll get to that in a minute.
The digits on both the hands and feet are connected by extensive webbing. You might recall from the article on gekkotan digits that gecko scansor pads and their associated blood vessels and musculature are quite complex. One peculiarity of geckos is that many of them possess accessory, rod-like elements (termed paraphalanges) that project from the sides of the phalanges (the cylindrical bones that make up the digits). Surprisingly – in view of their extensive digital webbing – parachute geckos lack paraphalanges.
If you’ve ever read anything about Ptychozoon (or seen it on TV, or in real life), you’ll be familiar with the idea that these integumentary structures are all used to help these geckos to glide. It’s perhaps somewhat surprising, therefore, to find that it’s sometimes been doubted or even flatly denied that Ptychozoon is truly capable of any sort of aerial activity; to be fair, however, these assertions all date from the early 1900s. Theodore Cantor (in 1847) and Georges Boulenger (in 1890) had noted a parachuting function for the skin flaps somewhat earlier [Boulenger was so confident about this that he had P. kuhli illustrated in a gliding pose: this picture is shown below]. However, the flaps are also used to help break up the body outline and conceal these animals when they press their bodies against bark. Some authors have asserted that, because this cryptic role is used more often than the parachuting one, the flaps and fringes are “primarily cryptic” in function.
Russell (1979) argued that all of the features involved in parachuting – like the enlarged lateral skin folds on the body, jumping behaviour and extensive digital webbing – are seen in related gekkonine gekkonids that use them for crypsis, and not for parachuting or gliding. Hence, the characters allowing the behaviour are, in Russell’s view, ‘protoadaptational’. This makes Ptychozoon an excellent example of exaptation (where structures that initially served one role were co-opted during evolution for another).
Parachuting or gliding behaviour, or both
Experimental (and photographic) evidence does of course confirm that Ptychozoon definitely does engage in ‘parachuting’ behaviour at least. But does it do more: can it actually glide? ‘Parachuting’ can be defined as falling at a slowed rate and in controlled fashion, while gliding is usually imagined as descending at an angle of less than 45° to the horizontal. The two kinds of behaviour grade into one another (Pennycuick 1972) and parachute geckos seem capable of both.
Heyer & Pongsapipatana (1970) tested the parachuting skills of Ptychozoon by dropping test subjects from 41 m up in a meteorological tower at the Sakaerat Experimental Station, Thailand (they also dropped a variety of non-parachuting geckos, and in their work on gliding snakes also dropped an Agkistrodon pit viper and even a specimen of the aquatic snake Enhydris plumbea). On being dropped, the parachute geckos stretched their legs out, held their tails out straight, and travelled for a much greater distance than did the control geckos. Sometimes the parachute geckos went with the wind, but on other occasions they travelled at right angles to it [P. kuhli below from here]
Marcellini & Keefer (1976) concluded that specimens of P. lionotum didn’t just parachute, but actually glided. The geckos did this (on average) for about 5 m (one specimen made a glide of 9.35 m) and performed well in terms of the rate of descent. The style of descent seen in parachute geckos is pretty distinctive: they drop vertically for 1.2-3 m, then adopt the characteristic ‘spread-eagle’ posture and parachute/glide in a descending arc. As they approach the chosen landing site, they pull upwards and stall sharply. Young et al. (2002) also regarded parachute geckos as gliders and argued that their feet were particularly important in their aerial movements. In fact, Young et al. (2002) proposed that the geckos might be more similar – in terms of aerodynamics – to flying treefrogs than to other squamates capable of aerial behaviour (like the lacertid Holaspis). Most recently, Vanhooydonck et al. (2009) compared the aerial behaviour of P. kuhli with other lizards: they concluded that parachute geckos are able to generate lift and described P. kuhli as a “true glider” (p. 2479).
Little known is that Ptychozoon is not the only gecko that engages in gliding behaviour. Luperosaurus – identified in some studies as the sister-taxon to Ptychozoon – has similar flaps and folds and glides too, and so do at least some species of Thecadactylus, like T. rapicauda (Pianka & Vitt 2003). Thecadactylus is not at all close phylogenetically to Luperosaurus and Ptychozoon (it seems to be a phyllodactylid, and not a gekkonine gekkonid like Luperosaurus and Ptychozoon). Still other gecko species are able to leap and drop great distances (as much as 6 m) when escaping predators.
As usual, there is more than one
As is so often the case, you only ever ordinarily hear about a single species of Ptychozoon (this being P. kuhli from Java, Sumatra, peninsular Malaysia, southern Thailand, Borneo, the Nicobar Islands*, Sulawesi, and possibly Myanmar) and hence might assume that it’s the only one. But, no, it’s actually one of six. The others are P. lionotum (often incorrectly spelt P. lionatum) from Thailand, Myanmar, peninsular Malaysia and India (Pawar & Biswas 2001), P. rhacophorus from northern Borneo, P. horsfieldii of Thailand, peninsular Malaysia, Borneo, Sumatra, Java and Myanmar, P. intermedium from the Philippines, and P. trinotaterra from Thailand and Vietnam (and only named in 1999). They differ markedly from one another, especially in tail anatomy and in the details of their scalation.
* The Nicobar Islands population was named last year as a new species: P. nicobarensis Das & Vijayakumar, 2009. So, there are currently seven species, not six. See the comments for more.
In the Bornean species P. rhacophorus, the tail tapers to a point (it looks fairly typical for a lizard) and the lateral lobes on the tail are very small. Rather than having regularly spaced, rounded tubercles on its flanks, it possesses randomly aligned large scales (some of which are semi-conical) that are scattered across its sides. The amount of digital webbing present in P. rhacophorus is small compared to that of the other species, and its lateral body fold is also smaller. In fact, P. rhacophorus looks like an ideal ‘intermediate’ between the more extreme parachute geckos and members of the probable sister-taxon to Ptychozoon, Luperosaurus (Russell 1979).
The tail tip of P. rhacophorus [A in the adjacent composite; from Brown et al. (1997)] doesn’t have any sort of flattening at its tip, but what seems to have happened in other species is that some or many of their lateral lobes have become partially or completely fused, thereby forming a terminal tail flap that might be rounded or squared-off at its tip. P. horsfieldii resembles P. rhacophorus in having a tapered tail [its tail is B in the adjacent composite], but its lateral tail lobes are bigger than those of P. rhacophorus and their shape (like scalene triangles) means that they look ‘swept back’ compared to the lobes of the other species. The lobes are bigger and have more rounded tips in P. intermedium [C in adjacent composite]; three or four of the lobes are fused to form the terminal tail flap in this species, and it also differs from the others in having small, slightly convex tubercles scattered across its sides. P. lionotum lacks tubercles on its sides and has a wide tail [D in adjacent composite] where about eight lobes near the tip are partially fused together, giving the tail a squared off, serrated tip. The weirdest tail is seen in P. kuhli [E in adjacent composite]: here, the lobes at the tail-tip have become imperceptibly fused, forming a rounded, paddle-like tail tip. Two obvious rows of enlarged, rectangular tubercles are obvious on the sides of this species; it also has large, rectangular scales lining its sides (as do P. horsfieldii, P. intermedium and P. lionotum). P. trinotaterra has a tail similar to that P. kuhli, but narrower. This species also has just a single row of flattened tubercles, or lacks tubercles entirely [scale bars in adjacent photos = 5 mm].
As usual, relatively little is known about the behaviour and lifestyle of parachute geckos. They are presumably nocturnal insectivores that frequent tropical evergreen habitats, but there are some indications that they can make a living around places where people live and they may be reasonably good at dispersing through disturbed areas (Pawar & Biswas 2001). Some authors say that parachute geckos sometimes walk around with the tail curled up over the back, perhaps to mimic scorpions (a behaviour previously reported for various other gecko species). How often they engage in the parachuting or gliding behaviour is not entirely clear: do they only employ this behaviour as an escape tactic, or do they deliberately leap from tree to tree while foraging or searching for mates? One of my favourite facts about these geckos is that specimens are sometimes captured in mist nets set for bats (Brown 1999), and this at least implies that their parachuting behaviour is a normal part of their daily life [captive P. lionotum shown below; from here… though note that its mostly smooth-edged tail is rather different from what’s shown in D above. Hmm].
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)
- Lamellae, scansor pads, setae and adhesion… and the secondary loss of all of these things (gekkotans part IV)
- The incredible leaf-tailed geckos (gekkotans part V)
- 300 years of gecko literature, and the ‘Salamandre aquatique’ (gekkotans part VI)
- Whence Uroplatus and… there are how many leaf-tailed gecko species now?? (gekkotans part VII)
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 – –
Brown, R. M. 1999. New species of parachute gecko (Squamata: Gekkonida; genus Ptychozoon) from northeastern Thailand and central Vietnam. Copeia 1999, 990-1001.
– ., Ferner, J. W. & Diesmos, A. C. 1997. Definition of the Philippine parachute gecko, Ptychozoon intermedium Taylor 1915 (Reptilia: Squamata: Gekkonidae): redescription, designation of a neotype, comparisons with related species. Herpetologica 53, 357-373 [version below included for Research Blogging purposes].
Brown, R. M., Ferner, J. W., & Diesmos, A. C (1997). Definition of the Philippine parachute gecko, Ptychozoon intermedium Taylor 1915 (Reptilia: Squamata: Gekkonidae): redescription, designation of a neotype, comparisons with related species Herpetologica, 53, 373-373
Heyer, W. R. & Pongsapipatana, S. 1970. Gliding speeds of Ptychozoon lionatum [sic] (Reptilia: Gekkonidae) and Chrysopelea ornata (Reptilia: Colubridae). Herpetologica 26, 317-319.
Marcellini, D. L. & Keefer, T. E. 1976. Analysis of the gliding hehavior of Ptychozoon lionatum [sic] (Reptilia: Gekkonidae). Herpetologica 32, 362-366.
Pawar, S. & Biswas, S. 2001. First record of the smooth-backed parachute gecko Ptychozoon lionotum Annandale 1905 from the Indian mainland. Asiatic Herpetological Research 9, 101-106.
Pennycuick, C. J. 1972. Animal Flight. Edward Arnold (London).
Pianka, E. R. & Vitt, L. J. 2003. Lizards: Windows the Evolution of Diversity. University of California Press (Berkeley).
Russell, A. P. 1979. The origin of parachuting locomotion in gekkonid lizards (Reptilia: Gekkonidae). Zoological Journal of the Linnean Society 65, 233-249.
– ., Dijkstra, L. D. and Powell, G. L. 2001. Structural characteristics of the patagium of Ptychozoon kuhli (Reptilia: Gekkonidae) in relation to parachuting locomotion. Journal of Morphology 247, 252-263.
Vanhooydonck, B., Meulepas, G., Herrel, A., Boistel, R., Tafforeau, P., Fernandez, V. & Aerts, P. 2009. Ecomorphological analysis of aerial performance in a non-specialized lacertid lizard, Holaspis guentheri. The Journal of Experimental Biology 212, 2475-2482.
Young, B. A., Lee, C. E. and Daley, K. M. 2002. On a flap and a foot: aerial locomotion in the ‘flying’ gecko, Ptychozoon kuhli. Journal of Herpetology 36, 412-418.