I have a lodger. To be more precise I have at least two lodgers. They are tokay geckos. Living where I currently do, in semi-rural southern Thailand, tokay (sometimes spelled tockay) geckos are pretty much everywhere, and they let you know it. Each evening I hear their calls echoing across the tracks that lead to my house. ‘Tok-kaaay ……tokaaay’ booms in the darkness, sounding more like a demented parrot than a shy lizard. Among lizards, geckos are well known for being the noisy buggers.
Most reptiles are fairly quiet, maybe the angry hiss if you disturb them, but otherwise you don’t hear them much. Geckos are the loud guys at the party, and they achieve this by having true vocal chords. We’ve known geckos have vocal chords for a long time, since 1839 in fact, when the German physician and anatomist Friedrich Gustav Jakob Henle published the monograph: Vergleichend-Anatomische Beschreibung des Kehlkopfs: Mit Besonderer Berücksichtigung des Kehlkopfs der Reptilien (Comparative anatomical description of the larynx: With special consideration of the larynx of the reptiles). For the biologists among you, this is the self-same Henle that the ‘loop of Henle’ within our kidneys is named after. Although primitive compared to mammals or birds, the larynx, vocal chords and associated structures found in geckos are quite sophisticated compared to other reptiles. This adds not only volume to their calls, but also allows a wide range of sounds to be made, with specialised warning calls and mating calls. The loud, two syllable ‘Tok-kay’ bellow from tokays is produced by the male, aimed at attractive females with his authoritative, manly voice and simultaneously deterring male competitors with his .auth … you get the idea. Where I live I clearly hear ‘Tok-kaay’ when my resident male tokay advertises his presence. But other people here something different. Many people here ‘Gek-koh’; indeed, this vocalisation is the reason behind its scientific name, Gekko gecko. Now this may be partly that we interpret sounds differently, but there is more to it than that. Recent studies have shown not only considerable range in sounds emitted by tokays, but that there appears to be significant regional variation (Yu et al., 2011). Simply put, tokay geckos have local accents. So maybe my southerner tokays really do say ‘tok-kay’ while those rough northerners say ‘gek-ko’. I’m sure there’s a research paper in there for someone.
As geckos go, tokays are big. A full grown male can be 30cm (12 inches) long. The largest known gecko, Leach’s giant gecko, can grow to about 36cm (14 inches) long, so tokays are not that far behind. But more than that, they are quite beautiful (which you’d find hard to say about Leach’s giant gecko). The ones here are light blue-grey and covered in vivid orange-red spots, complete with huge dark green eyes. Two colour morphs are known; to the north and east of their range tokays have black rather than red spots. Currently only two subspecies have been identified; the common or garden tokay: Gekko gecko gecko, which occurs all the way from India to Southern China (and incidentally includes red and black spotted morphs) and the rather elusive Gekko gecko azhari, described by Mertens in 1955 and known only from Bangladesh. Once more genetic studies are completed it may be that we find that tokays are really a ‘species complex’ rather than a single species.
Tokays have yet another unusual feature, a rudimentary third eye (known as a pariental eye) on the top of their heads. Pariental eyes are found in a great many creatures: many reptiles and amphibians and some fish species (but not in birds or mammals). It does not form an image, like the paired visible eyes, but is sensitive to changes in light levels. The pariental eye is essential an outgrowth of the pineal gland, a small endocrine gland deep within the brain. The pineal gland is especially known for producing melatonin, the hormone, many of you will know, responsible for regulating sleep patterns (and sometimes sold as a supplement to counteract jet-lag or insomnia). The pariental eye, via the pineal gland, is thought to regulate circadian rhythm (day-night sleep cycle) and seasonal rhythms in activity, by reacting to changing light levels. Although we do not have a pariental eye, we do have a pineal gland, indeed almost all vertebrates do, and it preforms the same functions in us, letting our bodies know when to sleep and when to wake up, as it does in tokays and other reptiles. The difference appears to be that we get our visual cues to light levels through our paired eyes. Which then begs the obvious question; so… er geckos, and monitors, and frogs have paired eyes as well; so why do they need to growth a third eye in order to monitor light levels? It’s a tricky question. It turns out that maybe the pariental eye isn’t just about providing information on light levels to regulate sleep patterns. Numerous studies from the mid-1970s on have shown that, in many different lizards, the pariental eye is directly involved in regulating behaviour related to thermoregulation. Lizards are cold-blooded (ectotherms, to use the scientific term). That means they cannot internally regulate their body temperature; they take on the temperature of their environment. Consequently, in order to keep their body temperature within the range where their muscles will work well and the body’s chemical reactions occur at the correct rate, they must adopt specific behaviour patterns. Basking in sunlight to warm up, moving to shade to cool down. Well it turns out that without the information from the pariental eye, lizards don’t do that; their behaviour becomes a little more random. For tokays it’s a little different. They are nocturnal, so they can’t really move to bask in sunshine to warm up … or move to shade to cool down. In fact, they are known as thigmothermic, which is a fancy way of saying they warm up or cool down by touching warmer or cooler surfaces. An interesting observation is that the two major groups in which pariental ‘third’ eyes are never found are birds and mammals. Of course the other key feature these two share is that they are warm blooded (endotherms: they regulate their internal temperatures irrespective of external temperate). Recent studies of fossil evidence tend to suggest that, in the predecessors of modern mammals, the pariental eye disappeared around about the same time as warm-bloodedness evolved. (Benoit et al. 2016) As luck would have it, the disappearance of a third eye is fairly easy to identify in the fossil record as the connecting nerves fibres pass through a small hole at the top of the skull. So it would seem, for reasons still not fully understood, the real need for a third eye is to allow cold-blooded animals to thermoregulate.
I’ve digressed quite a long way for geckos though, so let’s move back to them. Probably the ‘superpower’ that geckos are most famous for is the ability to run up, and cling to, smooth sheer surfaces. Apart from my tokays, I have many house geckos living, surprise-surprise, in my house. If you’re going to be picky, these are spiny-tailed house geckos (Hemidactylus frenatus). They’re mostly pretty unobtrusive fellas. They eat a fair few mosquitoes, so that’s always a good thing, if fact they eat most things smaller than themselves, including juvenile house geckos.
But they’re quiet and generally well-behaved lodgers, apart from leaving lots of gecko crap on the floor for me to sweep up every morning. I mostly see them running across walls, or across my windows chasing prey and squabbling with each other. They cut an eerie form when caught in the light, on the outside of frosted windows.
There have been various theories advanced over the years as to how exactly geckos achieve this. From suction pads to Van der Waals forces (intermolecular forces created by fluctuating polarisations of nearby particles as a consequence of quantum dynamics …. can you tell I’m out of my depth here?). Each toe is covered in rows of wrinkled skin forming parallel ridges (like they’ve stayed in the bath too long); these in turn are covered in tiny spatula-like bristles (lamellae). Capillary action (think of two sheets of glass stuck together with a thin film of water between) between these spatula-like surfaces and the walls is believed to part of the answer. Van der Waals forces (description above, I’m not repeating it) is also thought to play a part. However, recent studies suggest that actually the major force is electrostatic, and that is what primarily allows to gecko to cling to impossible surfaces (Izadi, Stewart and Penlidis, 2014). So currently, our best guess is it’s a combination of things that give the geckos their ‘superpower’. Whether this is the definitive answer, or whether someone will come up with new evidence for a different explanation, time will tell. One thing we do know is that it is pretty impressive. Researchers William Stewart and Timothy Higham, of University of California, Riverside, found that tokay geckos were in a league of their own here. In lab experiments where they attached pulley weights to tokays that climbed up an acrylic sheet, they found that it took up to 20 times their bodyweight before they started to slip. That’s the equivalent of an 80kg ((~180lb) man gripping on to vertical acrylic, with a saloon car strapped to his back. This is real Marvel superhero territory. But even more extraordinary still, the researchers found that the grip remained just as strong in dead geckos. So when I look at geckos on my wall, and they seem to be hanging on with no physical effort whatsoever, well that’s probably true. So next time you see a tokay gecko looking striking with blue-green skin and bright orange-red spots remember, that’s it’s superhero costume it’s wearing.
All text and photographs © Colin Munro
Benoit, J., Abdala, F., Manger, P.R., and Rubidge, B.S. 2016. The sixth sense in mammalian forerunners: Variability of
the parietal foramen and the evolution of the pineal eye in South African Permo-Triassic eutheriodont therapsids. Acta Palaeontologica Polonica 61 (4): 777–789.
Stewart, W.H. and Higham, T.E. 2014. Passively stuck: death does not affect gecko adhesion strength. Biol. Lett. 10:20140701. http://doi.org/10.1098/rsbl.2014.0701
X. Yu, Y. Peng, A. Aowphol, L. Ding, S.E. Brauth & Y.-Z. Tang (2011). Geographic variation in the advertisement calls of Gekko gecko in relation to variations in morphological features: implications for regional population differentiation. Ethology Ecology & Evolution, 23:3, 211-228, DOI: 10.1080/03949370.2011.566581