Now that the main gekkotan groups have been introduced, it’s time to get down to some of the details. We begin with stuff on lifestyle and behaviour… [gekkotan motley below - mostly assembled from wikipedia - features (top, left to right) Aeluroscalabotes felinus, Pachydactylus bibronii, Rhacodactylus ciliatus and (bottom, left to right) Nephrurus amyae, Phyllodactylus xanti and Phelsuma madagascariensis].
Gekkotans are mostly insectivorous, but some of the large species prey on other lizards, and even on snails, birds, mammals (including bats) and small snakes. Nectar and pollen is eaten by species of Phelsuma, Hoplodactylus and Naultinus. In general, geckos are predominantly visual predators with large, sensitive eyes. Some species have elaborately shaped, ‘notched’ pupils that close down to form a series of dots connected by zig-zagging lines. All gekkotans except eublepharids have done away with moveable eyelids, and the eyes are covered by immobile, transparent eyelids termed spectacles. They keep these clean by licking them. So it’s not that the eyelids are absent: rather, they’re fused, and the lower one is entirely transparent. Does anyone know how animals with transparent eyelids sleep?
Gekkotans generally have short, rounded tongues that aren’t used much in detecting or discriminating prey.
The majority of gekkotans are nocturnal, and nocturnal behaviour is so widespread across the clade that it is likely primitive for the gekkotan crown-group. However, diurnality has evolved in many gekkotan clades (most famously in the Phelsuma day geckos of the western Indian Ocean). Some day geckos (like the recently extinct species P. gigas from Rodrigues) are or were nocturnal, meaning that these animals switched from diurnality (primitive condition for Squamata) to nocturnality (primitive condition for crown-Gekkota), then to diurnality (primitive condition for Phelsuma), and then back to nocturnality (derived condition within Phelsuma) within their evolutionary history (Austin et al. 2004). Oh, and some extant diurnal species within Phelsuma may be members of a clade that was ancestrally nocturnal, in which case they went from diurnality to nocturnality to diurnality to nocturnality, and finally back to diurnality [the day gecko Phelsuma inexpectata shown here; from wikipedia].
Lizards with loud voices
About 80 gecko species are highly vocal, and some species make clicks, chirps, quacking noises or loud growls or barks. The word gecko is supposedly a corrupted onomatopoeic reference to the call of the Tokay Gekko gecko, which sounds like ‘Tuck-too’ or ‘Tow-kay’ [Tokay shown below, from wikipedia].
Gecko calls mostly function in social and/or defensive behaviour: geckos in general are territorial reptiles and, in many vocal species, males use their calls to advertise their presence and to keep rivals away. Some species live colonially and have a dominance hierarchy. Among such social species, large choruses with an ‘almost deafening’ volume have been reported.
How do geckos vocalise? By vibrating vocal cords housed within a cartilaginous larynx. In this respect they differ from other squamates: when non-gekkotan squamates make noises, they (mostly) do so by forcing air past the glottis, or by vibrating the tissues around the glottis. Laryngeal anatomy has proved complicated and diverse within gekkonid geckos: in Ptenopus, for example, the larynx is anteriorly placed in the head, asymmetrical in males, and superficially recalls the vocal apparatus of some frogs (Rittenhouse et al. 1998). Teratoscincus – the wonder geckos or frog-eyed geckos – stridulates by rubbing the armour plates on its tail together. Like many geckos, Teratoscincus species can autotomise (= shed or drop) their tails, and a thrashing autotomised tail also makes a loud hissing noise (Pianka & Vitt 2003).
Most geckos of both sexes possess paired, sac-like structures located posterior to the cloaca – the function of which is unknown – and males also possess bony cloacal rods… the function of which is also unknown (they might be something to do with widening the cloacal aperture during mating). Cloacal sacs and bones are absent in sphaerodactylids (Kluge 1982, Gamble et al. 2007).
Eggs, eggshells and cervical sacs
Most gekkotan species produce just one or two white, round or oval eggs. In contrast to many other lizards, they don’t exhibit parental care of any sort. The eggshell is soft and parchment-like in eublepharids, diplodactylines and pygopodids, but (once dry) hard, highly mineralised and impermeable in most gekkonids (and in the diplodactyline Eurydactyloides). While carphodactyline, diplodactyline and pygopodid eggs are elongate (including those of Eurydactyloides), those of some gekkonids (particularly larger species) are more spherical (Kratochvíl & Frynta 2005).
Gekkonid eggs are adhesive when first laid, and stick hard to the surface they’re laid on. This might be the inside of a hollow branch, under bark, on the underside of a rock or – in species that associate with humans – the side of a window-frame or cavity within a wall or roof. Females will sometimes lay communally, with as many as 186 eggs being recorded from one location.
Thanks to their mineralised shells, gekkonid eggs can withstand exposure to sea water. This feature – combined with the small size of most geckos and their habit of associating with humans – means that some species have been transported inadvertently on boats and in timber. The Pacific gecko or Common house gecko Hemidactylus frenatus [shown above, from wikipedia] occurs on islands across the Indian and Pacific Ocean, throughout tropical Asia, Australia, on St. Helena in the South Atlantic, in Somalia, and in the USA, Mexico and Panama. It’s also been reported on occasion from Brazil, England and elsewhere. It’s therefore become both a human commensal and an alien invader, and may in fact be responsible for out-competing, and causing the local extinction of, other gecko species (Bauer 2000). Some species (like the Mourning gecko Lepidodactylus lugubris of the Pacific region) are parthenogenetic, and this has also helped them to be good colonisers of new places. Viviparity is present in the diplodactylines Naultinus and Hoplodactylus from New Zealand, and Rhacodactylus from New Caledonia.
As just mentioned, those hard-shelled gekkonid eggs have highly mineralised shells. Animals that produce calcareous eggshells have to somehow store and mobilise the calcium they need. Female birds produce a special kind of bone (medullary bone) that acts as a calcium reservoir: it forms on the inside surfaces of their bone walls, and when the time is right, they’re able to mobilise huge amounts of calcium for eggshell generation. How do geckos produce all the extra calcium they need for eggshell production? The answer seems to be that they possess special, calcium-storing sacs or pouches – called endolymphatic glands – on either side of the neck (the sacs of Phyllodactylus shown here, from Kluge (1987)). Prior to egg-laying, these sacs enlarge and fill up with a sort of ‘calcium milk’. Because some geckos have semi-translucent skin, the sacs are visible in some individuals at some times. I’m sure there must be some funky x-ray images of reproductively active female geckos out there, but I can’t find any right now.
I should point out that endolymphatic glands are not unique to gekkotans – they’re actually widespead within squamates – and enlarged neck sacs are not unique either, as they’re also present in such iguanians as chameleons and agamids.
More on gekkotans to come!
For previous Tet Zoo articles on gekkotans (woo-hoo – finally I can say that!) see…
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.
Gamble, T., Bauer, A. M., Greenbaum, E. & Jackman, T. R. 2007. Evidence for Gondwanan vicariance in an ancient clade of gecko lizards. Journal of Biogeography 35, 88-104.
Kratochvíl, L. & Frynta, D. 2005. Egg shape and size allometry in geckos (Squamata: Gekkota), lizards with contrasting eggshell structure: why lay spherical eggs? Journal of Zoological Systematics 44, 217-222.
Kluge, A. G. 1982. Cloacal bones and sacs as evidence of gekkonid lizard relationships. Herpetologica 38, 348-355.
– . 1987. Cladistic relationships in the Gekkonoidea (Squamata, Sauria). University of Michigan Museum of Zoology Miscellaneous Publications 173, 1-54.
Pianka, E. R. & Vitt, L. J. 2003. Lizards: Windows the Evolution of Diversity. University of California Press (Berkeley).
Rittenhouse, D. R. & Russell, A. P. 1998. The larynx and trachea of the barking gecko, Ptenopus garrulus maculatus (Reptilia: Gekkonidae) and their relation to vocalization. South African Journal of Zoology 33, 23-30.