Arboreal adaptations
Geckos can be roughly divided into either arboreal or terrestrial species, with the two types being differentiated, among other things, by the structure of their feet.
Arboreal species, which perhaps represent the stereotypical picture of geckos, are excellent climbers and can scale even the smoothest of surfaces, such as glass, with ease. They are able to do this as they possess digital lamellae: essentially toe pads that enable to the gecko to adhere to the surface that it's climbing. These lamellae, which give the underside of the gecko's toes their characteristic plated appearance, are formed by thousands of hair-like setae; in the case of the tokay gecko (Gekko gecko), Autumn and Peattie (2002) estimate that there are 14,400 such setae per mm2 on the gecko's foot, equating to approximately 6.5 million setae on a single gecko. The setae are in turn are divided into hundreds of flat-tipped spatulae, which measure about 0.2 microns in length, and are tiny enough to take advantage of the weak electrodynamic forces that exist between molecules (van der Waals force), thus enabling the gecko to stick to underlying surface. It has been estimated that a single setae can yield an adhessive force of 200 microNewton (200 μN); a single gecko could therefore generate 130 kg force. In addition to digital lamellae, arboreal geckos frequently possess prehensile tails, which are also used to assist climbing.
In contrast, geckos of the superfamily Eublepharoidea, the so-called eyelash geckos, lack the well-developed digital lamellae of arboreal species. Instead, these terrestrial geckos possess claws, which are covered by scales forming a sheath.
Photographs showing the differences in the structure of the feet of arboreal and terrestrial gecko species. The plate-like lamellae that enable arboreal species (in this case Rhacodactylus chahoua) to climb even smooth surfaces are clearly visible in the photograph of the toes of this species shown on the left. In contrast, terrestrial geckos such as G. luii lack these lamellae, but possess scales that sheath their claws.
References
Autumn, K. & Peattie, A.M., 2002. Mechanisms of adhesion in geckos. Integrative and comparative biology, 42 (6), pp. 1081 - 1090.
Defecation
A further feature that often distinguishes between terrestrial and arboreal geckos are their toilet habits. Terrestrial geckos frequently establish a defecatoria; this is essentially a localised area that is used as a toilet rather than faeces being randomly distributed throughout the enclosure. If more than one gecko is housed in the enclosure, then they often use the same communal site. In contrast, most arboreal species simply scatter faeces randomly within a vivarium, so that the floors, walls and surfaces of plants etc can all become soiled.
In their study into the behaviour of the terrestrial western banded gecko (Coleonyx variegatus), Carpenter and Duvall (1995) hypothesised that the establishment of defectaria fulfilled several functions. Firstly, it was suggested that, by placing a distance between itself and its discarded faeces, the gecko reduced the risk that the smell of the faeces would attract the attention of predators. Additionally, it was suggested that, as geckos can differentiate between the scent of their own faeces and that of other individuals, the faeces were actually being used as territorial markers, with the deposition of additional faecal matter not only strengthening the chemical signal, but also increases its duration. The establishment of defectoria may therefore enable geckos to distinguish between familiar local individuals inhabiting adjacent territories and potentially threatening intruders. A third possibility suggested in the paper is that, as defectoria were often located in hides, their production may reflect the fact that, following nocturnal hunting, geckos return to daytime shelters where the higher diurnal temperatures increase their digestive rate and result in the deposition of faecal matter in the hide.
Irrespective of whether the species is terrestrial or arboreal, the general appearance of healthy faeces is broadly similar. Healthy droppings consist of two parts; a dark brown / black component, which represent the faecal matter and a paler, white region, which are in fact urates (unlike mammals, reptiles excrete uric acid as a dry mass rather than a liquid. However, in a well-hydrated individual, small amounts of liquid can be excreted as well). Overall, the faeces of a healthy animal should be well-formed and slightly moist, but not too dry (excessively dry faeces, especially if accompanied by yellowish urate can indicate dehydration) nor too runny. Any faeces that appear abnormal, whether in terms of texture or colour or because they seem excessively smelly could indicate potential health problems; it is therefore advisable to have a faecal sample tested by a vet. Examples of both healthy and abnormal faeces can be found on the excellent pages relating to leopard gecko health on the SleepyDee Geckos website.
References
Carpenter, G.C., & Duvall, D., 1995. Fecal scent marking in the western banded gecko (Coleonyx variegatus). Herpetologica, 51 (1), pp. 33 - 38.
Defensive behaviour
Geckos can display a range of defensive behaviour; in captivity, this is often most apparent in either young animals, who have yet to become accustomed to interactions with humans, or when older animals have been disturbed or startled unexpectedly. Frequently, the first response is flight; the animal will attempt to run away or seek shelter in a hiding place. This can sometimes be accompanied by an attempt to distract a perceived predator. In many species this takes the form of slow deliberate wagging of the tail; the tail is raised vertically and waved from side to side in a slow, sinusoidal motion. The aim of the tail wagging is to transfer the attention of the predator, causing it to attempt to strike the tail rather than the body of the gecko. In extreme cases, the gecko can then drop its tail (a process known as caudal autotomy) and, while the predator is distracted by the tail, which continues to writhe for several minutes after being dropped, the gecko uses the opportunity to escape. Tail waving is frequently accompanied by the gecko standing with its legs extended, giving the impression that it's standing on tip toes, while at the same time arching its back in an attempt to make itself appear as large as possible. Marcellini (1977) observed that similar displays also occur during aggressive territorial encounters between geckos of the same species and suggested that the aim of the display was to intimidate a rival before fighting commences. Many species will also gape and hiss / bark; if the threat persists, geckos can (and will) bite. Although gecko bites are generally superficial, those of larger species, such as leopard geckos, can be painful and, while they may not do any lasting damage, they can certainly draw blood.
Some species display more complex responses to threats. For example, geckos of the genus Strophurus, including eastern spiny-tailed geckos, employ two characteristic responses (Bustard, 1964). Firstly, the animals display conspicuous warning colours by gaping to reveal the electric blue lining of their jaws, which contrasts to the dark purple colouring of the tongue and roof of the mouth. Secondly, these species secrete a pungent, sticky liquid from a gland that runs along the tail. When attacked, this liquid is exuded through the skin, with more forceful ejection sometimes resulting in the liquid reaching a distance of 60cm from the gecko. Analysis of these development of the distinctive features has led Melville et al (2004) to suggest that the caudal glands of Strophurus evolved around 25 million years ago.
References
Marcellini, D., 1977. Acoustic and visual behaviour of Gekkonid lizards. American Zoologist, 17 (1), pp. 198 - 200.
Bustard, H.R., 1964. Defensive behaviour shown by Australian geckos, genus Diplodactylus. Herpetologica, 20 (3), pp. 251 - 260.
Melville, J., Schulte, J.A., & Larson, A., 2004. A molecular study of phylogenetic relationships and evolution of antipredator strategies in Australian Diplodactylus geckos, subgenus Strophurus. Biological Journal of the Linnean Society, 82, pp. 123 - 138.
Senses
Hearing
The exterior of the ear of geckos appears in the form of a small pit immediately behind the jaw line. A notable feature of some species, such as leopard geckos, is that a light shone behind the gecko will be visible through the aperture of the ear, as shown in the adjacent image.
It has been established that the hearing of geckos is typically less sensitive than that of mammals, ranging between approximately 100Hz and 10,000Hz, with the greatest sensitivity to audio stimuli obeing between about 100Hz and 3,000Hz (Peterson, 1966; Wever and Hepp-Raymond, 1967; Wever and Werner, 1970).
References
Peterson, E.A., 1966. Hearing of the lizard: some comments on the auditory capacity of a non-mammalian ear. Herpetologica, 22 pp. 161 - 171.
Wever, E.G & Hepp-Raymond, M.C., 1967. Auditory sensitivity in the fan-toed gecko, Ptyodactylus hasselquistii puiseuxi Boutan. Proceedings of the National Academy of Sciences, 57, pp. 681 - 687.
Wever, E.G & Werner, Y.L., 1970. The function of the middle ear in lizards: Crotaphytus collaris. Journal of Experimental Zoology, 175, pp. 327 - 342.
Sense of smell
Unlike in humans, where the nose is the only olfactory organ, geckos can also detect scents using Jacobson's organ. This vomeronasal organ is located in the roof of the mouth and acts as a chemoreceptor to detect pheromones in order to locate a possible mate, to warn of potential predators and to detect food items. Scent particles are picked up by numerous tongue flicks or labial (lip) licks and are then transferred into the mouth, where fluids then carry the chemical signature to the Jacobson's organ.
Geckos use their tongues to transfer scent particles to their vomeronasal organ.
Interestingly, research has suggested that the hunting method employed by different gecko species is intimately related with the use of Jacobson's organ (Cooper, 1995; Cooper & Habbeger, 2000). In this research, various geckos species were exposed to swabs of different scents, including crickets, plant material and control chemicals, such as de-ionized water. It was observed that in species that actively forage for their prey, for example leopard geckos and Chinese cave geckos, there was a direct relationship between tongue flicking and a biting response to the scent of prey items, while plant material and other stimuli were ignored. However, in ambush predators, such as leaf-tailed geckos and tokay geckos, biting occurred in response to prey items without prior tongue-flicking. The researchers hypothesise that a foraging gecko therefore uses tongue flicking to detect the chemical scent of its prey, thus enabling the gecko to locate its food. However, for a gecko that relies upon ambushing passing insects, no advantage is to be gained from using tongue flicking.
References
Cooper, W.E., 1995. Prey chemical discrimination and foraging mode in Gekkonoid lizards. Herpetological Monographs, 9, pp. 120 - 129.
Cooper, W.E., & Habegger, J.J., 2000. Lingual and biting to food chemicals by some Eublepharid and Gekkonid geckos. Journal of Herpetology, 34 (3), pp. 360 - 368.
Vision
Unlike other geckos, geckos of the subfamily Eublepharinae, including leopard geckos and Chinese cave geckos retain movable eyelids. When closing their eyes, it is always the lower eyelid that moves upwards. The eyelids of other species have fused to form the brille; a transparent disc that covers and protects the eye. Nocturnal gecko species can be distinguished by the shape of the pupil, which invariably forms a distinctive narrow, vertical slit that dilates in low light levels. This contrasts to the pupils of diurnal species, whose whose eyes have circular pupils.
The photograph above left shows the lidded eye of a Chinese cave gecko; the species, G. luii, belongs to the subfamily Eublepharinae. In contrast, the eastern spiny-tailed gecko on the right shows an eye more typical to geckos on the whole, in that a moveable eyelid is absent and the eye is instead protected by a transparent brille.
The eyes of geckos have only one type of photo-sensitive cell, these being cones, the same type of cells that are used by mammals to differentiate between colours in bright conditions. Unlike mammals, they lack rods; the cells which detect light at low levels, but only only in black and white. Experiments have shown that specimens of Tarentola chazaliae were able to distinguish between grey and blue cards when light levels were insufficient for humans to be able to differentiate between the two (Roth and Kelber, 2004). Further research has suggested that the vision of T. chazaliae is approximately 350 times more sensitive than human cone vision at low light thresholds (Roth et al, 2009).
References
Roth, L.S.V., and Kelber, A., 2004. Nocturnal colour vision in geckos. Proceedings of the Royal Society of London B, 271, S485 - S487.
Roth, L.S.V., Lundström, L., Kelber, A., Kröger, R.H.H. and Unsbo, P., 2009. The pupils and optical systems of gecko eyes. Journal of Vision, 9 (3), pp. 1 - 11.
Shedding
Like all reptiles, geckos will periodically shed their skin. The frequency with which they do so is highly variable; hatchlings and juveniles that are still growing may shed as frequently as every two weeks or so, whereas mature animals may shed only once every four to six weeks. The skin of an animal that is about to shed takes on a dull, almost tissue paper-like appearance, often with a bluish-grey tinge. Often this is the only indication that a keeper will have that their gecko is going to shed, as the animals are frequently quite secretive at this time and hide themselves away from view during the shedding process itself.
Shedding usually starts at the snout, with the gecko often rubbing and scrapping its body along the rough surfaces of hides, pieces of cork bark etc in an attempt to dislodge the skin that's being jettisoned. Although the skin of a gecko may be dull for a day of so before shedding actually begins, the shedding process itself is relatively rapid, often taking less than half an hour to complete in a healthy animal. Occasionally however, a gecko can experience problems removing all of its skin; the toes, tip of the tail and the area around the eyes are all particularly prone to retaining shed. Advice on how retained shed can be removed can be found here.
The photograph above left shows a tessellated gecko just starting the shedding process, with the old skin, which displays a clear bluish-hue, just starting to detach from its snout. The leopard gecko in the right hand image is starting to remove the shed from lower down its body; note the contrast between the pale areas yet to shed the old skin and the darker, fresher looking newly shed regions.
One noticeable trait among many gecko species is that they will consume the shed skin, a characteristic known as dermatophagia. Just how widespread this feature is was confirmed by Weldon et al (1993), whose study showed that the majority of geckos species exhibited this behaviour. It is noticeable though that Australian species, including Strophurus sp and Diplodactylus sp simply discard their skin. Although the reasons for this behaviour are not known for certain, Bustard and Maderson (1965) suggested that it the shed skin represented a source of protein for the animal in question.
References
Bustard, H.R., & Maderson, P.F.A., 1965. The eating of shed epidermal material in squamate reptiles. Herpetologica, 21 (4), 306 - 308.
Weldon, P.J., Demeter, B.J. & Rosscoe, R., 1993. A survey of shed skin-eating (dermatophagy) in amphibians and reptiles. Journal of Herpetology, 27 (2), 219 - 228.
Tail signals
In addition to the defensive posturing with the tail mentioned earlier, geckos can also undertake other tail movements for a variety of reasons. Geckos, especially hatchlings and young juveniles, frequently vibrate their tails rapidly from side to side when excited, for example when they spot prey. Additionally, male geckos will rapidly shake their tails against the floor of their enclosure if they perceive that their territory is being encroached by an intruder: if the 'intruder' signals back in a similar manner and is therefore another male (or a female that is unwilling to mate), then this is a prelude to fighting; conversely, if no 'reply' is made, the other gecko is generally female and mating will occur.
Thermoregulation
Like all reptiles, geckos are poikilothermic; their internal body temperature is determined by the ambient temperature of their environment. It is therefore essential to provide species with a range of temperatures in their enclosures, as this temperature gradient will enable the gecko thermoregulate; in other words, to select the optimum temperature it requires at any given time.
As temperature affects so many aspects of the physiology of geckos, one cannot over-emphasise just how crucial it is to provide a correct temperature range in enclosures. Incorrect temperature can adversely affect the digestion of food and, therefore, the growth rate of geckos (Autumn & DeNardo, 1995); it can even impact upon the auditory sensitivity of animals and hence reduce their ability to catch prey in the first place (Werner, 1976).
One final point to make is that many care sheets suggest that a night-time drop in temperature should be provided for some species of gecko, including leopard geckos. However, research suggests that the preferred body temperature for many species (including leopard geckos) remains constant, with no nocturnal decrease being discernible (Angilletta & Werner, 1998; Autumn & DeNardo, 1995). For that reason, I maintain a constant level of heat in all my vivaria, with no reduction at night.
References
Angilletta, M.J. & Werner, Y.L., 1998. Australian geckos do not display diel variation in thermoregulatory behavior. Copeia, 1998 (3), 736 - 742.
Autumn, K. & DeNardo, D.F., 1995. Behavioral thermoregulation increases growth rate in a nocturnal lizard. Journal of Herpetology, 29 (2), 157 - 162.
Werner, Y.L., 1976. Optimal temperatures for inner-ear performance in gekkonid lizards.Journal of Experimental Zoology, 195, pp. 319 - 352.
