Thick-tailed gecko

About the species

Underwoodisaurus milii is a compact gecko with a large head, large lidless eyes and a tail that looks like a little carrot. The tail is the animal's fat store, and a well-fed animal has a thick, plump tail. The legs are slender and the toes long and thin, without the adhesive lamellae that many other geckos have. It therefore cannot climb glass, but runs quickly across rock and soil.

The back can be dark purplish brown, reddish brown or pale sandy with small white, yellow and black spots, and the belly is white. An original tail is often dark with pale bands, while a regrown tail is smoother and more uniformly coloured.

Size varies between populations. Adults reach 12-14 cm including the tail and about 8-10 cm from snout to vent, and animals from the east are usually slightly smaller than animals from the west. In captivity, adults typically weigh 14-25 g. Young males and females look alike, and sex can only be determined reliably after several months.

The species was described by Bory de Saint-Vincent in the 1820s and has since been placed in several genera, including Phyllurus, Gymnodactylus and Nephrurus. The genus name Underwoodisaurus dates from 1965, and today the genus is placed in the family Carphodactylidae together with the knob-tailed geckos. The species name honours the French naval officer Pierre Bernard Milius. The genus's other species, U. seorsus from the Pilbara, was described in 2011.

The best-known feature is the defence. If the gecko feels threatened, it rises high on its legs, arches its back, opens its mouth wide, barks and throws itself forward. This is mostly bluff, and the tail is normally only shed when the animal is really seized. Many juveniles are aggressive, but most calm down with age. Like other geckos without eyelids, it licks its eyes clean with its tongue.

Care

The enclosure

An adult can be kept in a low enclosure of at least 60 × 40 × 30 cm (length × width × height), and bigger is always better. For a male with two females, the enclosure must be correspondingly larger, because each animal needs its own hide. Floor area matters more than height, but the lid must fit tightly, because the gecko can climb a branch and escape.

The substrate should retain moisture and at the same time be suitable for digging. A mixture of roughly half fine, washed sand and half unfertilised soil or coconut fibre in a layer of 3-5 cm works well. Avoid calcium sand. Make a dry, warm zone at one end and a cooler and slightly damper one at the other.

Provide at least two hides, one at each end, and a damp hide with moist moss in the cool end, so that shedding goes easily. Stones, bark and cork tubes provide hiding places and a natural look. Place stones directly on the floor of the enclosure or glue them in place, so that they cannot topple when the animal digs beneath them. Ventilation should be good, but not so strong that the moisture disappears completely. Damp and stagnant is the worst combination.

Temperature

The species needs a clear temperature difference between the two ends. Keep the cool end at 22-26 °C and the warmest spot at the floor at 28-32 °C. At night the temperature may fall to 15-20 °C. The animal's behaviour is the best guide: if it lies in the warm end all the time, the enclosure is too cool, and if it always hides in the cool end, it is too warm.

Preferably heat from below with a heat mat under one end, controlled by a thermostat. A lamp from above mostly heats the air and dries out the substrate. Measure with a digital thermometer at both ends, and do not let the temperature stay above 32 °C for long.

Many keepers give the animals a 6-12 week cool winter with 18-20 °C by day and 12-15 °C at night. It has not been shown that this is necessary for breeding, but it does no harm to a healthy, well-fed animal.

Humidity and water

The species comes from dry regions and thrives at a humidity of 30-50 %. The enclosure should be neither constantly damp nor completely bone dry. Mist the cool end lightly two or three times a week, so that the gecko can lick droplets from the glass and stones. The substrate in the warm end should be dry, so that mould does not develop.

The damp hide with moss is most important during shedding. If the animal has difficulty shedding, keep the moss a little wetter for a week or so. Provide a shallow, stable water bowl and change the water regularly, even though many animals prefer to drink droplets. A hygrometer shows whether you are hitting the right level.

Lighting and UVB

The species is nocturnal, and most keepers manage without UVB as long as the animal gets calcium with vitamin D3 through the food. Some nevertheless use a weak UVB tube for shade-dwelling species (Ferguson zone 1), so that the UV index is at most 1-2 at the most brightly lit spot, because it gives a clearer day-night rhythm and an extra safeguard for the bones. If you choose UVB, there must always be shade and hides in the rest of the enclosure, and the tube must be replaced according to the manufacturer's instructions, typically after 6-12 months.

Keep the light on for about 12 hours a day and dark at night. Avoid direct sunlight and strong spotlights, which stress the animal and dry out the enclosure. Coloured night light is unnecessary.

Feeding

The gecko feeds exclusively on insects and other small arthropods. Offer a varied mix of crickets, locusts, dubia roaches and black soldier fly larvae, and occasionally mealworms or superworms. Prey must not be larger than the distance between the animal's eyes, and the feeder insects should themselves have been given nutritious food over the last day.

Juveniles get two or three small insects every day or every other day. Adults get four or five larger insects every third or fourth day. The tail shows whether the amount is right: if it is thin, give more, and if it is very broad, cut back.

Dust the insects with calcium at every second or third feed, with calcium containing vitamin D3 for juveniles and egg-laying females, and give a multivitamin once a week. Feed with tweezers or in a shallow dish, so that the insects do not hide, and remove live insects that are not eaten.

Behaviour and handling

Many young animals are suspicious and bark, lunge forward and try to bite, but this is mostly bluff, and most calm down. Adults often become trusting and tolerate calm handling. Approach from below with a flat hand rather than grabbing from above, and let the animal walk up by itself. Never grab the tail, because it is easily shed and regrows shorter and more uniformly coloured. Keep handling short, and leave the animal alone while it is shedding.

The species is peaceful and happily shares hides with conspecifics. An adult male can be kept with one or more females, and females can usually be kept together. Two adult males must never be kept together, and juveniles should not be kept with adults or other species. Make sure all animals get food and are not crowded out, and separate them if unrest arises.

Breeding

Sexual maturity and sexing

Males are recognised by a distinct bulge at the base of the tail, where the hemipenal pouches sit. The species grows slowly, and sex can only be determined reliably when the animals are about 9-10 months old. The age at sexual maturity is poorly documented, so use size and condition as a guide. Do not breed a female until she is fully grown, weighs well over 14 g and has a plump tail. Small and slender females are drained of calcium by egg-laying.

Mating

In the wild the species breeds in the southern spring and summer, but in captivity the season can stretch over most of the year. Many keepers give the animals a 6-12 week cool winter or vary the day length to kick-start breeding. It has not been shown that the cooling is necessary, so if your animals already breed, you can skip it.

Introduce the male to the female, and watch them at the start. The male bites the female on the neck during mating, and there may be small scratches. Both sexes can bark during the mating period. The female can lay several clutches after one mating, but the pair can also stay together throughout the season. Give the female a rest if she begins to lose weight.

Egg-laying and clutches

The female usually lays two eggs at a time, occasionally only one. The eggs can be seen through the belly skin one to two weeks before laying. She then lays a new clutch about every 30 days during the season. This can amount to up to 9 clutches in a long season, but often fewer, and the first clutch is often infertile.

Place a laying box in the enclosure, for example a plastic tub with a hole in the lid and damp, but not wet, soil or sphagnum. Otherwise the female buries the eggs under a hide. Give her plenty of calcium and vitamin D3 throughout the season, and keep an eye on her weight.

Incubation

Carefully move the eggs to a small box with damp vermiculite or perlite without turning them. Roughly equal parts water and substrate by weight is a good starting point. Mark the top with a pencil, place the eggs half-buried in the substrate, and air the box once a week.

At 27-29 °C the eggs hatch after 58-65 days, and at lower temperatures it takes longer. It has not been studied whether temperature determines sex in the species. Eggs that collapse and go mouldy are infertile or dead.

Rearing the young

The young are 3-4 cm long and weigh about 1 g at hatching. Keep them singly or in small groups of the same size in small plastic tubs with damp kitchen paper, a hide and a water bowl. They begin to eat after the first shed. Give two or three small crickets or dubia nymphs of about 6 mm every day or every other day, dusted with calcium containing vitamin D3.

The young grow slowly. Keep them separate from adults, weigh them regularly, and separate young males when the sex is certain. Move them to a larger enclosure when they are about half grown.

Health and common mistakes

The species is hardy, and most problems are caused by wrong humidity, wrong heat or too little calcium. The typical mistakes are:

• Shed remnants on the toes and tail tip when the air is too dry. Provide a damp hide with moss, and gently loosen the remnants with lukewarm water.
• Too little calcium and vitamin D3, especially in juveniles and egg-laying females. This causes soft jaws, bent limbs and tremors.
• Over- or underweight. A thin tail means the animal is not getting enough or is ill, and a very broad tail that it is getting too much.
• Mould and fungus when the substrate is constantly wet and ventilation poor.
• Tails lost after handling or fights. The tail regrows, but becomes shorter and more uniformly coloured.
• Sand swallowed with the prey. Feed in a dish.

A healthy animal has clear eyes and a plump tail and is inquisitive in the evening. Seek a vet with experience in reptiles if the gecko loses weight, does not eat for several weeks or seems lethargic. Only a few surveys of diseases in the species exist, so the advice is based mostly on the practice of experienced keepers.