|The Spiny-Tailed Monitor (Varanus acanthurus)
by Jeff Lemm
Whether it's due to their proud nature, their struggle for survival in their native lands, or because they're simply more available, monitor lizards are quickly becoming popular in both zoos and private collections. Within the last couple of years, one species in particular has become increasingly popular-the spiny-tailed monitor (Varanus acanthurus). Due to their small size, intelligence, calm demeanor and incredible reproductive output, "ackies" are on their way to being America's new favorite lizard.
First described in 1845 as Odatria ocellata by Gray, the name "spiny tail" wasn't given until Boulenger renamed the animal Varanus acanthurus in 1885. In 1942, Mertens placed the species in the subgenus Odatria along with most of the other small- to medium-sized Australian varanids.
Two subspecies of spiny-tailed monitors may be found throughout much of the northern half of Australia, from the northern half of Western Australia, all of the Northern Territory (except possibly where the closely related V. baritji is found), western Queensland, and the northern third of South Australia. Animals from the northwest of the range are generally described as V. a. acanthurus, while animals from the east of the range are described as V. a. brachyurus (although some authors believe the subspecies to be divided from the North to the South). A third subspecies, V. a. insulanicus, may be found on Groote Eylandt and Marchinbar Island in the Gulf of Carpentaria off the coast of the Northern Territory (DeLisle, 1996).
Because the acanthurus group has such an incredibly large range, a great deal of morphological, color and ecological differences may be present. Some northern animals may be jet black, while in other areas they may be yellow, brown, tan or red. Body size also varies greatly, with some animals reaching total lengths of 30 inches (11.8 centimeters) and possibly longer, and in some areas animals have short tails, while in other areas the tails may be longer. Ecology may even vary tremendously within the range. Animals from tropical areas have been found in trees, while in drier areas they have been found in burrows, rock outcrops, or in spinifex grass (Bennett, 1998). In addition, no true zones of segregation or overlap exist within the mainland forms, making the taxonomy of the species very confusing and in need of revision. Furthermore, species such as V. baritji, which was once considered part of the acanthurus group, are present within the range where other acanthurus forms are absent. Together, these characters and habits are forcing researchers to believe a greater number of acanthurus subspecies exist than previously believed (H. DeLisle, B. Eidenmueller, D. King; pers. comms).
Two acanthurus forms are readily available in the hobbyist trade and are generally known as "red ackies" and "yellow ackies." Although dealers and breeders know that several differences exist between the two, few differentiate between them, while some breeders even interbreed the two. After a great deal of research and conversation with the leading experts, it has become apparent that these two color morphs are most likely the two mainland subspecies of acanthurus. The red form being V. a. acanthurus from the west, while the yellow form represents V. a. brachyurus (meaning "short tailed") from the east. Most books on varanids disagree about where the taxa are from and which color morphs come from what range, but the type specimen found by Gray was from the northwest coast and was named acanthurus, while the type brachyurus specimen was found in central Australia. Recent field observations, along with morphological data such as tail length and size were also considered in confirming where each color form comes from.
First, red specimens may have longer tails than yellows in some areas. Red ackies also grow larger than yellows and have a distinct cross and spot head pattern which is absent in the yellow ackie. Further evidence that suggests the red ackie is, indeed, V. a. acanthurus is that on a recent trip to Australia, wild red ackies were found and photographed abundantly near 80-mile beach and the eastern Kimberleys to the Victoria River and Halls Creek. An enormous 34-inch red was also found near Wyndham, an area also within the range of V. a. acanthurus. Yellow ackies were commonly found in the east, especially near Mt. Isa (F. Retes, pers. comm.).
In whatever microhabitat they exist, ackies generally prefer arid, flat terrain near rocky outcrops. Rock crevices or burrows under large boulders serve as retreats and areas where animals may thermoregulate without being exposed to predators such as raptors, snakes and other monitors. The flat body and spiny tail are perfectly evolved for living in burrows and cracks, as the animal is able to fill itself with air to avoid being extracted, while the spiny tail serves to cover the more vulnerable body parts that may be exposed at any given time. The streamlined shape of the animal also makes living in burrows much easier, especially in group situations. Large monitor lizards are generally considered asocial, whereas acanthurus and other small monitors have been found living in underground colonies (S. Irwin, pers. comm.). This interesting social behavior has also been noted in captivity, where alpha males and females rank the highest in group situations.
Most small Australian monitors survive mainly on small lizards and insects, and V. acanthurus is no exception. Losos and Greene (1988) found that in 127 museum specimens, orthopterans, beetles, cockroaches and lizards were the main prey items. Lizards included agamids, geckoes and skinks. No relationship between monitor and prey size was found, but larger lizards usually contained more items in the stomach. A small ackie, weighing only 119 grams, was found to contain seven large grasshoppers in its stomach. Given the reproductive output seen in acanthurus in captivity, this is not surprising, because animals that produce multiple clutches of eggs require a great deal of food.
Little information exists on reproduction in wild acanthurus; however, my friend and colleague Grant Husband examined a wild nest in the Northern Territory during the month of January (Husband, 1979). It had been dug into a mound with an S-shaped tunnel underneath that consisted of a 40-centimeter egg chamber. The tunnel had been refilled and the eight young were found digging themselves out. From this and other data it has been reported that V. acanthurus mates in the late dry season from August to November (King and Rhodes, 1982). Clutch size in the wild is believed to range from two to 11 eggs and is dependent upon the size of the female. Hatchlings emerge during the wet season from December to March after a three- to four-month incubation period.
Captive Husbandry and Reproduction
Given the proper requirements, ackies will flourish in captivity. In fact, they adjust so well to captive situations that they are commonly coined "the perfect beginner monitor." Rarely exceeding 30 inches (76.2 centimeters) total length in captivity, a pair of adult acanthurus can safely and comfortably be housed in terraria measuring 24 by 36 inches (.60 meter by .10 meter). Of course, any monitor would benefit from a larger enclosure, especially a species as active and curious as acanthurus. Substrates commonly used are sand, dirt, or a mixture of the two, while the depth and substrate type depends on whether or not a nest box will be used during the breeding season.
Because they come from hot environments, high temperatures are necessary to keep acanthurus healthy and fit. A temperature gradient from one end of the cage to the other is effective in monitor husbandry. Ambient temperatures should range from 100 degrees Fahrenheit (37.8 degrees Celsius) on the warm end to the low 80s Fahrenheit on the cool end (high 20s Celsius).
A basking spot on the warm end, usually provided by a spotlight, should range anywhere from 140 to 170 degrees Fahrenheit (60 to 76 degrees Celsius) on the substrate surface (not air temperature). These temperatures may seem extremely high for any animal, but these temperatures are common on the surface of most substrates when the air temperature is greater than 85 degrees Fahrenheit (30 degrees Celsius). As an example, on a 82-degree (Fahrenheit) day in Southern California, the surface temperature of hard-packed dirt in the sun measures roughly 140 degrees Fahrenheit (60 degrees Celsius), whereas on days that were hotter than 90 degrees Fahrenheit (32.2 degrees Celsius), I measured dirt at up to 200 degrees Fahrenheit (93.3 degrees Celsius). In ackie habitat in Australia, which gets as hot as 100 degrees Fahrenheit (37.8 degrees Celsius) or more, the rocks the lizards bask within probably reach temperatures over 200 degrees Fahrenheit (93.3 degrees Celsius) on the surface. I have yet to hear of or see a burn on an acanthurus. The proper way to ensure that an animal will not be burned is to make sure the ambient cage temperature stays warm. Thermal burns occur when a cold animal in a cold enclosure sits under a hot basking light that is directed on less than 75 percent of the animal's body.
Provide multiple hide areas throughout the temperature gradient, especially if more than one animal inhabits the enclosure. A hide area is very important under the spotlight, because acanthurus and other Odatria monitors prefer to thermoregulate under cover. A method that proves very effective is a stack of plywood boards, separated just enough for the animals to squeeze between them. This method, developed by Frank Retes, simulates a burrow or caprocks with a vertical temperature gradient within which the animals may be found thermoregulating in the wild. A heating pad situated under the substrate at the bottom of the woodpile ensures the animals will not only stay warm at night, but that they will use the entire woodpile to thermoregulate.
As with any diurnal, basking animal, providing ultraviolet light is important. Although it is still uncertain whether monitors can acquire enough vitamin D and calcium from their diet to remain healthy, most keepers provide full-spectrum lighting to be safe. Provide basking areas within 6 to 10 inches (15.2 to 25.4 centimeters) of full-spectrum bulbs to ensure proper vitamin metabolism. Finally, provide a water bowl at all times, while under some of the hide areas the substrate should be lightly sprayed a couple of times per week. Although they come from hot areas, humidity in much of northern Australia is very high, as is the humidity in any subterranean burrow. Proper humidity levels for acanthurus enclosures should range from roughly 65 to 85 percent.
Because of the speedy metabolism associated with high temperatures, acanthurus should be fed five to seven times per week, with younger animals being fed daily. Insects such as crickets, wax worms, mealworms and roaches make up most of the diet of captives, but pink mice and frozen-thawed ground turkey are also readily taken-especially during the breeding season. Adults have been known to become obese when fed pinkies more than three or four times per week, but if kept on an insect diet they may be fed every day. Bits of fish and scrambled egg have also been accepted in the authors' group of ackies. Live insects and pinkies are grabbed swiftly at mid-body and then brought to "killing stations." These stations, usually the rocks or boards in the cage but sometimes cage walls, serve to give the animal a lever in which to break the neck of the pinky or squash the insect so that it may be handled and swallowed more easily. Ground turkey or frozen-thawed pinkies may be offered on rocks or small dishes near hide areas. If the prey isn't moving, ackies may not accept it, but tong feeding for the first couple of weeks may get the animals used to unmoving prey.
The spiny-tailed monitor has been bred in captivity for years, with some successful breeding taking place in Europe as long as 25 years ago (H.G. Horn, pers. comm.). Multiple clutching, believed to be a fairly new concept to captive reproduction of monitors, also occurred over a decade ago in private collections (Erdfelder, 1984). In the last five years, however, breeding success of many monitors has been tremendous and captive-produced animals have been bred to at least the fourth generation.
When planning to raise ackies for reproductive purposes, one of the most important things is to purchase a group of hatchlings that hatched together or are the same size and age, and raise them together from as early an age as possible. Trying to assemble a group of adult animals that don't know each other usually results in injury or death to one or more animals. Not only males fight, but the social system of ackies also seems to include an alpha female who will not only attack other females, but smaller males, too. Females raised together may be introduced to a larger single male raised separately, but the animals should be observed for the first few days to make sure they are compatible (J. Lemm, pers. observ.).
Like most other varanid species, ackies are difficult to sex, and hatchlings are next to impossible to sex, although the well-trained eye can ascertain gender at four or five months of age. Males develop clusters of enlarged scales on either side, and just below, the vent. Females may have slightly enlarged scales, but they are much smaller. Male ackies also have a more developed, muscular neck region with a wider head, whereas females have a more slender head that may be more pointed. I have noticed that females of both subspecies in my collection also tend to be less shy towards their keepers than males, although both male and female reds can be snappier than yellows, which never seem to bite.
One interesting behavior common to immature animals, even hatchlings, is they may show mating behavior, but this does not mean they are a certain gender. Female ackies will "mate" other females as well as males, and males may mate other males, although this behavior seems to decrease with maturity.
Ackies reach sexual maturity very quickly and have been known to lay fertile clutches at as little as 5 or 6 months of age. King and Rhodes (1982) discovered that females with a snout-to-vent (SVL) between 10 to 12 centimeters (3.9 to 4.7 inches) contained enlarged ovum in the wild which corresponds to the size that acanthurus can reach in less than a year if kept at the correct temperatures. Basking temperatures are very important during breeding, and copulation may cease if the basking site dips below surface temperatures of 135 degrees Fahrenheit (57.2 degrees Celsius). If kept warm and fed daily, spiny-tailed monitors will begin to copulate many times a day, usually in the spring and summer months, and ending around October; although this pattern differs greatly depending on temperatures, food and light cycles, as well as where the animals are geographically located. Animals seem to suddenly "turn off" even when kept warm and well fed, and will show no interest in one another until the start of the next breeding cycle. At this time, I normally reduce temperatures by roughly 20 degrees Fahrenheit and shut off one of two basking lights. Light cycles are reduced to 10 hours of light and the animals are only fed two to four times per week. This will last until the animals show signs of restless activity, usually one to three months later. I then feed the animals daily, offer more heat and increase the light to 14 hours a day. Breeding usually commences within a month or two. Some breeders never change any of the conditions in their enclosures, and the animals begin to breed on their own again after a short shutdown, but I prefer to mimic natural conditions in order to guarantee that the animals shut down completely.
Eggs have been laid in every month of the year in captivity, and many animals may
Bennett, D. 1998. Monitor Lizards: Natural history, biology and husbandry. Chimaira, Warlick Druck, Meckenheim, Germany.
DeLisle, H.F. 1996. The Natural History of Monitor Lizards. Krieger publishing, Malabar, Florida.
Erdfelder, K.H. 1984. "Haltung und Zucht des Stachelschwanzwarans Varanus acanthurus Boulenger, 1885. Sauria 6(1):9-11.
Husband, G. A. 1979. "Notes on a nest and hatchlings of Varanus acanthurus." Herpetofauna 11(1):29-30.
King, D. and L. Rhodes. 1982. "Sex Ratio and Breeding of Varanus acanthurus. Copeia pp.784-787.
Losos, J.B. and H.W. Greene. 1988.
"Ecological and evolutionary implications of diet in monitor lizards." Biological Journal of the Linnaen Society 35:379-407.
breed and produce clutches for up to six months of the year. Copulation to oviposition normally takes from two to four weeks, and females are usually ready to reproduce again within a month's time. Some females have been known to copulate within a day or two of laying and will lay another clutch within a couple of weeks. Clutches are usually laid between two weeks and two months apart, depending on the health and age of the female, and some females, usually the younger ones, have been known to lay as many as six clutches during a single breeding season (F. Retes, pers. comm.).
Provide a moist area, either in a nest box or in the substrate of the enclosure. Moist dirt at a depth of roughly 10 inches (25.4 centimeters) works well as a substrate for nesting because it enables females to dig long burrows and deposit eggs in a small chamber at the end. The female will usually guard the nest from other animals, and for this reason males should be removed from the cage or the female should be placed in a laying cage. After a couple of days the female will start to wander away from the nest area and may look slightly thin (not always, because many females continue to feed up until laying). The greatest clue to whether a female has laid is if the burrow is filled in and she is sitting near the entrance or on top of the lay box. Ackies will burrow even when not in a reproductive mode, but usually the entrance to the burrow is left open.
Clutches for both mainland subspecies may contain as many as 18 eggs, depending on the size of the female, but average clutch size is seven to 10 eggs (F. Retes, pers. comm.). Eggs measure roughly 3 centimeters (1.2 inches) and weigh 4.5 to 6.5 grams for brachyurus and up to 2/3 larger than this for acanthurus.
Gently excavate the eggs from the nest and place them in sealed plastic boxes on a substrate of perlite at a ratio of one part water to one part perlite, by weight. Do not add water to the mix unless it is extremely dry, and never add water during the last two weeks of incubation. Adding water during this time is believed to be the cause of young fully formed animals dying in the egg as a result of "drowning." Open the egg boxes once a week throughout incubation to allow for proper gas exchange, and repeat this process two or three times a week during the last two weeks of incubation.
Incubate the eggs at temperatures ranging from 84 to 86 degrees Fahrenheit (28.9 to 30 degrees Celsius), and they may take between 12 and 15 weeks to hatch. Hatchling brachyurus weigh from 4 to 6 grams on the average and measure 5.5 to 7 centimeters (2.2 to 2.8 inches) SVL, while acanthurus are slightly larger. Upon hatching, place the young monitors in plastic boxes lined with moist paper towels and put them back in the incubator for up to two days, or until the umbilical heals and the animals are actively exploring their surroundings.
Young ackies may be kept in groups in aquariums or plastic tubs with one area of the cage reserved for basking at a temperature up to 120 degrees Fahrenheit (49 degrees Celsius) on the substrate surface. Spotlights shining down on retreats are optimal, whereas a number of retreats in various spots throughout a temperature gradient within the cage will allow for proper thermoregulation. The temperature gradient should be similar to that of the adults, and as the monitors age you may increase the basking site temperature. Spray retreats inside or underneath with water daily, and provide a shallow water dish because young monitors dehydrate easily. Hatchling ackies may begin to feed within 24 hours of hatching. They will voraciously hunt for small crickets and other insects. Bits of ground turkey and chopped pink mice may take some time to get used to, but usually are accepted without a problem.
Spiny-tailed monitors continue to grow in popularity and availability every year. As more people learn how gentle and curious these animals are, they may soon become the next bearded dragon of the pet trade. Perhaps with some education and persistence, people will learn that these small captive-bred lizards not only make excellent, manageable pets, but they can help take pressure off larger species that are still being imported from the wild today.
I would like to thank Frank Retes for his many useful comments. I would also like to thank Allen Repashy and Pete Kuhn for allowing me to photograph their collections.