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Food & Supplements Print E-mail
Captive Care of Monitors
Extract from A Little Book of Monitor Lizards © D. Bennett 1995. Viper Press, UK

No expense should be spared when raising animals destined to be fed to monitor lizards. They should be fed only fresh foods and kept under the best conditions possible. For guidance on breeding insects, rodents, snails and other suitable prey items you should consult members of your local herpetological society. With the exception of Gray's monitor (whose diet in captivity is discussed in Chapter 6), plants do not figure in the diets of monitor lizards.

Vitamins and Minerals
The regular use of good vitamin and mineral supplements is indispensable. Feeding monitors on a varied diet should, in theory, provide all the nutrients, vitamins and minerals required for good growth and health. But vitamin and mineral deficiencies are very common in captive monitor lizards, even when they are housed under ideal conditions and their prey animals are reared on ambrosial fodder. Horn & Visser (1990) suggest that monitor lizards increase their mineral input by rubbing their prey into the ground before consuming it. All food items should contain supplements and many people find the use of soluble vitamins in drinking water to be advantageous. Invertebrates can be dusted with vitamin and mineral powders whilst dead vertebrates can be injected with liquid supplements. The micro-nutritional requirements of reptiles are complex and beyond the scope of this work (an excellent discussion of the subject can doubtless be found in Frye (1991). A surfeit of certain vitamins (particularly A and D3) and minerals are known to cause problems, but in general monitor lizards are not prone to health problems associated with the overuse of supplements, providing suitable ones are chosen. A bewildering variety of vitamin and mineral supplements are available, many made specially for reptiles and all containing slightly different proportions of different micronutrients. Balance between calcium and phosphorus is particularly important. The most authoritative works on breeding monitor lizards in captivity recommend the use of vitamin and mineral supplements with all food items and suggest the use of a calcium:phosphorus ratio of 2:1. (Horn & Visser 1990; Eidenmuller 1992).

Insects make up the most important part of the diet of many monitors in the wild and should form the staple diet of most species in captivity. Their movements trigger the monitors' predatory natures and because they are difficult to catch they make the lizards work for their food, sometimes the only exercise lethargic specimens ever have. Crickets, locusts, grasshoppers, cicadas, butterflies, moths, beetles, cockroaches, mantids, stick insects, maggots, June bugs and beetles will be accepted by many monitors. Some of these foods can be cultured but many suitable insects will be available locally at different times of year. When foods are collected from the wild you should ensure that they have not been exposed to pesticides and that you are not collecting protected species. Monitor lizards can eat large amounts of insects in a single meal which would be impossible to collect outside in any season. They can be bought locally or obtained by mail order but the cheapest and most reliable option is usually to breed them at home. Many initial attempts at breeding crickets result in mass escapes that make further attempts impossible due to household hostility. If the crickets escape outside during warm weather they may serenade the neighbourhood for the rest of the summer. Therefore, colonies should be housed in a very deep, smooth sided containers and kept well covered. One of the best orthopterans to breed for monitor lizards is the black cricket, Gryllus bimaculatus. Unfortunately its song is not as melodic as that of the less nutritious members of its family, but it is a large meaty insect that few lizards can resist. Many exotic invertebrates are available through the post from entomological suppliers and a few of them may be inexpensive enough for occasional inclusion in the diet. An excellent food for monitor lizards is the Argentinean wood cockroach (Blabtica dubia). They reach a large size (adults can weigh up to 5g) and are easy to keep and breed (providing the temperature never falls below 18oC). Although considered unsightly by many people, this large insect cannot fly, nor can it climb smooth surfaces, making escapes unlikely. They can be obtained from entomological suppliers, or from other reptile keepers.

There is little or no danger of introducing disease with insect prey (although they are known to act as vectors of disease in wild monitors (e.g. Mackerras 1962)), but many insects and invertebrates are poisonous to some animals, and so their use should be avoided. They include centipedes, millipedes, scorpions, spiders, and many insect larvae. Although all these creatures are preyed upon in the wild there is no guarantee that monitors are immune to the poisons of alien species. All invertebrate prey should be dusted with a vitamin and mineral supplement just before they are fed to the monitors.

Other invertebrates
A wide variety of land and water dwelling molluscs and crustaceans are devoured with relish by many monitors. With the possible exception of the slugs and snails none can be propagated, so they must be bought or collected. Slugs, snails, crabs and crayfish can often be found locally. More exotic species such as giant African land snails and hermit crabs are available from some pet shops. The shells of molluscs and crustaceans present no difficulties to monitor lizards. Small prey are swallowed intact whilst the larger ones are crushed and their shells discarded or swallowed separately. Saltwater crabs and molluscs should only be offered to lizards that are known to eat marine organisms in the wild and can therefore tolerate high salt intake. Earthworms make a good food for many monitors and are very easy to collect.

Rodents, rabbits and other mammals make excellent foods for most monitors lizards, but they should not be used to the exclusion of other prey types. Domestic varieties of many mammals are inexpensive, widely available and very easy to breed. However, some insectivorous and molluscivorous monitors seem unable to cope with large amounts of fur in their diets and their use as food for these species should be restricted to the occasional treat. Mammals should be fed sparingly to all monitor lizards because if overused it may be difficult to get them to accept the other foods essential to their health. Very small monitor lizards can be offered chopped up portions of baby rodents.

Reptile keepers have a bad reputation with rodent keepers, and it appears that the latter rarely impart their knowledge to the former. Mice and rats will live anywhere and eat almost anything, but those destined to become reptile food should be treated with particular compassion. They should be given sufficient space, kept clean and warm, and fed on the finest foods available. Even then their corpses should be injected with vitamin supplements just prior to use. Frozen rodents can be bought in bulk from some biological suppliers and thawed out when required but is unwise to use animals that have been frozen for over six weeks as food and, if available, freshly killed foods are always to be preferred. Night time walks down country roads often yield an abundance of fresh car victims. They make perfectly acceptable foods as long as there is no danger that the animals have been exposed to poisons. Using live mammals to feed monitor lizards is probably unnecessary, because the lizards "kill" dead food before swallowing it and therefore seem unable to tell the difference. Furthermore there is always the chance of an indignant rat biting back and causing serious injuries.

Birds and their Eggs
Most monitors will eat birds, ranging in size from day-old budgerigars to adult ducks and chickens. They should not be collected from the wild, but obtained through hatcheries, bird breeders or butchers. Vitamin and mineral supplements should be injected into the body cavity of dead prey animals before they are given to the lizards. Eggs make a good occasional food for many monitor lizards. Smaller specimens may crack the shells and let the contents run down their throats, whilst larger ones will swallow them whole. There is some danger of Salmonella infections with many farmed eggs which can be eliminated by boiling them until the yolks set, although this doubtless compromises their nutritional value.

Many monitors, including those not normally thought of as aquatic, are very partial to fishes. Many species of fish from the fishmonger are suitable as occasional foods for monitor lizards known to feed around seashores. They include trout, whitebait, herrings and mackerel. Freshwater fishes (such as goldfish and tropical species such as cichlids, barbs and rainbowfish) can be fed to all monitor lizards. Fish with sharp spines (which are often invisible) are unsuitable because they are adept at lodging themselves in their predators' throats. Livebearing fish such as platys and swordtails are ideal, because they are easy to keep and breed at home.

Reptiles and Amphibians
Although monitors eat many reptiles and amphibians in their natural environments, wildcaught animals cannot be recommended as food for captive varanids because of the very real possibility of disease transmission. On the other hand frogs, lizards and snakes which have been bred in captivity make good food, so long as they are known to be disease free. In practise it may prove very difficult to obtain sufficient quantities of animals to forma regular part of the monitors' diets. The best amphibians for use as monitor food are the frogs of the genus Rana, especially the European common frog (Rana temporiaria) and the north America bullfrog (R. catesbeiana). Both of these animals can be easily propagated and are extremely prolific. Although some monitors are known to prey on toads, the toxins of unfamiliar species may prove fatal, so they are not a suitable food. The same applies to a few frogs. Both the yellow monitor and the rough-necked monitor are known to eat amphibian eggs in the wild, but whether they would do so in captivity is unknown. Turtles and their eggs are eaten with great enthusiasm by monitors in the wild, but because of the dangers of amoebic infections they cannot be recommended as food for captives. All lizards and snakes will be eaten by monitor lizards. Certain species of geckoes, skinks, anoles and garter snakes are available at low prices, so captive bred specimens are readily available. However the dangers of passing on disease by introducing unquarantined reptiles and amphibians (dead or alive) into the monitor enclosure cannot be overestimated and only animals known to be free from disease should be used..

Unsuitable Foods
Monitors should be fed whole fresh animals whenever possible. Processed meats, offal and meat derivatives such as mince and dogfood are very poor and messy substitutes for the real thing. Great care must be taken to ensure that the monitor lizards do not eat food animals that have been treated with pesticides or other hazardous chemicals. Animals that appear sick or wasted should not be offered. Poor quality, intensively farmed foods (particularly birds and their eggs) pose a risk of Salmonella and other food poisonings. Some processed foods are sold specifically for feeding to carnivorous lizards, but I am unaware of any reports of captive breeding in monitors maintained on such a diet.

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Butaan start to visit fruiting trees before they are large enough to swallow the fruits. They make repeat journeys to trees, perhaps to reinforce memory of the position of the tree. If the youngster survives it may continue to use this tree for many decades. Fruiting trees like this are a vital resource for entire populations of butaan. Learn more >

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