Diet

Providing high quality food for monitor lizards is unquestionably one of the most important factors in their successful care. The diets of monitor lizards are very varied in the wild. Many are opportunistic feeders that will eat any animal they are able to overpower but most feed mainly from two or three classes of prey. The studies conducted to date show that most of the food eaten by even the large monitors is comparatively small. Of course the animals' diet in the wild does not indicate their preferred choice, merely what is available. A monitor looking for large prey such as crabs will come across many smaller animals such as insects in its search. As a result the bulk of its diet may be the smaller animals which are encountered in the search for larger prey. When larger animals are abundant they make more efficient foods. because they offer more nutrition for less energy expenditure. Monitors are intelligent reptiles and in captivity they quickly become used to certain foods and will ignore others that are less sustaining. or are too much trouble to catch or eat. Most published accounts of monitors reproducing in captivity relate to animals which have been fed a diet comprising almost exclusively of rats, mice and chicks, and there is not enough data available to determine how much difference a more varied diet would make. However the rarity of captive breedings indicate that more attention should be given to the animals' lifestyles in the wild, and certainly the diets of free living monitors are drastically different to those provided for most captive specimens. Wherever possible the lizards should be given a varied diet which includes animals of the prey types known to be taken in the wild. Most foods can be offered alive but it may be necessary to give the lizards only dead birds and mammals to keep on the right side of the law. Captive monitors kept on a diet of large animals often ignore smaller foods (Kaufman et al 1994), including those that form the bulk of their prey in the wild, and it is very likely that this laziness is detrimental to the lizards' well· being. After visiting many zoos and private keepers to ask questions and photograph their animals, an eminent monitor breeder was kind enough to invite me to his house. Virtually all of the captive varanids I had photographed previously were either extremely shy or else indifferent to my efforts, but the first specimen I attempted to photograph here. a rather young sand goanna. behaved in a way which I was not at all prepared for, despite the warnings of my learned friend. After several flicks of the tongue it emerged from its retreat. launched itself at me. seized my finger and made a most laudable attempt to amputate it. Since then I have seen the same vigourous behaviour. albeit from a slightly greater distance. in many groups of well-kept captives. all of which reproduced. I have no doubt that careful feeding plays a major role in the successful maintenance of these animals and that, after neglect of initial parasitic infestations. overfeeding is the main cause of premature demise and failure in attempts to breed monitor lizards in captivity.

Ideally all prey items will have been raised in captivity but in practise many suitable food animals may be available locally. Before taking them it is imperative to ensure that it is legal to do so and that the animals are not contaminated with poisons or disease harmful to the monitor lizards. Never take more animals from the wild than are needed to satisfy the lizards' immediate needs.

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 ellception of Gray's monitor. 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. Hom & 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 complell and beyond the scope of this work (an ellcellent 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
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 18°C). 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.

Mammals
Rodents. rabbits and other manunals make excellent foods for most monitors lizards, but they should not be used to the exclusion of other prey types. Domestic varieties of many manunaJs 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. Manunals 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 (all he 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 partilcular 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.

Fish
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 environment~, 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. caresbeiana). 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. Cenain 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

Frequency and Quantity of Food
Juvenile monitor lizards have enormous appetites. grow very quickly and should be given as much food as they will eat. As their growth rate slows however their appetites remain undiminished. As adults many species become prone to obesity in captivity. Most monitor lizards (probably all of them) are able to store huge amounts of fat in the body and around the base of the tail. If they eat more food than they need fat continues to accumulate until it becomes detrimental to their health. Obese animals are unlikely to reproduce and are much more prone to disease and earlier demise than those of normal weights. Obesity is seen least often in animals that are fed a diet containing plenty of invenebrate prey and is usually most extreme in seasonal species deprived of their annual period of reduced activity. Furthermore, males may be more prone to obesity than females. In some species of monitor lizards fat reserves are used in different ways by the sexes. In nature, females' fat reserves are sometimes used to produce eggs whilst males expend very little energy in sperm production and expend most energy during the mating season. In captivity, whilst females may be able to produce eggs unhindered, the males' ability to search for mates and fight off rivals is severely limited. Because of this females should be fed more generously than males, especially prior to and after reproduction.

It is impossible to give precise details of the optimum weight and amount of food required by adults, because these parameters are influenced by many factors. This provides only a very rough guide. In the wild monitors' weights fluctuate from season to season and it may be necessary to reproduce this pattern in order to maintain the animals successfully. In general, insects and other small invertebrates should be offered to the lizards at least several times per week, whilst vertebrate prey are given less often (once per week or fortnight). However some monitors will not be satisfied with this, whilst it may cause others to put on too much weight. Small meals provided regularly are better than large meals given at longer intervals, and carerul attention should be given to the condition of the animal and their food intake adjusted accordingly. It may be possible to cure some obese animals with careful feeding.

Feeding Problems
Initially. wild-caught monitor lizards are often reluctant to feed in captivity. They should he offered as wide a choice of prey animals as possible in the hope that something will stimulate their appetites. Adult lizards in good condition are quite able to withstand periods of fasting of a month or so, but if the fast persists much longer action should be taken. Force feeding an animal that has refused to feed for some time may be of little value. because the food is simply regurgitated rather than digested. Regular injections of amino acids and minerals often help the lizards to recover. Veterinary supervision is highly recommended.

Sometimes a monitor lizard that appears to be healthy will suddenly refuse to eat. Often this is related to their need for a period of reduced activity each year. Lizards used to periods of inactivity in the wild may go off foorl altogether despite the provision of favourable environmental conditions. As long as the lizards are not maintained at too high a temperature and do not become emaciated. long fasts are not dangerous, but a loss of interest in food may he a sign of disease. A lack of suitable light is often cited as a reason for loss of appetite in reptiles. If the enclosure is illuminated with cheap light bulbs now is the time to change them for more expensive light sources that imitate the sun's spectrum. If disease is suspected the animal should be taken to a vet for examination. If lizards from temperate environments and still fasting after a month, despite the new lights, and the vet can find no sign of illness. it may be wise to reduce the ambient temperature of the enclosure by several degrees and allow the lizards to become inactive for a few weeks. Any animals that become very thin should be force fed as described above. Often an animal that refuses to feed can be tempted eventually by offering a wide range of prey items. Once the lizards begin to feed they become less fussy about the food they take, and soon accept the usual monitor fare. Heavily gravid females are often unable to eat because they are literally stuffed with eggs. If neither eggs nor appetite have appeared after a month veterinary advice must be obtained. The only feeding problem experienced by many monitor lizards in captivity is that they get too much of it.

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.