Considered very intelligent, monitors are often claimed to recognize their keepers. Recent experiments on Varanus albigularis by John Phillips of the San Diego Zoo suggest that these lizards may even be capable of counting.
Monitors are moderate- to large-sized lizards of the genus Varanus within the family Varanidae. Most African species, including the white-throated or Cape monitor (Varanus albigularis), belong to the subgenus Polydaedalus. Among African monitors, white-throats are believed to be most closely related to savannah monitors (V. exanthematicus) and Yemen monitors (V. yemenensis) to which they show a strong resemblance in limb proportions, and body and head shape. Yellow monitors (V. flavescens) also show a strong physical resemblance to white-throats, but they are actually more closely related to Bengal (V. bengalensis), rough-necked (V. rudicollis and V. dumerilii) and Asian water monitors (V. salvator) than any of the African species.
White-Throated Monitor Characteristics
Generally, white-throated monitors have shorter, blunter, more bulbous snouts than savannah monitors, and the top of their heads and necks are dark brown while their backs are a dark gray-brown. They typically have raised and almost conically shaped neck (nuchal) scales.
White-throated monitors normally show a pattern of five to six rows of pale-yellow, darkly bordered, blotchy spots on the dorsal surface. Limbs may also be spotted with some pale yellow, and the tail may have an alternating pattern of dark brown with off-white bands from where it attaches to the body all the way to the tip. Their bellies are dirty-yellow with some spotting, and they are usually much more colorful than the pale whitish or tannish belly of the savannah monitor. Juveniles can even show some degree of banding.
Color patterns vary depending upon where lizards are found. White-throats from more northern locations show color flecking; whereas individuals from areas close to the equator have large white or yellow eyelike spots arranged as pairs or tetrads along the dorsal side. In Namibia near the Etosha area, these lizards have miniscule flecking and appear very pale in general coloration.
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White-throated monitor (Varanus albigularis albigularis) at the Oakland Zoo in California
As far as size is concerned, larger individuals tend to be found among the Varanus albigularis albigularis and V. a. microstrictus subspecies. Males are larger and heavier than females. Their tails are slightly longer than their snout-to-vent length. These lizards average a total length of just more than 3 feet to nearly 5 feet, and they show SVL ranges between 2 and 31⁄2 feet. Specimens with a TL more than 6 feet are probably rare, but the biggest white-throated monitors, V. a. microstictus, come from Kenya. Weights for males of reproductive age range between 11 to nearly 18 pounds, and females weigh about 10 to 14 pounds. Hatchlings are typically about 41⁄2 inches in SVL and weigh 0.7 to 0.9 ounces.
In the wild, white-throated monitors are mostly terrestrial species, but they also show a facility for being arboreal, particularly to escape predation and also during the breeding season. They also climb trees to hunt their prey, to escape the heat during the day and often to safely slumber at night. They are seen living within many of Africa’s varied habitats, including savannahs, verdant shrub lands, woodlands, rocky outcrops and combinations of these habitats. Many also seek refuge within burrows, especially in the cooler months.
White-throats are diurnal, and during the summer they are known to be active most of the day. Ambient environmental temperatures at midday become high and force them to seek refuge.
Considered very intelligent, monitors are often claimed to recognize their keepers. Recent experiments on Varanus albigularis by John Phillips of the San Diego Zoo suggest that these lizards may even be capable of counting. Monitors were offered a constant number of food items for a period of time. Later, the number of items was suddenly reduced. The lizards demonstrated that they still expected to receive the additional missing items, and apparently they only became calmer after the missing number was supplied.
White-throated monitors usually have large territories utilizing any and all habitats available. Male domains average about seven miles, and females’ turf averages about two miles. Traveling as many as three miles per day when foraging, monitors may use their entire home range during the summer rainy seasons. During the winter dry seasons, they only patrol 10 percent or less of their entire range. Interestingly, a 1-1 sex ratio of these lizards can be encountered while patrolling territories during the rainy seasons, but during the mating period, males show greatly enhanced activity over females, which skews the encountered sex ratio by as much as 6-1 in favor of males.
Monitors shed their teeth and grow new ones throughout their lives. As they mature, most African species — except desert monitors (Varanus griseus) and ornate monitors (V. ornatus) — replace most or all of the recurved and pointed teeth along the sides and back of the mouth with larger peglike teeth. These are used to crush and macerate the tough or hardened shells of invertebrates.
Recognized Subspecies
The white-throated monitor is subdivided into three recognized subspecies.
- Varanus albigularis albigularis has 37 to 167 midbody scale rows and 85 to 110 ventral scale rows from its neck to its groin area.
- Varanus albigularis angolensis has 109 to 144 midbody scale rows and 74 to 98 ventral scale rows.
- Varanus albigularis microstictus has 122 to 152 midbody scale rows and 88 to 94 ventral scale rows.
As might be expected, V. a. angolensis is almost completely restricted to Angola, but V. a. microstictus has a more westerly range from as far north and west as Sudan, Ethiopia and Somalia to as far south and west as Zambia and Mozambique. Varanus albigularis albigularis ranges throughout much of southern Africa from at least as far north as Zambia, and their distribution overlaps with V. a. microstictus in eastern Zambia and Malawi, and slightly with the very restricted penetration by V. a. angolensis into western Zambia and northern Namibia.
Although they can tolerate cooler temperatures than most other monitor species — they show no ill effects even when the ambient temperature goes a bit below 60 degrees Fahrenheit — long exposures to such low temperatures can result in illness or death. Generally this species may be relatively inactive during their winters (roughly October through February with a temperature range of 60 to 72 degrees), and this seems to result mostly from the lack of forage. Most regions in Africa can show annual dry (winter) and wet (summer) seasons per year, and African monitors have generally adapted to these seasonal fluctuations by shifting back and forth between their burrows and the ground surface depending on temperature and humidity levels. Naturally, white-throated monitors can loose as much as 30 to 50 percent of their total body mass during extended periods of inactivity.
Monitor Caging Musts
White-throats require relatively large enclosures — the larger the better. Homemade cages can be constructed of wood with a built-in heavy-glass (or Plexiglas, but it can scratch more easily) front. The top should have screening to provide ventilation and to allow for light placement. Any screens used must be resistant to monitor claws and attached securely to the cage. Balance ventilation against escape potential. Plastic-coated, welded, heavy wire is likely to be the best if screening is used along the sides and any other areas the lizard may be able to easily reach.
The door can be in the front, on the sides or on the top. I prefer to reach down to pick up a monitor — so long as the cage is not more than 3 feet high — rather than coming toward it from the front, especially with frightened or aggressive specimens. Hinged, locking doors are preferred to doors that slide up and down or laterally to avoid the potential to “guillotine” animals.
Probably the best solution for caging larger individual monitors is to provide them with small rooms or room-sized enclosures containing some sort of drainage in the floor for easy cleaning. There are also high-quality enclosure manufacturers specializing in reptile housing, and some of the largest cages can be good choices for monitor lizards.
Minimal enclosures are, of course, not recommended, and any monitor lizard housed under such conditions may exhibit stress or become aggressive. They will certainly not breed. A minimum cage size for any monitor should be no less than one-and-a-half to two or more times the SVL of the lizard; two-thirds to one times the length of the animal in width; and one to one-and-a-half times the length of the animal in height. Be sure the lizard cannot peer over the top.
Soft substrates in a relatively deep layer (at least 18 inches or more) are recommended. Lightly moistened topsoil of some sort provides good humidity. White-throated monitors are prone to constructing burrows and enjoy digging in the substrate. With proper care and drainage, this substrate may only need annual or semiannual replacement so long as it does not emit any foul odors. Certainly, if the entire substrate cannot be soil, providing some area where soil is available and into which these monitors can dig would be useful to the lizards.
Monitors tend to rub their prey in and through soil, and this activity may be related to their need for certain trace elements and vitamins. However, avoid topsoil for juveniles because its dusty quality may dehydrate them, and there is a high chance for impaction. Also, potting soil shows a very high risk for rotting and encouraging mold growth.
White-throats thermoregulate by shifting back and forth between warm basking areas and significantly cooler retreats. Enclosures require heating in such a way as to provide substantially warmer, tightly constrained basking areas, and cooler areas allowing retreat as far from the basking area as possible. Basking areas should be a relatively small section of the enclosure. Properly heated enclosures provide a thermal gradient between the extremes of very warm and very cool, allowing the lizards to select optimal temperatures as they go about their daily activity.
Remember, all the important physiological processes, including digestion, immune response and reproduction, are profoundly influenced by temperature. The coolest areas could be somewhere around 75 degrees, and a hotspot for basking can be up to 105 degrees, especially if breeding is desired. Remember that the sun and spotlights shining into enclosures heat rocks and substrates, so these cage objects will be hotter than ambient.
Although white-throats come from somewhat dry climates, they still drink regularly. Provide a heavy, large container of fresh water. The lizards enjoy an occasional soak, but excessive soaking might indicate that the cage is too warm. Make sure the water temperature is comfortably warm but not too hot and does not rapidly and significantly cool down. Remove feces-contaminated water as soon as possible. Disinfect and thoroughly rinse any such water container before reusing it.
Enclosures should be humid but not kept damp or continuously wet. Also avoid keeping the pen too dry. For African monitors, provide about 20 to 50 percent humidity.
A Stomach for Ambush
White-throats are active foragers, but they show the digestive physiology of ambush predators. This might be because their digestive tracts show significantly reduced function due to fasting during the long dry season. White-throats also consume huge meals (apparently up to 10 percent of their total body mass) in one sitting when food is plentiful.
When seeking prey, monitors recognize it visually and through the use of chemical cues, which are apparently obtained as a result of tongue-flicking. Olfaction linked with the use of the Jacobson’s organ is strongly developed in monitors and appears to be used in both foraging for prey and finding mates.
Wild white-throated monitors living in Etosha National Park in northern Namibia have a diet consisting mostly of land snails, various insects, and bird and reptile eggs. They hardly ever consume small mammals. In many places throughout Namibia, white-throats often share burrows with ground squirrels, but they do not seem to attack or eat them. Some captive white-throated monitors have been observed crushing and eviscerating rodent prey, but then eventually ejecting them and refusing to actually ingest them.
During the wet seasons, white-throats can find abundant invertebrate prey. Grasshoppers and crickets are the bulk of their quarry at this time. Generally, however, this species will eat anything it can catch, including snakes. Sand racers (Psammophis spp.) and various venomous species, especially local species of viper (Bitis spp.) and cobra (Naja spp.), are commonly consumed. Young chicks of ground- and tree-nesting birds are also taken.
In the more southern region of Africa, the white-throated monitor’s diet consists mostly of insects: millipedes and tenebrionid beetles.
There are reports of some species of African monitors, including white-throated monitors, accepting and eating fruit. To date, however, little is actually known about this herbivorous behavior.
Food in Captivity
In captivity white-throated monitors accept a varied diet of crickets, large roaches, king or giant mealworms (Zophobus spp.), silkworm larvae, locusts and hornworms. Other invertebrates could include shellfish such as shrimp, crawfish, crabs and large land snails. Feeder insects should be dusted or — better yet — gut-loaded with calcium and vitamin supplements for reptiles.
Monitors are also fond of eggs, but only use egg yolks or — even better — fertilized eggs with embryos in them. The whites of nonembryonated eggs contain avidin, which can induce a biotin deficiency, and various complications involving the skin, intestinal tract and nervous system can occur. This is particularly the case if almost the entire diet consists of these eggs, which is not a good idea anyway because diets should contain a variety of foods.
There is also a potential risk of Salmonella when feeding raw eggs. A safer option is to cook the eggs, which should eliminate both the avidin and Salmonella problems.
White-throated monitors are a large and fascinating monitor species. They are a dominant form of monitor species in southern Africa where their range only overlaps with the Nile monitor, a species with very different habitat preferences.
Contrary to popular belief, Varanus albigularis does not normally consume rodents, but it seems to prefer mostly invertebrate prey. Although they are available in the pet trade, most specimens available are still largely wild-caught animals.
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