Nile Monitors

By Paul Donovan

They say big is beautiful. And you can’t get much bigger than the Nile monitor, Varanus niloticus. Distributed throughout much of sub-Saharan Africa, this is the largest lizard found on the African continent, only surpassed in terms of bulk by the Nile crocodile, Crocodylus niloticus. With its sturdy body, powerfully built limbs and 2m (6½ft) length, it is a lizard deserving of a great deal of respect. 

For good reason, they are sometimes called ‘Land crocodiles’ for they have the mannerism of a Nile crocodile in the way in which they walk. It’s a very deliberate, gated walk, with a sinuous motion. Another reason perhaps for the name, is that they can often be found mingling amongst Nile crocodiles, especially during the breeding season when they become infamous thieves of croc eggs and young. 

As one may imagine of a lizard with such a reputation, they are massive, powerfully built creatures. The head is quite angular with a blunt, rounded snout, small eyes set beneath a raised bony ridge, and obliquely slit nostrils situated central to the snout and eyes, and located quite high on the head. The skull has evolved into a heavily built ‘box’ structure with thickened bones. The lower jaw is bowed, and both the upper and lower posterior teeth have developed into efficient crushing tools. These features are married to a jaw muscle structure which imparts maximum force towards its rear, enabling the monitor to deal with hard shelled molluscs. 

Near the wedding tackle

I had a rather amusing encounter with a good sized individual I had in the snake park I once owned. I was standing in the enclosure holding this monitor, as I was talking to a group of people about it. As I released it and before I knew what had happened, it ran to the other side of the enclosure, turned tail and shot up my leg before clamping its jaws around my leg just below the wedding tackle. Well, the visitors had a good laugh while my wife at the time said “don’t move let me get a picture”. Trust me, a lizard like that impaling its teeth into your leg, not only brings a great deal of pain with it, but also tears to your eyes. And, of course, the realisation of how lucky I’d been it had missed latching onto something else. Well, at least the visitors took a tale away with them to recount of their visit! 

Aggression aside, I think the Nile Monitors ranks as one of the most attractive of all the Varanids. The brownish-grey or greenish-brown upper body adorned with dark reticulations and yellowish spots or stripes on the back, head and limbs really sets this monitor apart from the rest. The underside is yellowish with blackish cross bars. Juveniles are conspicuously black with yellow spots arranged in crossbars. This may seem a rather garish colour to be, but it actually serves remarkably well as a form of camouflage. It is believed that a forest subspecies from western African, V. n. ornatus may exist which differs marginally in the number of cross-bars and spots. Though from what I can gather, the jury is still out on that one. 

The cranial scales are small and smooth as are those of the supraocular (those directly above the eyes). The dorsal scales along the body are also small, oval in shape, of marginally different sizes, and what is called tectiform; shaped a bit like the roof of a house. 

During the day, this monitor can be found residing in crevices amongst rocks or in burrows in close proximity to water; often even dug into the embankments of rivers. The burrow has a very distinctive semi-circular shape to it. 

Periodically they will emerge to bask in the sun’s warmth, by sprawling out on the river banks, rocky outcrops or drape themselves over branches or tree stumps. If startled, they are quick to take to the water where they feel at home. With their powerful tail, they propel themselves along like they had an outboard motor bolted to it. Equally, they can ascend a tree at a rate a top ranking mountaineer would be envious of. The huge razor-like claws grip the trunk, like crampons do to ice. 

Despite their name and penchant for water, I have found them some considerable distance from it. When I was living in the city, on numerous occasions  I’d receive a call to relocate a vagrant lizard who’d taken up residence in someone’s garden. Many of these properties were in the centre of the city, where the nearest water source was kilometres a way. So it does seem to be a wandering species which can find itself in a lot of unwelcome predicaments.  

Trying to capture these wanders’ is a task in itself. They are belligerent lizards, with an attitude. When confronted, they raise their body on all fours, arch the back, distend the throat while hissing loudly, and then unleash a flurry of belligerent lashes with that muscular tail. Anyone who has been on the receiving end of these, can attest to the pain they deliver. Even a small monitor can deliver a whack that will result in a stinging red welt. As for a large one, they are capable of breaking a small animal’s leg.

Feeding habits

For all intents and purposes, when it comes to feeding, there is not much that goes unnoticed when it comes to filling a Nile’s belly. Everything from earthworms, slugs, snails, insects, birds, rodents, frogs, toads, molluscs, crabs, fish, lizards, snakes, carrion, as well as being a predatory nest robber of crocodile eggs, and their young are considered with relish. They have even been observed nest robbing in full view of the parent crocodile. I have only ever witnessed this on one occasion, and it was a fascinating cycle of a scavenging predator trying to outsmart an apex predator. In this instance, perseverance paid off, and the monitor had its fill. 

Such is their craving, they have even been witnessed trying to swallow bones with only the smallest piece of meat attached. Any bone too large to be swallowed whole, will be smashed against a rock to aid its ingestion. They will even rob food directly from the mouth of another monitor. Furthermore, such is their self-confidence, that they will even venture into houses and bush camps in search of food. 

Breeding 

In Botswana, the breeding season begins around June, and ends early October. This is not the hot summer season where the mercury can hit 95-104F (35-40C), but the winter months where daytime temperatures can be pleasant, but chilly. Typically temperatures average 52/54F (11-12C). 

Males establish quite a large territory, which they defend with retribution against fellow intruding males. When a male wanders into another’s territory, it is confronted with what is often referred to as a ‘combat’ dance. In effect, they are actually tests of strength, where they grapple with one another trying to assert their authority. The weaker of the two must move off to find a ‘patch’ he can call his own. Interestingly, mating between the sexes does not appear to be as brutal as that often witnessed in other Varanids. 

Around twenty-eight days following mating, the female goes in search of a suitable nest site to lay her leathery eggs. This is usually a patch of sandy ground in the vicinity of water, in which she will excavate a hole to a depth of 27in (70cm). It is not uncommon to find these nests in accompaniment to those of Nile crocs. 

That being said, Nile Monitors are notorious for digging out a hollow in a termite mound, and then laying her eggs in it. These termite mounds can be a considerable distance from water but mounds serve a useful purpose for the development of eggs. Once the female has excavated a hole, laid her eggs and departed, the busy little termites are then left to repair the damage. They seal the developing eggs inside, well away from predation. Not only does this offer protection, but the temperature inside the mound remains fairly constant, while outside the mound, the temperature can fluctuate wildly. Just what the eggs need. 

Typically, around 35 eggs are laid per clutch, with each measuring 2in x 1.5in (6cm x 4cm). When I worked in a zoological collection back home in the UK, we bred a pair of Nile monitors. We incubated the eggs in vermiculite at a temperature of 80.6 – 84.2F (27-29C), and humidity of 90%. This resulted in an incubation period of 28 weeks. 

The brightly coloured hatchlings measure around 30cm. When they emerge, it is the job of those within the termite mound to begin excavating, and making their way up the chimney from where they eventually emerge. It may take the little monitors several days to dig their way out, but they do not immediately disperse. They may remain close to the mound and scurry back inside at the first sign of danger. Not until the hatchlings have departed, can the termites begin the process of repairing the damage the young monitors have caused. Those born in a ‘conventional’ nest, once free, will scurry to the nearest area of protection, or even take to the water. Needless to say, few survive to reach adulthood, as predation is very high, particularly from predatory birds such as Yellow-Billed Kites. 

Captive care 

If you are looking for a large monitor which is easy to care for, has an attitude, but a challenge to breed, then the Nile monitor will fit the bill nicely. Even small individuals can be a challenge. Mishandling can result in deep lacerations to a misplaced finger. And as for those claws, well, they are as sharp as scalpel blades and capable of metering out nasty flesh wounds. Magnify that several times, and you can imagine what damage could be inflicted by a large individual. A snappy temperament, razor-sharp claws, whipping tail, and a writhing body which spins around in your hands as you try and hold it, are not easy to control. I have seen leather welding gloves ripped to shreds during handling these monitors. So, think carefully before you take the plunge and buy one. 

The criteria for keeping Niles in captivity, follows that of most large reptiles, plus a little bit extra. A very large cage, with ‘trees’ for climbing, and a large ‘swimming pool’ for soaking in are the prerequisites for this monitor species. They should be maintained at a temperature of 26-27°C  with a heat lamp suspended over a basking area where the temperature can be allowed to reach 30-35°C. In some areas of their range, (not here in Botswana) the monitor may go into partial hibernation between April and August. This need not be replicated in captivity. Neither is it a prerequisite to their breeding. 

Although feeding is not a problem, with large insects, chicks, mice, and rats being readily taken, one should be ever vigilant for the onset of obesity. There are two reasons as to why this can set in. Firstly, in the wild these monitors are active, continually on the search for food or a mate, and this burn’s off a lot of energy, even where individuals gorge themselves. Secondly, because of constraints on the limitations imposed by the cage size, activity levels will not be as high. Therefore, many hobbyists tend to over-feed their animals, and this entire nutritional intake is thus converted into fat which is stored along the sides of the body, a condition I have witnessed this in a lot of post-mortem specimens. I have even known people that suspected their animals to be pregnant, when in fact they are simply suffering from gross obesity. Good feeding management needs to be practiced to maintain a healthy individual. 

Tail-end  

If you can provide this monitor with the space it requires, and are prepared to put up with the belligerence, then it is an impressive lizard to look after. 

Did you know

Most lizards cannot run and breathe at the same time, due to something called Carrier’s constraint. Because most lizards move by flexing their bodies in a side to side motion, this flexing causes one lung to expand, while the other contracts, thus reducing the amount of oxygen getting into the body. It also moves ‘stale’ air from the compressed lung, to the expanding one. Again, reducing its efficiency. Most lizards also cannot use the same muscles for two actions – running and breathing. 

Monitor lizards have overcome this problem, by using an accessory ventilatory pump to initiate “gular pumping”. This effectively forces air into their lungs while running. The system is very similar to the diaphragm in mammals which works independently of movement.