Florida’s Invaders: The Nile Monitor

In some parts of Florida, you may find yourself surprised to encounter a very large and very unfriendly dinosaur-looking lizard from Africa.

Nile Monitor

There are about 80 species of Monitor Lizard in the world, ranging across Africa and Asia and through the Indonesian Islands into Australia. They are an enormously varied group, ranging in size from small Tree Monitors only a foot long to the Komodo Dragon, the largest living lizard at around ten feet (though an extinct version in Australia was twice that size), and inhabiting everything from deserts to tropical rainforests. All are in the single genus Varanus (though this group’s taxonomy is undergoing a lot of changes, and it is likely that a number of new species and genus may become accepted).

Unlike most lizards, which are slow, plodding, and somewhat dim-witted, the Monitors are active and inquisitive animals, quite birdlike in their actions. In lab experiments, some species have even demonstrated an ability to judge numbers and can count up to “six”. Other species have been observed acting cooperatively to raid crocodile nests: one lizard lures the mother croc away, while the others then move in to dig up the eggs. In zoos, Komodo Dragons are able to distinguish individual zookeepers, and often develop bonds with their favorite humans. Because of their alert intelligence and their active diurnal lifestyles, several Monitor species have become popular in the exotic pet trade. And one of the most popular is the Nile Monitor, Varanus niloticus.

The Nile is a strikingly attractive lizard, with a greyish yellow throat and a glossy black body with yellow spots, stripes, and circular markings. The younger ones have the brightest colors. An adult will measure somewhere between 4 and 5 feet from nose to tail (exceptionally large ones of 7-8 feet have been recorded in the wild), and weigh around 15-20 pounds. They are noticeably more slender than most other Monitors, and their snakelike appearence is emphasized by their long forked tongue. And indeed some similarities in their skull bones indicate that snakes are evolutionary descendents of a lizard group related to the Monitors–as are the ancient marine Mosasaurs. Another unique adaptation is the presence of a muscular wall between the ventricles of their heart, which makes all of the Monitors much more efficient in extracting and using oxygen to power their high-energy active lifestyle.

Because it is so tough and adaptable, the Nile is one of only five Monitor species that are not in conservation danger. The lizards range from the Nile River valley all the way down to South Africa, and although they avoid dry savannah and desert areas, they can be found along virtually every major river system.

With their bright colors, inquisitive natures, and active lifestyles, young Nile Monitors were very popular in the pet trade and were captive bred by the thousands. Sadly, though, they had a rather large drawback, of which most of the people who purchased them were unaware: unlike many of their fellow Monitor Lizards, which quickly learn to accept humans, most Nile Monitors are nervous, belligerent and defensive animals which can capably defend themselves with teeth, claws, and a long whiplike tail, and they never really become tame. A bite from even a young Nile Monitor is painful; a tail lash from a large adult can break bones. It is a lizard not to be trifled with. And so, after being bitten a couple times, many would-be keepers decided that discretion would be the better part of valour, and turned their now-unwanted pet loose.

In most areas, the released lizards were unable to survive the winters. But the Nile Monitor is adapted for a life in warm water: they spend nearly all their time along tropical African rivers, either sunning themselves on the banks, hunting for fish or small mammals, or raiding crocodile nests for eggs and hatchlings. They found Florida, with its subtropical climate and its vast network of rivers and canals, perfectly suitable. Females dug nests in the sandbanks with up to three dozen eggs at a time. By 1990, wild populations were observed in the area around Cape Coral, and they spread quickly from there. Today they can be found from the Florida Keys up to Ft Meyers.

For the most part, Monitors will run from humans, usually by dropping into the water and swimming away. If cornered or caught, they are vicious in self-defense and can cause some damage. But their real danger is to Florida’s native wildlife. It didn’t take them long to take up their old nest-raiding ways, and in some areas Nile Monitors are significant predators on the nests and hatchlings of both American Alligators and endangered American Crocodiles (adult Alligators probably return the favor by eating the adult Monitors). But the big lizards are opportunists and will also eat ground-nesting birds, rodents and other small mammals, frogs, fish, crabs, and stray cats. (On the plus side, though, they also raid and destroy Burmese Python nests.)

Fortunately, the lizards seem to have become limited in their range by the weather, and have not penetrated very far up the peninsula. In some local areas, extensive programs of trapping have eliminated them. But in other areas, they seem to be here to stay.




6 thoughts on “Florida’s Invaders: The Nile Monitor”

  1. Komodo is the largest living Varanus, huh?

    Well, feast your eyes on — Varan the Unbelievable!! :>)

    (btw, “Rodan”, which in the Japanese original was “Radon”, and changed for English-language release so as not to be confused with the periodic table, was also a borrowed term; in that case, a somewhat anagramatic abbreviation of Pteranodon)

  2. We had plenty of these monitors on the farm where I grew up (or perhaps not exactly the same species; I’m not sure, and too lazy to go look it up now). Our dog once tore one up quite badly, and in turn got thoroughly whipped by the monitor’s tail, before the latter made its escape down a hole. We were sure it wouldn’t survive, but we saw it again some weeks later, scarred but otherwise in good health – monitors are tough.

    I once saw one in the space under a heap of rocks on a hill. It was mid-winter and the poor thing was too cold to move. When I poked it with a stick, all it could do was hiss at me. It does demonstrate that they can survive perfectly well through frosty winters though, so perhaps they’ll spread up Florida in due course. I don’t know how far north one has to go there before you encounter really hard winters.

    Where I grew up temperatures dip well below freezing every night during winter, but then warm up to around 15 to 20 degrees Celsius during the day. Not sure what the situation is in your neck of the woods, but our monitors seem to have simply hibernated through winter and could tolerate quite low temperatures as long as they had warm summers.

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