How do you sex and breed monitors?
Q. I recently bought a baby Nile monitor. I’m having a hard time finding much information about it. What I need is a way to check its sex for possible breeding later on. Is it realistic to even consider breeding this species in captivity? Also, exactly how do you tell a Nile monitor from a water monitor? They seem much the same to me.
Terre Haute, Ind.
A. Interesting array of questions, and I’ll try to answer all of them. First, regarding identification, pet shops sometimes confuse Nile monitors (Varanus niloticus) and water monitors (V. salvator). They look alike as youngsters because both are brightly spotted and have crests on their tails. However, a close look at the face will tell you which species you have. Water monitors have nostrils where you would expect to find them: at or near the tip of the snout. In Nile monitors, however, the nostrils sit much further back — almost half the distance from the snout tip to the front of the eye. This difference is easy to see when you look closely.
Now regarding sexing monitors, this is tough, and often it is not really possible. Like other lizards, male monitors have a pair of hemipenes at the base of the tail. When the animals are young, it sometimes is possible to carefully “roll” the hemipenes. While a friend holds the lizard securely, start about an inch or two below the vent and slowly and gently roll your thumb and first finger up the tail toward the hind legs. Roll your thumb as if you were rolling a pen in your fingers. This puts some pressure on the hemipenes in their pouches behind the vent and may push them forward enough so they can be seen exiting through the vent. Your dealer or vet should be able to show you how to do this. Note that this procedure will only work safely on young specimens. Adults just won’t tolerate it and could get hurt. You could, too, from all the potential biting and slashing.
Complications arise as a monitor becomes older. In adult females, often a pair of scent glands behind the vent can be erected much as can the hemipenes, which causes confusion. Some advanced keepers have used veterinary X-rays of the tail base to look for calcium spines on the hemipenes. The organs show on an X-ray, but even this doesn’t always work.
Regarding breeding, I guess the realistic answer would be: "Good luck." You have to remember that these commonly 5- to 7-feet-long, aggressive lizards not only need a large cage, but also require a large pool for swimming. Although fighting among a small group is not exceptionally dangerous, it is hard to combine a group of young specimens and let them grow up and naturally find mates. Assuming you can find a properly sexed pair and give them the large cage and pond they need, as well as temperatures in the 90 to 95 degrees Fahrenheit range, you could have some success. It has been done.
After mating, the female digs a shallow nest in sand or dry dirt near the pond and lays a dozen to several dozen large white eggs. She covers the nest, which uses natural sunlight for incubation heat, and then leaves the eggs alone. It may take Nile monitor eggs six to seven months to hatch, producing beautifully colored babies that immediately head for the water. Breeding Nile monitors is a challenge, but perhaps it is not an impossible one.