To predict what the monitor lizard will do in the future, Stephanie Dowell from Fordham University decided to look at how it behaves in its native home. Together with her supervisor, self-confessed museum geek Evon Hekkala, Dowell sequenced the DNA of dozens of Nile monitors from all over Africa—both freshly caught individuals and those preserved in museum collections.
She found several surprises. A supposedly distinct species called the ornate monitor turned out to be the Nile monitor under a different name. So two lizards became one. But the canonical “Nile monitor” actually comprises three genetically distinct lineages from the west, north, and south of Africa. The western one should count as a distinct species: It separated from the others 7.7 million years ago and its genome differs by up to 9 percent, a far greater gulf than exists between us and chimpanzees. So one lizard became two again.
The newly minted West African Nile monitor isn’t just genetically distinct, but also genetically narrow. Based on the similarities between different individuals, Dowell estimated that the lizard’s population crashed between 1,000 and 1,800 years ago. That coincides with the rapid expansion of the ancient Malian city of Djenné-Djenno—a major hub for trans-Saharan trade. Archaeological evidence confirmed that people were indeed catching and trading monitor lizards, probably for food. That’s why the lizard’s numbers plummeted.
Such exploitation continues today. Every year, around 500,000 Nile monitor skins are shipped around the world to be made into shoes, bags, and accessories. Another 10,000 live individuals are exported for the pet trade. And some of them have made their way to Florida.
When Dowell analyzed the DNA of 25 Florida monitors, caught by colleague Todd Campbell, she found that all of them belong to the West African species. All are descendants of lizards that had been captured somewhere between Liberia and Cameroon and shipped across the Atlantic. This happened on at least three separate occasions, giving rise to populations now living in Cape Coral, West Palm Beach, and Homestead.
Knowing where the lizards came from, Dowell could predict where they’ll go. Their West African home has a remarkably similar climate to southern Florida, so assuming that the invasive lizards are adapted to such environments, they’ll likely stay put. “I don’t think that they’re going to spread that far into the north,” says Dowell. “If we really want to target the surveying and eradication efforts, we should look further south.”
But the other species of Nile monitor is another matter. It can survive through the cold and frosty winters of southern Africa by hibernating. If it got into the U.S., Dowell’s simulations predict that it could make itself comfortable across the eastern and western seaboards, especially if the climate continues to warm.