Iguanas on Mexico’s Clarion Island likely native, not introduced by people: Study

Researchers have long speculated that humans introduced spiny-tailed iguanas to Mexico’s remote Clarion Island about 50 years ago. However, a recent study suggests the Clarion iguanas are likely native to the island, arriving long before human colonization of the Americas.

Clarion Island is the westernmost and oldest of a small group of islands in Mexico’s Revillagigedo Archipelago. Despite its remoteness, the island is home to endemic wildlife, including two snake species and a lizard species, and at least three species or subspecies of birds.

In the 1970s, the Mexican military brought some nonnative animals to Clarion, including pigs, sheep and rabbits, which transformed the island’s native flora. Along with the domestic animals, biologists speculated the military also introduced a population of spiny-tailed iguanas (Ctenosaura pectinata) sometime between the 1970s and 1990s.

Wildlife records from earlier expeditions to Clarion hadn’t mentioned the lizards. When Daniel Mulcahy from the Museum of Natural History in Berlin and first author of the new study visited the island in 2013 and 2023 to study snakes, he began to suspect the Clarion iguanas were different from those on the mainland.

Genetic analysis confirmed his suspicion. The team found the island iguanas diverged from their mainland relatives roughly 425,000 years ago. According to some recent estimates, humans arrived in North America much later, roughly 23,000 years ago.

The researchers hypothesize that spiny-tailed iguanas likely arrived on Clarion from the Mexican mainland, a distance of about 1,100 kilometers (700 miles), by floating on vegetation mats across the Pacific. Once there, the lizards would likely have evolved in isolation. Based on their genetic analysis, the researchers consider the island’s iguanas “an evolutionarily significant unit” but add that more data is needed to determine if the lizards could be considered a subspecies or even a species of their own.

As to why previous surveys hadn’t spotted iguanas, the researchers pointed to the island’s dense cactus thickets and tall grasses. Thick vegetation historically covered the rock outcrops and burrows that spiny-tailed iguanas scurry away to when approached, which could explain why early visitors to Clarion hadn’t seen them, the authors write.

The domestic animals brought over in the 1970s cleared much of the vegetation, as did a large fire in 1984, likely bringing the iguanas into view. The findings of the study have important conservation implications, the researchers say; the Mexican government, assuming the iguanas were an invasive species, has been exploring plans to eradicate them from the island.

“Our research is of critical importance in demonstrating that iguanas are native to Clarion and should be considered part of the natural fauna, which should be conserved rather than eradicated,” the authors write. “This type of work is fundamental to conserving some of the world’s most unique and imperiled diversity,” Rayna Bell, an amphibian and reptile expert from the California Academy of Sciences, U.S., told The New York Times.

Banner image: Spiny-tailed Iguana from Clarion Island. Image courtesy of D.G.