Stephan Halloy was conducting surveys on plants and wildlife on the high plateaus around San Miguel de Tucumán in northwestern Argentina in the 1970s when he first encountered lizard Popsicles.
The mountains around the Argentine city climb rapidly to elevations of 13,000 to 16,400 feet, packing a multitude of ecological niches into a relatively small area. The plateaus at the top can be hot in the afternoon, but quickly drop to below freezing at night — not exactly the type of place you would expect to find a lizard.
Nonetheless, Dr. Halloy, now a senior adviser with the New Zealand Ministry for Primary Industries, caught a few and placed them in a box outside his tent overnight. “When I opened the box the next morning they were hard as wood — you couldn’t bend them,” Dr. Halloy recalled recently. “They looked absolutely dead.”
But once the sun came up, the lizards began to thaw and were soon running around in the box just like normal.
“Obviously I found that very surprising,” Dr. Halloy said.
In the 1990s, Robert Espinoza, a biologist at California State University, Northridge, heard this story from Dr. Halloy, and he has been studying lizard Popsicles ever since. The lizards belong to the genus Liolaemus, and research by Dr. Espinoza and his colleagues has revealed that the lizards are indisputably the coolest on the planet. Whereas tropical lizards like iguanas fall from trees when it gets cold, Liolaemus can supercool their bodies, tolerate freezing and live farther south and at higher elevations than any other known lizard species.
“They’re real record holders,” Dr. Espinoza said.
Liolaemus species have been found on the island of Tierra del Fuego, at the southern end of the Americas, and one researcher has even heard stories of them walking on Perito Moreno, a glacier in Patagonia. Most Liolaemus are found in Argentina and Chile, although some are found as far north as Peru. Containing more than 272 documented species, Liolaemus is the second-largest genus among all mammals, birds and reptiles, after only anoles, another type of lizard.
Dr. Espinoza is still investigating how these lizards survive such cold climates. In one experiment, his team fitted models of lizards, made of hollow copper, with temperature loggers and placed them at one area at 13,369 feet in Salta Province. The models recorded temperatures as low as minus 11.2 degrees Fahrenheit on the surface and 15.8 Fahrenheit underground. (The lizards usually spend the night in burrows.)
The team then tested the cold adaptations of six species from varying elevations. They found that some could survive cooling as low as 21.2 degrees Fahrenheit, although Dr. Espinoza suspects that wild lizards can withstand colder temperatures. Liolaemus huasihuasicus, the species that Dr. Halloy initially encountered, lives on a mountain about 1,640 feet higher than the highest species Dr. Espinoza looked at — a presumably colder area.
Dr. Halloy noted in a 1989 publication that Liolaemus huasihuasicus could survive freezing at 14 degrees Fahrenheit, but only when at an elevation of 13,944 feet; the lizards died when cooled to 26.6 degrees Fahrenheit at tests conducted at 1,476 feet.
Dr. Espinoza and his co-authors found that Liolaemus lizards have adapted abilities to deal with the cold through three mechanisms. Some lizards avoid extreme cold by going underground. Others use a process of supercooling; by staying completely still, they can allow their bodies to drop below freezing without actually freezing solid. Finally, some can also tolerate full-body freezing for short periods of time. Dr. Espinoza said that some Liolaemus species likely made use of more than one mechanism, depending on the conditions.
The strategy of full-body freezing is likely similar to that seen in North American wood frogs, which stay frozen over winter thanks to an antifreeze-like glucose solution that protects the cells; Dr. Espinoza still needs to investigate this hypothesis to be sure. The world’s southernmost gecko, Darwin’s marked gecko, another Argentine lizard that Dr. Espinoza has studied, most likely adopts the supercooling strategy.
Lizards of many colors
The reason Liolaemus lizards can withstand such cold temperatures and high elevations may also explain why there are so many of the lizards. Whereas there were only about 50 described to science when Dr. Halloy worked on them in the late 1970s, there are now 272 species.
Dr. Espinoza and others have discovered a number of species, and his occasional co-author Fernando Lobo, a zoologist at the National University of Salta in Argentina, has discovered 30 or more species of Liolaemus and its close cousin, the genus Phymaturus. In one case, Dr. Lobo discovered a species under his tent, in cloudy, frozen weather in the Argentine province of Santa Cruz near the Chilean border.
“They didn’t look like any of the others,” Dr. Lobo said. “We suspected they were a new one. We’ve had that excitement dozens of times in these 25 years.”
At the current rate of discovery, Liolaemus will likely become the most numerous genus of living mammals, reptiles and birds in coming years.
‘A species pump’
The large number of Liolaemus species may be related to the mountainous region where they live, Dr. Espinoza said. The Andes are relatively young — about the same evolutionary age as the lizards. He believes that as the Andes pushed out of Earth’s crust, the genus splintered into myriad ecological niches that eventually resulted in new species.
“The Andes are just kind of a species pump creating all these new types,” Dr. Espinoza said.
Most of the lizards are similar in size, but they differ greatly in color and even in diet and birth strategies. Dr. Lobo related a story about an expedition in Argentina’s Jujuy Province. During their work, a local woman appeared from a small sheep- and llama-herding village in the mountains.
“She told us very clearly which one was which species with their Indian names, and said ‘That one lays eggs and that one gives birth,’” Dr. Lobo said.
Dr. Espinoza said that half the lizards give birth to live young, perhaps because laying eggs in cold temperatures is likely not a recipe for success for some species. In 2016 he also described a new species, Liolaemus parthenos, in which the females reproduce through virgin birth, without fertilization from a male.
Melisa Olave, a researcher with Argentina’s National Scientific and Technical Research Council, who heard about the lizards on Perito Moreno, conducted a recent study showing that the rise of the Andes may not be the only factor driving Liolaemus evolution and diversity. Liolaemus species have very low extinction rates relative to other lizards. Their variation in habitat use, generalist approach to diet — some species are herbivorous while others are omnivorous or carnivorous — and different forms of reproduction may be critical to explaining Liolaemus species’ richness and survival. She said that being a generalist is typically considered advantageous, because it is easier to find suitable habitats in the highly varied landscapes of South America’s southern cone.
In other words, the high diversity of Liolaemus may be more a product of low extinction rate than of habitat splintering.
Dr. Espinoza agreed that species persistence over time could be a contributing factor in species richness, but he also believes that alone cannot explain the diversity.
In any case, Dr. Olave shares the general sense of wonder infecting many of the researchers who have worked with these lizards.
“Liolaemus species have an extraordinary ability to survive through time,” she said.