Lizards in New Orleans have highest blood-lead levels ever recorded, study finds

The brown lizards scampering on New Orleans porches and sunning themselves on sidewalks should be dead by any other standard.

That’s because the Cuban brown anoles live with the highest blood lead levels ever documented in a vertebrate, according to a Tulane University study published this month. Despite lead concentrations that would kill humans and other animals, the reptiles thrive, darting across fences and blending into the city’s leafy yards.

What started out as an effort to assess the impact on animals of high levels of lead in New Orleans took a turn when the scientists discovered just how high the levels are, shocking the researchers so much they repeated the initial test, thinking there might have been a mistake. 

“I don't think anyone would have assumed lizards would be rockstar heavy metal-tolerant animals, but here we are,” said Alex Gunderson, assistant professor of ecology and evolutionary biology at Tulane and a co-author of the study.

Record-breaking levels

The study, published in Environmental Research, found that lizards caught across the city averaged nearly 1,000 micrograms per deciliter of lead in their blood, with one animal topping 3,000. By comparison, health effects in people appear well below 100, and there is no safe level of exposure. The scientists working on the study were so shocked by the numbers that they repeated the tests.

In a person, the levels of lead the lizards would mean they were “probably dead,” said Gunderson. Lead is a potent neurotoxin that damages brains, hearts, reproductive systems and kidneys. It is especially prevalent in older cities such as New Orleans because of the lingering legacy of lead paint in houses and gasoline and industrial waste in the soil.

The other wild vertebrates known to have higher concentrations — scavenging birds like condors and vultures or Nile crocodiles — have died or shown symptoms at far lower levels.

Dr. Donald Smith, professor of microbiology and environmental toxicology at the University of California, Santa Cruz, called the findings "interesting and extraordinary." 

“The levels in these lizards are extraordinarily high, and the fact that they show no measurable signs of toxicity is surprising, because in other vertebrates I’m familiar with, blood lead above 500 micrograms per deciliter is associated with obvious illness or even death," he said. 

In condors that had lead poisoning from ammunition fragments, death could occur when blood lead levels reached around 100 to 150.

Testing the limits

The Tulane team captured about 100 lizards from across New Orleans, including neighborhoods with known high and low soil contamination. They tested the reptiles’ balance, sprinting and endurance using treadmills made for dogs and small dowels for the lizards to run across. The lizards continued to act like lizards. 

“We found no evidence that these extreme lead levels were physiologically affecting them in any way,” Gunderson said.

Researchers gave the lizards extra doses of lead to see when effects might appear. Only when blood levels reached about 10,600 micrograms per deciliter — more than 10 times the amount they were found with — did endurance begin to falter.

What it means for humans

To understand why the lizards are resistant to lead’s effects, the researchers examined gene expression in the brains and livers of the lizards. Several altered genes were linked to ion transport — the way cells move metals — and to oxygen delivery. Lead typically disrupts red blood cells, reducing oxygen flow and causing anemia. The lizards’ genetic shifts suggest they may be compensating by boosting their ability to move metals and carry oxygen, said Gunderson.

The study also found that lead levels in the bones of lizards were lower than the blood levels. 

"That suggests to me that the lizards are somehow binding or sequestering the lead in their blood, in their circulation, that may be somewhat protecting other organs that could be sensitive to lead," Smith said.

Their survival was shocking to researchers, but that doesn’t mean that the city’s soil is safe for humans. It is unclear how the lizards got such high levels of lead, and it may be that the source is insects that store lead in their exoskeleton. But it's likely the soil is also a source. 

“If you catch a lizard and it has high lead, it probably means that humans living in that area have higher risk of exposure,” Gunderson said. “They can be used as a proxy for exposure in humans and other animals.”

The animals that eat those lizards — cats and birds — are likely getting a dose of lead that isn’t good for them, said Gunderson.

Lead remains a worrying public health concern in New Orleans, particularly for young children who often play in soil or are likely to put their hands in their mouths. Decades of studies have mapped contamination across the city, often concentrated in older neighborhoods where lead paint and gasoline residues persist. 

The discovery of a vertebrate so resistant to lead poisoning raises new scientific questions that could lead to a better understanding of the lizards’ protective mechanism.

“We didn’t expect them to be particularly resistant,” Gunderson said. “That means there is some physiological thing that they have that we don’t. That could be important.”