Tiny lizards in New Orleans are packing the highest levels of lead any vertebrate on the planet—and it doesn’t seem to phase them in the least.
The new record comes from a study recently published in the journal Environmental Research that measured the volume and impact of lead on brown anoles (Anolis sagrei) from two different city neighborhoods. “We were shocked by the data, then it just kind of snowballed from there,” says Annelise Blanchette, who worked on the study as a Ph.D. student at Tulane University in New Orleans. “The fact that they are thriving is unique to [these lizards].”
Monitoring anoles that carry such “unprecedented” levels of this heavy metal could help public health officials identify city neighborhoods with higher potential human health risks. “Animals really are these sentinels that show what is there in the environment,” says Jesse Berman, an environmental epidemiologist at the University of Minnesota who was not involved in the work.
How lizards pick up lead
Previous research from Tulane linked lead tolerance with aggression in mockingbirds around New Orleans, and Blanchette wanted to expand on this. “We have a decent understanding of how heavy metals affect humans but a less well-rounded understanding of how it affects wildlife,” she says.
(Why do most U.S. eagles have lead poisoning?)
So Blanchette and her colleagues collected brown anoles from an older urban area uptown near Tulane University and another in Lake Shore, a more suburban area by Lake Pontchartrain. In the lab, the researchers found extremely high lead levels in the lizards’ blood. One individual had 3,192 micrograms of lead per deciliter of blood. “I remembered getting that data back and being very surprised that it was alive,” Blanchette says. The lizard average was 955 micrograms per deciliter—still extremely high compared to results from other vertebrates, including humans.
“If you were anywhere near 10 micrograms [per deciliter], doctors would be concerned,” says Blanchette, now a biologist at the U.S. Environmental Protection Agency. Levels that these lizards could tolerate would definitely be fatal for humans.
The lizards in the uptown area had higher levels on average than those from the suburbs. Blanchette says this is likely due to a couple of factors. For one, the Tulane area is a little older, meaning the houses are more likely to still have lead paint. Conversely, Lake Shore has younger buildings and less lead paint, but Blanchette also notes that its yards and other green spaces have a lot more remediated soil brought in from places with less lead contamination.
In neighborhoods, paint is a big source of lead. It erodes over time, and tropical storms can also knock flecks off the walls of houses into the surrounding soil. Brown anoles then ingest lead through direct contact with paint on houses or contaminated soil or by eating small arthropods.
Blanchette says that industry along the Mississippi is also likely responsible in part—it’s not just paint from old houses. The city “is unfortunately a hot spot for a lot of potential avenues for contamination, but it’s not a problem unique to New Orleans,” she says, adding that older cities likely have similarly high levels of lead contamination in the soil.
Yet, even the more contaminated uptown lizards didn’t seem affected by the lead in tests of balance, endurance, and sprint speed. They didn’t find any significant physical differences between lizards with higher lead levels and those with lower levels.
(Heavy metals give these spiders a powerful bite.)
Testing lizards’ lead limits
To determine how much lead these lizards can ingest safely, the team dosed Lake Shore lizards for 60 days at levels ranging from no lead to 500 milligrams of lead per kilogram of body weight per day. The highest two levels were too much for the lizards. “They had ulcers that developed around their mouths, and they had stopped eating,” Blanchette says.
But those receiving daily doses between one and 10 milligrams of lead per kilogram of body weight had no discernable symptoms. At the end of the test period, one lizard from the 10 milligrams per day group had a blood level of 10,600 micrograms per deciliter. “That was the highest that we could confidently say we aren’t seeing any negative effects,” Blanchette says.
Measuring only balance, speed and endurance may not be enough to definitively say these lizards can tolerate high lead exposure. Helle Bernstorf Hydesov, a wildlife veterinarian at the University of Copenhagen who has studied lead in mammals but was not involved in this recent research, says there is “no such thing as safe lead exposure” and questions whether it’s worth talking about tolerance. “You can kind of say, that if you’re not finding any side effects to lead, it’s because you haven’t investigated the right health parameters,” she says.
For now, the numbers Blanchette’s team found in the lab and in the wild put anoles in a league of their own. Marc Humphries, an analytical chemist at the University of the Witwatersrand in Johannesburg found 1,310 micrograms of lead per deciliter in one Nile crocodile in in South Africa—previously the highest ever recorded in a free-ranging vertebrate.
“Reptiles appear to possess a remarkable capacity to tolerate high levels of contaminant exposure with minimal apparent physiological effects,” Humphries says. In both the crocodile and lizard cases, “these lead concentrations would be lethal to any mammal or bird, but reptiles somehow manage to survive.”
(These lizards use bubbles to breathe underwater.)
How lizards survive lead poisoning
In a final test, Blanchette and her colleagues collected more anoles from Lake Shore and the Tulane area. DNA analysis a few different genes were turned on in lizards with high lead levels than those with lower levels. It’s intriguing that some of the genes that expressed differently have been previously implicated with heavy metal tolerance, says Blanchette.
Leaded anoles seem to express more genes related to oxygen carrying capacity, for example. Lead can interfere with the ability of cells to take in oxygen, so this may represent an adaptation in the lizards, Blanchette says. “If we can understand what’s happening in the cellular level in animals, then we can use that knowledge and apply it to humans,” she says.