In the Fenway Victory Gardens, just minutes from Northeastern University, lizards reign supreme. The Italian wall lizard, Podarcis siculus, is a non-native species that recently established itself in the gardens. This small green and tan lizard has remarkably managed to survive and thrive in Boston — and in several other cities and towns in the northeast of the United States — despite the region’s differences from its native Mediterranean habitat.
Evolutionary ecologist Colin Donihue learned of Fenway’s P. siculus population in 2016, when a gardener emailed him a blurry photo of a lizard she’d spotted in her plot. A postdoctoral fellow at Harvard University at the time, Donihue was interested in the ways that species adapt to novel and human-dominated environments — a new population of lizards in Boston was the perfect case study. “I’d been jumping around the world studying this kind of question,” said Donihue, “and lo and behold, right here in our backyard was a prime example.” So, he grabbed his lizard-catching gear, hopped on the train, and headed to Fenway to investigate.
An Italian wall lizard perched on a rock in the Fenway Victory Gardens. Photo by Ella Messner, Biology & Mathematics, 2023
The Italian wall lizard population was in its infancy when Donihue began studying it. In the fall of 2016, he estimated that 15 to 20 lizards lived in the gardens — likely transported to Boston the prior year from a well-established population in New York City. Since then, the Boston population has exploded, growing to hundreds of individuals. As generalist insectivores, the lizards eat all sorts of bugs found in the garden; Donnihue’s team identified everything from ants to earthworms to beetles in their stomachs. In fact, the diversity of invertebrate life in the gardens may explain why the lizards have been able to flourish there. Although the Fenway Victory Gardens are located along a system of connected parks, which seem like an appropriate P. siculus habitat, the lizards have not spread beyond the gardens, possibly because food is harder to come by in the surrounding areas.
“Native to southern Europe, P. siculus does not normally encounter the frigid temperatures characteristic of winter in Boston — how this Mediterranean lizard survives the extreme cold remains a mystery.”
Food availability is likely not the only factor confining P. siculus to the Fenway Victory Gardens; the lizards’ survival strategy in winter may also explain the population’s limited range. Native to southern Europe, P. siculus does not normally encounter the frigid temperatures characteristic of winter in Boston — how this Mediterranean lizard survives the extreme cold remains a mystery. One hypothesis is that the lizards spend the winter in the garden’s massive compost piles. When microbes break down the organic material in compost, they generate heat, possibly creating a warmer haven for the lizards. Dependence upon compost piles could explain the population’s failure to expand beyond the Fenway Victory Gardens.
It is possible that the lizards have also developed the ability to dramatically slow their metabolism in the winter or be supercooled, enduring temperatures below the freezing point of water without actually freezing. Donihue believes that Boston’s wall lizards survive the cold through a combination of physiological and behavioral adaptations, but admits that scientists know little about what the population does during the winter. He fantasizes about solving this mystery by outfitting lizards with a GPS backpack of sorts that would track their location and activity throughout the cold months. Unfortunately, he hasn’t found a tracking device small enough for the lizards to carry, so their winter whereabouts remain unknown.
Although P. siculus is not native to the northeast of the U.S., Donihue does not believe the lizards pose a threat to the region’s ecosystems. There are no native lizards in Massachusetts, so P. siculus is not displacing or threatening another species. Furthermore, wall lizard populations in the Northeast are confined to human-dominated areas. “If they were to really jump those areas and go into more natural ecosystems, I think there would be more cause for concern,” said Donihue. Such a jump seems implausible; due to their likely reliance on human-created heat sources, Donihue suspects that the lizards would struggle without proximity to humans.
“So what do we do about our new reptilian neighbors? According to Donihue, we should simply watch and enjoy!”
So what do we do about our new reptilian neighbors? According to Donihue, we should simply watch and enjoy! On sunny days in spring, males can be spotted perched on rocks, flicking their heads, or doing push-ups to impress potential mates. By July, tiny baby lizards scamper across the gardens. “They’re just doing their own thing and they’re not going to bother you,” said Donihue. “They’re just on their own little journey.”
