In ancient mythology, salamanders could withstand fire. In modern times, though, just a small warming has been enough to dramatically change them—and perhaps threaten their future. Salamanders in the southern Appalachian Mountains of Tennessee, North Carolina, and Virginia have gotten smaller over the past 50 years due to increasing temperatures in their habitats, a new study has concluded. It’s the first confirmation that climate change can alter body size, a connection that had only been hypothesized in the past, and one of the fastest studied responses to changing temperatures on record.

“This is helping us understand yet another way in which climate change could play out,” says ecologist Michael Adams of the U.S. Geological Survey in Corvallis, Oregon, who was not involved in the new work. “In this case, we’re learning how it can change the life history of an organism.”

Over the past decade, scientists have documented several population declines in amphibians, including salamanders. But the causes of these declines have been hard to work out; some are due to disease, others to habitat loss or invasive species. Karen Lips, a biologist at the University of Maryland, College Park, wanted to determine what was behind dropping numbers of Plethodontidae, or lungless salamanders, across the eastern United States. The Appalachians are home to more salamander species than any place on Earth, many unique to the area. The Plethodontidae, the most numerous, represent a large and diverse family of salamanders that all respire through their skin. The salamanders play a key role in the ecosystems of the mountains, consuming insects that are too tiny for most other vertebrates.

Lips and her colleagues analyzed more than 9000 salamanders from a dozen different Plethodontidae species, some from museum specimens collected as early as 1950 and others from 85 current locations across the southern Appalachians. Although they didn’t find widespread evidence of disease, they did notice another trend: The salamanders had gotten smaller. Salamanders collected after 1980 were 8% smaller than those from decades past. The trend was most significant in places that had seen the largest shifts in climate during that time: low elevations with an increase in temperature and a decrease in rain, the team reports online today in Global Change Biology. On average, the salamanders shrunk by 1% per generation.

Lungless salamanders, Lips explains, are highly sensitive to changes in their surrounding environment because of the way they breathe through their skin. When the temperature rises, their body speeds up. Using computer modeling, Lips’ collaborators showed that temperature increases in the areas of the Appalachians that have warmed over the past 50 years could speed the salamanders’ metabolisms by more than 7%.

Lips’ team hasn’t yet confirmed the mechanism by which the change in body size has occurred, although she hypothesizes that it’s driven by the salamanders’ metabolisms. “If it’s a little bit warmer, the animal is going to burn through its calories much faster,” Lips says. If availability of extra calories or extra time to forage is limited, the salamanders will grow to a smaller adult size due to this increased calorie burn. And smaller salamanders, she says, lay fewer eggs and are more susceptible to predators. “In general, bigger is better for these guys,” Lips says.

The scientists haven’t yet confirmed that the salamanders’ smaller size is behind the documented population declines, though they are confident that climate change is what’s driving the shrinkage in body size.

Joseph Milanovich, an ecologist at Loyola University Chicago in Illinois, says that an ability to alter their body size could be what has allowed salamanders to survive previous temperature fluctuations. “These organisms have been on Earth for hundreds of millions of years,” he notes. “They’ve been through climate warming and cooling cycles before.” So the flexibility to alter their metabolism and body size may be an adaptation that’s arisen throughout these cycles. But that doesn’t mean that their current decrease in size will allow them to survive the faster pace of climate change that’s occurring now, Milanovich says.

Lips and her colleagues next plan to study how the change in body size affects the lungless salamanders’ interactions with each other and other species. The change in size, she says, could have broader effects on the Appalachian forest habitats they live in due to their key role in the food chain.

“These guys are really important in the food web of the forest,” Lips says. “They are extremely abundant and they play an important role in nutrient cycling by eating bugs and converting those into food for birds and mammals.” So understanding how salamanders respond to climate change is likely one piece of the larger story of how entire ecosystems respond, she says.