It’s dusk on the Monday evening after commencement, and campus is unusually quiet — except near the Hesburgh Library’s reflecting pool, where a moonlight serenade is just getting underway. Loud, high-pitched, musical trills come from nearby trees and shrubs. Br-r-r-r-r-r-it! Br-r-r-r-r-r-r-it! As the night sky appears, the calls become louder and more frequent.
Amid this cacophony, tiny visitors about the size of a half-dollar coin appear one by one on the pool’s smooth granite ledge. Some hop down to its inner copper edge just above the waterline. They inflate their vocal sacs with air, creating a vibration across their vocal cords that can carry for a mile.
This sound is the mating call of the gray tree frog. The library reflecting pool is the local gathering spot for their singles mixer. Under the watchful eye of Touchdown Jesus, a whole amphibian world unfolds around and beneath the water’s glassy surface.
The pool’s residents are of particular interest to Gracie Hedgpeth, a fourth-year doctoral student in biological sciences. Hedgpeth’s goal is to develop a preventative treatment for chytridiomycosis, a deadly fungal disease, called “chytrid” for short, that is threatening amphibian populations around the world. The microscopic fungi travel through the water, burrow into amphibians’ skin and kill them.
Chytrid can spread from pond to pond on boots or equipment that aren’t properly cleaned, Hedgpeth notes. “It’s essential to clean boots, nets and gear before visiting a new freshwater site, because a few minutes of disinfection can protect entire amphibian communities,” she says.
Amphibians are the most threatened animal group in the world and are in decline, Hedgpeth says. Many species eat nuisance and disease-carrying insects such as mosquitoes, helping to keep those populations in check. Two out of every five amphibian species are at risk, and chytrid, their leading killer, has spread almost everywhere. “We need to figure out a way to protect these animals if we want to prevent extinctions,” she says.
Amphibians are threatened, she further explains, in part because their permeable skin makes them highly sensitive to pollution, habitat loss and pathogens. Because they are so sensitive, frogs serve as a key indicator — like a canary in a coal mine — of ecosystem health.
“If my research shows positive results in this species, maybe that’s indicative that [the treatment] can be used on a wider scale,” she adds.
Gray tree frogs (hyla versicolor) are common throughout the eastern United States, including Indiana. They are frequently found in forests, swamps and backyards and around bodies of water. Some people first learn of gray tree frogs when they see the amphibians, which have sticky toe pads, attach themselves to trees, windows or other vertical surfaces.
In late spring and early summer, when evening temperatures consistently rise above 60 degrees, the tree frogs gather at sundown at the reflecting pool. Passersby might not see them unless they know to look.
Hedgpeth learned of this tree frog habitat from a colleague. At first she thought it wasn’t likely, because the water is shallow, and she knew that University maintenance crews treat it with a mix of EPA-registered blue and yellow dyes to restrict photosynthesis and reduce the growth of weeds and algae. “Then I went out at nighttime and, sure enough, I could hear them calling,” she says.
The herpetologist grew up in San Antonio, Texas, never expecting frogs to become a central part of her life. “Definitely not. I’d never gone camping or anything outdoorsy,” she says. While earning a bachelor’s degree at the University of Oklahoma, she worked in a disease ecology lab and eventually decided to focus on amphibians.
Only male frogs make the calls, which they emit to attract females and establish territory near a breeding pool. The females — a bit like undergraduates pondering a prospective SYR date — listen, consider proposed partners and then approach the one they favor.
James Mitchell ’24, ’26M.S., visited the pool one night in May to study the frogs up close. “I’m trying to learn the different calls of the different types of frogs. I recognized these calls were associated with gray tree frogs,” he says. He first noticed them gathering while he left the library one evening a few weeks earlier. An aerospace engineer by training, Mitchell developed a strong interest in campus wildlife during his undergraduate years. Already familiar with most local bird calls, he is expanding his hobby to include frogs.
University researchers offered a public opportunity to observe the frogs at a free springtime event called Frog Fest. Hundreds of people — mostly families with children — turned out to learn about frogs, toads and other animals at science booths and through activities led by students and faculty on the library lawn. Hedgpeth, who organized the event, is conducting her research in the Rohr Laboratory of Ecology and Public Health, which is affiliated with the College of Science.
The chytrid fungus is believed to have originated on the Korean peninsula and to have spread via humans who sell and trade frogs for food, scientific study and medical use. Gray tree frogs haven’t been hit as hard by the fungal disease as some other species of frogs and salamanders, Hedgpeth says. Scientists want to determine why some amphibian species seem to be more tolerant of the fungus and how such tolerance might be extended to other species.
The antifungal treatment she is studying could be applied to bodies of water and absorbed through the frogs’ skin. Deployment would occur during breeding season, when eggs and tadpoles are in the water. Field trials are proceeding this summer in ponds at Notre Dame’s Linked Experimental Ecosystem Facility (ND-LEEF) in St. Patrick’s County Park near South Bend. So far, none of the adult frogs around the reflecting pool or at ND-LEEF have tested positive for chytrid.
Plentiful on campus, gray tree frogs aren’t endangered, and they make for a good test species, Hedgpeth says. Adults live in the crevices of trees and other sheltered places, usually not traveling more than a few hundred feet during their lifetime. They return to the same breeding ponds to reproduce.
The frogs who rely on the library’s reflecting pool likely have no idea about the two lakes on campus because they never hop that far, Hedgpeth says. And the reflecting pool offers a safer environment, because it doesn’t contain fish that would eat the frog eggs.
Hedgpeth’s work involves extracting the fungus that causes chytrid or ordering a supply from a commercial laboratory. She filters it and uses it to treat the water in a breeding pool, providing the frogs with just enough exposure to the fungus for them to develop a resistance to it. The amount of the fungus is not sufficient to infect the frogs with the disease.
“I found [last summer in the lab] that it makes them more tolerant. They survive longer even if they have the disease. They maintain their weight better and they don’t experience symptoms as much,” Hedgpeth says.
When it comes to reproducing, gray tree frogs aim for quantity over quality. They mate frequently in late spring and early summer, Hedgpeth says, because few fertilized eggs yield tadpoles that survive to adulthood. Each male and female breeds “hundreds and hundreds of offspring. In order to have a stable population during their lifetime, they’ll have maybe two successful babies,” she explains.
Once the frogs mate, they lay their eggs in the water. The eggs become tadpoles over a period of about a month, then metamorphose into froglets over the next few months, sprouting arms and legs, then into adult frogs, which hop out of the pool and take up their life on land.
This summer Ariana Yeganeh ’26, rising junior Jacob Greenway and rising sophomore Kailana Williams are assisting Hedgpeth. They gather frogs, take them to the lab to harvest the eggs, weigh and measure the adults, then gently release them near the collection point.
In autumn, adult tree frogs seek out an opening in a tree or other safe shelter. They go into torpor for winter, then emerge in the warmth of spring for mating. Gray tree frogs have an average lifespan of seven to nine years in the wild. Their amphibious lifecycle, which starts in the water and moves to land, necessarily bridges aquatic and terrestrial ecosystems.
Hedgpeth is captivated by the frogs and that cycle of life which is repeating itself in the Hesburgh Library reflecting pool. Whenever she sees someone splash through the pool — not a regular occurrence — she wants to warn them of the fragile ecosystem at their feet. “I’m thinking, ‘Oh, be careful,’” she says. “People have no idea.”
Margaret Fosmoe is an associate editor of this magazine.