Liam Bowerman, 6, holds a juvenile spotted frog that lives in a small pond located in the Old Mill District in Bend.  

BEND, Ore. — In 2012, Jay Bowerman received an exciting email from a friend. The message contained a photo of an Oregon spotted frog that the friend’s 6-year-old nephew, Brett Judish, had caught in Bend’s Old Mill District.

But this was no ordinary frog.

“I told him what it was and I said it looked like a really big one,” said Bowerman, an expert on the threatened Oregon spotted frog species and a board member at Bend Science Station, where he also mentors students.

Bowerman, a biologist based in Bend, reached out to the U.S. Fish and Wildlife Service to relay word of Brett’s unusual discovery.

Biologists, dispatched to the pond where the frog was captured, began collecting more frogs and found that nearly all of them were tipping the scales at weights much higher than average.

“We joked when we first found them — are these frogs eating pizza or what? There’s a lot of restaurants in the area,” Bowerman said.

Insect smorgasbord

Take-out was not available to the frogs, but further research showed the pond in the Old Mill did have a smorgasbord of frog favorites, including dragonflies, damselflies, leeches, tree frogs, ants, yellow jackets and grasshoppers. Isolated from the flowing water of the Deschutes River, the pond was also found to be warmer than other areas near the river.

“The conditions in that pond seem absolutely perfect for rapid growth,” Bowerman says.

The pond was originally meant to be a catchment basin for runoff from surrounding areas during the development of the Old Mill District in the 1990s, but sediment eventually lined its bottom, allowing water to be retained year-round, said Bowerman. The area around the pond now has manicured lawns, town houses and parking lots.

The pond itself goes by various names. The Old Mill District administrators call it the “Hot Pond.” Bowerman refers to it as the “Casting Pond” because at one time it had rings where Bend’s fly-fishing community could practice casting.

The paved Deschutes River Trail is also nearby, but the frogs don’t seem bothered by the bike riders, dog walkers, skateboarders and joggers that pass daily.

Bowerman and student researchers have spent years investigating the life history and ecology of the Oregon spotted frog, a threatened species that is also at the center of water use disputes between conservationists and farmers who use Deschutes Basin water for irrigation.

The focus of Bowerman’s work has been on their size, diet, preferred habitat, growth, reproduction and survival habits. At last count there were seven to 15 breeding pairs of spotted frogs in the pond, said Bowerman.

Female spotted frogs in the pond average about 80 millimeters in length with some reaching up to 95 millimeters, or slightly more than 3½ inches. The average spotted frog found elsewhere is only around 60 to 70 millimeters in length. But the main difference was found in their weight. The female Casting Pond frogs can be as much as three times heavier than the average weight of females in other populations.

Sumo-sized frogs

“A 70-millimeter frog in Sunriver might weigh 30 grams,” said Bowerman. “A 95-millimeter frog from the casting ponds weighed almost 100 grams.”

It is not clear why individual frogs in this population have grown to sumo-wrestler proportions. The U.S. Fish and Wildlife Service acknowledges their existence but gives little explanation for their tremendous girth.

“The frogs at Old Mill pond are on the larger end of the adult size range for Oregon spotted frogs. The reason for this is not fully understood,” said Jodie Delavan, a spokesperson for the agency.

Bowerman believes their super size is a combination of factors, with the abundant food and warm pond waters playing primary roles. He explains that the warm water allows them to digest their food more quickly, allowing them to eat more and fatten up.

“Optimal growth can be achieved where there is sufficient food available in those warmer temperatures,” said Bowerman. “Those two things work synergistically.”

A spotted frog population is also located across the river in the areas near Hayden Homes Amphitheater, but those are normal-sized frogs, said Bowerman.

The Casting Pond frogs sometimes venture out of the pond to the river and will even swim across the Deschutes to the areas on the opposite bank. Many of them stay in the Deschutes during the winter months because the pond tends to freeze up for weeks or months. The large amount of ice in the water reduces access to oxygen.

‘Breathing’ through their skin

Contrary to popular belief, the frogs do not bury themselves in the mud. Instead, they will swim around under the ice in search of water that contains higher levels of oxygen, allowing them to take in oxygen through their skin. When oxygen levels drop, they will move to a new area.

“They have an exceptional ability to tolerate prolonged periods of low oxygen. We know they are capable of going at least seven days without oxygen,” said Bowerman.

Bowerman has been observing spotted frogs for 50 years and actively studying them since 1999. The past decade of study at the Casting Pond has been among the most informative, taking him back to his purpose of getting into biology in the first place.

“As a scientist, the process of learning how things work is why we do this,” he said.

Bowerman said the pond has also been useful in introducing student biologists to fieldwork.

“I get a kick out of it, especially when I get to work with young scientists and see them light up and grow in the process as we acquire information and evaluate it and draw conclusions. That has been fun.”

As for the frogs in the Casting Pond, they are expected to thrive as long as community members of the human variety observe them from a distance. And keep the pizza to themselves.

Scientific name: rana pretiosa

Status: In 2014 the Oregon spotted frog was listed as a threatened species under the Endangered Species Act. The main causes for their reduction in Central Oregon are habitat loss and invasive species, including bullfrogs and brown trout.

Range: Historically, they were found from Northern California to Southwest British Columbia. They are no longer found in California or most of Western Oregon and Western Washington. Currently, they are found in eight Oregon counties including Deschutes. Spotted frogs have been lost from over 78% of their original range.

Breeding: The spotted frog’s breeding period is late winter to early spring. Females lay around 1,000 eggs on average (as few as 200 but up to 2,000). Fewer than 1% of the eggs will make it to adulthood. Eggs may die from freezing or desiccation, and predators get many tadpoles or young frogs. For the population to remain stable, each breeding pair must replace itself once. 

Lifecycle: Larvae hatch into tadpoles between seven to 30 days. In Oregon, metamorphosis takes around 100 days. They begin to breed from one to three years of age. The lifespan is two to five years.

Highly aquatic: Oregon spotted frogs spend most of their life in the water, and as such their feet are fully webbed and their eyes are upturned, allowing them to peer out of the water.

How to help: When walking along riverbanks, make sure to avoid disturbing the habitat. Only enter the river at designated access points.

Oregon spotted frogs are highly sensitive to pesticides. When spaying your lawn or garden choose a nontoxic brand. That matters because even if you live far from the Deschutes River, chemicals can wash into storm drains and flow to the river.

It’s also worth noting that the Oregon spotted frog is a federally protected species. It is illegal to disturb or handle them without permits.