Deprecated: Function get_magic_quotes_gpc() is deprecated in /nas/content/live/clemsonglimpse/wp-includes/load.php on line 651
A gnawing problem | research and creative discovery | Clemson University

A gnawing problem

Peter Kent


Wildlife biologist Greg Yarrow was working in his yard when he realized he had forgotten something. It was Saturday, April 5, 2008, his daughter’s second birthday, but there was something else, something important like a doctor’s appointment. There was an appointment, and he was the doctor, a professor, about to miss his interview on National Public Radio.

Disheveled and grungy, Yarrow dashed to the Brooks Center theater on campus. “I got there and the place was full,” he says. “President Barker and his wife were there, so were the provost and lots of other people I knew. I was sick at what I saw.”

Thinking it was going to be a recorded interview, Yarrow had not imagined he was to be on a live radio show. Michael Feldman’s Whad’Ya Know? is heard on hundreds of stations by more than a million listeners.

Backstage, the producer asked Yarrow if he knew who Feldman was. “I said I never had heard of him,” Yarrow says. The producer explained that Feldman did ad-lib interviews, having fun with his guests.

“I heard my name,” Yarrow says, “the guy gave me a little push, and I was on stage, and Michael was setting me up. I was the scientist giving birth control to squirrels. I was screwed.”

Retelling has polished the story. Yarrow repeated it last March to a group of wildlife biologists attending a conference at Clemson.

“Michael, it’s a very delicate process,” Yarrow confided like a surgeon instructing an intern. “You need soft hands and a gentle touch. We trap the squirrels, take the males, and turn them belly up. Then we very carefully put on tiny condoms.”

The story always gets a laugh. Yarrow pauses, smiling, waiting to deliver the kicker. “Then I said, ‘Michael, that’s not what we really do. Let me tell you the real story about our research,’” Yarrow says. Then he begins to explain the eight-year project to find out if birth control can manage the university’s squirrel population.

Loveable and useless

Clemson has a gnawing problem. Squirrels chew the bark on tree limbs, weakening the tree, killing the limbs. Falling branches create a hazard to people and property, and could cause as much as $1.3 million in maintenance and replacement costs, say campus landscapers. About a hundred trees are damaged and another hundred are vulnerable.

It’s hard to imagine squirrels as formidable adversaries. For most of us, squirrels fall in the “lovable and useless” category, which is the second-best position for an animal to be in, according to animal-human relationship scientist James Serpell. The top spot is “lovable and useful,” which includes guide dogs, monkey caregivers, and winning horses. The worst place is “loathed and useless,” where rats and mosquitoes thrive. Different cultures assign different statuses. Dogs don’t get a whole lot of love in Saudi Arabia.

Status can change, and damage by Clemson’s squirrels has some people calling them “rats with bushy tails” and “day-shift rats.”

Animal damage has become a big problem. The wildlife biologists who Yarrow was telling the “tiny condom” story to were some of the researchers attending the 15th Wildlife Damage Management Conference, spotlighting a multi-billion-dollar conflict: humans vs. wildlife. Nationally, wildlife causes well over $28 billion in damages and loss—loss that includes lives. More than 200 people die each year in deer-vehicle collisions alone.

Shifts in human residential patterns, along with wildlife protection and restoration programs, have brought more people and wildlife closer together than ever before.

Kristina Dunn and Andrew Watson pack a pickup with squirrel traps. In June, their team captured and examined a hundred squirrels. Photo by Peter Kent.

The Squirrel Whisperer

On campus one warm June day, a squirrel meets a friend, a woman who wears a poker-chip sized silver heart pendant side-kicked by a charm of a small silver squirrel. Her friends tease her with the moniker “Squirrel Whisperer.”

She slows the rump-sprung, blue Ford F150, hoping the squirrel will hop into the bushes, but she has to brake and wait for it.

“Get out of the road, I don’t want you to die!” Kristina Dunn yells from the driver’s seat at the squirrel standing stock still on Jersey Lane just off Cherry Road.

Dunn cares about the squirrels not only as part of her personal philosophy— she’s a vegan and an outspoken animal conservationist—but also because they are her research subjects.

A Ph.D. student advised by Yarrow in the Department of Wildlife Biology and Fisheries, Dunn is in her third year of a four-year study of oral contraceptives as a way to manage Clemson’s squirrel population.

In 2005, the university landscaping service asked for Yarrow’s help when the number of dead branches in some varieties of trees alarmed groundskeepers. “We were having to prune limbs that we would not normally remove because they could fall on sidewalks and parking lots,” says Paul Minerva, Clemson arborist.

The culprits were caught gnawing, squirrels chewing on oaks, hickories, and other thin-bark trees. “I wasn’t going to have the squirrels dictate how we care for the trees,” Minerva says.

Nixing the air guns

Yarrow proposed deploying Clemson’s crack-shot air rifle team. The cull would come when campus was empty of students and faculty. University leaders nixed the idea. A place full of scientists surely could find alternatives.

Besides, a squirrel hunt would create a furor on a campus where feral cats are treated as sacred cows and landscaping services sends out emails to explain why a tree will be cut down.

As for nonlethal alternatives, repellents would not control births. Trapping was no-go, too. In an unconfined space, traps catch squirrels, but other squirrels simply move in. “It’s called the vacuum effect,” says Yarrow. Birth control was worth a try. If nothing else, it would provide research projects for wildlife biology graduate students.

Doctoral student Murali Pai started in 2005 to research GonaCon. The drug decreases mating and pregnancy in a wide range of animals, from white-tailed deer to horses and even prairie dogs. It works via the immune system, triggering antibodies that interfere with sex-hormone production. One or two doses can last for years. But GonaCon is far from ideal.

Pai found out how hard it is to give a shot to a squirming, biting, clawing, pound-plus animal redlining its flight-or-fight response. Pai managed to vaccinate only 317 squirrels. What’s more, the expense was startling. “It cost about sixteen dollars and fifty cents per squirrel,” says Yarrow, Pai’s adviser.

Pai graduated and moved on. The squirrels remained, breeding, gnawing. Yarrow had recruited a new doctoral student for the squirrel study.

Kristina Dunn began in 2011 by setting up a survey plot for a statistical count of the squirrels. She reckons that there are nine squirrels per acre—about 7,200 squirrels at Clemson. Wildlife biologists figure that five squirrels per acre is optimal in the Southeast.

Other researchers have concluded that squirrel boars do the chewing. Why they chew is not settled.

“It could be that males do it to establish territory, or wear down their teeth, or it could be because the bark contains nutrients, or it’s a stress behavior,” Dunn says.

Watson identifies a squirrel, wrapped for safety in a woven pouch of cloth, by reading its implanted microchip. Photo by Peter Kent.

Tricking the body

Motive is not Dunn’s line of investigation, though “I’d like to know,” she adds.

Her research focuses on the oral contraceptive DiazaCon and the timing to put it out.

DiazaCon mimics cholesterol. It tricks the body into sensing it has sufficient cholesterol, consequently reducing production. Cholesterol plays a pivotal role in reproduction, stimulating production of sex hormones.

The drug, though, did not begin as an animal contraceptive.

Pharmaceutical maker S.E. Searle developed DiazaCon as a human cholesterol reducer. Side effects, however, made it unpopular with men, causing some to grow breasts and lose interest in sex. Searle discontinued the drug.

Scientists at the U.S. Department of Agriculture’s National Wildlife Research Center in Fort Collins, Colorado, bought the inventory, using it for research.

DiazaCon is mixed with other ingredients so the squirrels eat it. Dunn and professional lab technician Wayne Chao follow a recipe that includes hulled sunflower seeds, corn oil, DiazaCon, and Rhodamine B, which is a purply pink dye that stains squirrel teeth, fur, and whiskers. “I’ve tasted it and it’s not bad,” says Dunn, quickly adding that her sample was DiazaCon free.

The contraceptive, which lasts several months in the liver, is put out in April and November, leading up to mating seasons. The bait mixture goes into aluminum L-shaped feeding bins mounted in trees in the five test sites. Dunn waits to see if the squirrels take the bait.

The full treatment

June is one of four data-collection months. For five to ten days, Dunn and student assistants catch a hundred squirrels to identify them by ear tag or microchip, much like the one used for pets, and exam them in a lab. “New ones get the full treatment,” Dunn says. “They’re tagged and logged in.” Within two hours of capture, the trapping process is reversed. The squirrels are set free.

Dunn alone, or with helpers, will do this as often as four times a day during daily trapping periods. At sunset traps are closed and upended.

The sturdy wooden box traps are roughly twenty-four inches long and eight inches square. One end has an opening and the other is covered in hardware cloth. Inside, the trigger is a simple tray baited with sunflower seeds. A squirrel steps on the bait plate, tripping the door release. Dunn prefers thick plywood traps that make the enclosure a dark lair. “It’s less stressful for the squirrels than the exposed, all-wire traps.”

Traps are set on five test sites on campus and the control site at the South Carolina Botanical Garden. Some sites are more productive than others. “The ones near Lehotsky, Fort Hill, and the library reflection pond always are good,” Dunn says.

The area around Lever and Martin dorms is the worst site. Construction and leaf-blower noise keep the squirrels away. But a big reason for empty traps is not around now. “Students shoo the squirrels away from the traps,” says Dunn. “I wish they wouldn’t. Let them know we release the squirrels.”

Of the more than one thousand squirrels Dunn has handled, seven have died, either from trap stress or because she put them down due to their poor condition. There have been no deaths in the other species Dunn captures. Red-tailed hawks are caught to see if DiazaCon affects squirrel predators. So far, the data do not show a problem.

Watson releases one of the research subjects. Photo by Peter Kent.

Trap check

This June day, Dunn does a trap check.

Riding shotgun is Andrew Watson, a junior in the Wildlife and Fisheries Department. He and Brittany Sumner, also a wildlife biology junior, help out and are working on a Creative Inquiry project using Dunn’s data to plot the squirrel population and range.

Ten squirrels take the bait.

Watson and Dunn lug the traps to the truck.

Next stop is Godley-Snell Research Center, where the squirrels become data points—sex, weight, blood, scrotum and teat observations, tags checked, and whisker samples snipped.

Dunn has dealt with a problem the previous researchers struggled with—squirming squirrels. “I found a seamstress who made a handling cone that attaches to the trap door with Velcro,” says Dunn. “We open the door, the squirrel runs into the webbing, and is bundled up.”

After the exam, the cone is reattached and the squirrel returns to the trap.

Dunn will graduate next year. When she finishes, the project will be over. Yarrow has been promoted to college administrator. He is not taking on new graduate students.

Will DiazaCon help with the squirrel problem?

Arborist Paul Minerva says he already has seen signs that Dunn’s project has reduced the gnawing. Dunn is reluctant to talk about results before she completes her research, but she offers one observation.

“I don’t know if squirrels have reached the biological carrying capacity on campus, but they have reached the cultural carrying capacity,” she says. “That’s the limit society sets when it won’t tolerate a species that’s a danger or doing damage. It’s a value judgment.”

Greg Yarrow is the chair of the Division of Natural Resources and a professor of wildlife ecology in the College of Agriculture, Forestry, and Life Sciences. Peter Kent is a news editor and writer in Clemson’s Public Service Activities.

, , , , ,