Sonoran Desert

Following the Fighting Females

by Dennis Durband

Hikers visiting Arizona's Mt. Graham should consider a long list of potential dangers before they hit the trail. Monsoon thunderstorms can roll over the mountain during a summer afternoon, pelting plants and animals with furious wind, rain, and hail. Javelinas and black bears wander the ponderosa pine forests of the 10,717-foot mountain, foraging for food. Snakes gobble mountain spiny lizards as they try to defend their territories from one another.

A good hiker would make plenty of noise to frighten bears away. Backcountry visitors would steer clear of javelinas and rattlesnakes to avoid provoking them. And no one would risk attracting lightning by carrying a metal fishing pole along a mountain trail during monsoon season.

No one except Sarah Woodley, that is.

Woodley is a biology doctoral student at Arizona State University. Her scientific instincts overrule her instincts as a hiker. That's why she sits quietly in the forest. Woodley accepts the risks in order to study the aggression of female mountain spiny lizards.

For the past three years, Woodley has studied the behavior of these five-inch-long reptiles. She collects blood samples to study their hormones and observes the animals to see how they behave.

Lots of mountain spiny lizards live along the wood-lined creeks of Arizona mountains like Mt. Graham. They do not fear humans. The lizards are good candidates for scientific studies because they are easy to watch and easy to catch.

Both male and female mountain spiny lizards are intensely territorial. They do furious pushups that send the message, "Here I am! Don't come into my territory!"

At least that is Woodley's interpretation of lizard body language.

Woodley studies how hormones influence female aggression in mountain spinys. Female mountain spinys are more aggressive than females of other lizard species. They will defend their territories by displays of pushups or by extending their dewlaps (throat flaps). They also charge and bite intruders.

In male animals, the hormone testosterone leads to aggression. But scientists don't know why many females also behave aggressively, Woodley says.

During her last round of field study, Woodley worked in a forested area about 6,000 feet up the side of Mt. Graham. The mountain is located in the Coronado National Forest about 60 miles northeast of Tucson.

Mt. Graham is black bear country, and Woodley has seen her share of the big mammals. The hiker in Woodley knew that she was supposed to make noise to scare them away. Scientist Woodley had to remain silent.

She was more wary of the javelinas—because of their sharp canine teeth—and the "jumpy" cattle that graze through the forest. Snakes often ate her lizard subjects while she was studying them.

Monsoon thunderstorms filled the sky late in the afternoons. Woodley got soaked many times. Despite the attractions and distractions, she made her observations and collected 60 female spiny lizards over a seven-week period.

In a laboratory at ASU, Woodley performed surgery on each of her captured reptiles. She removed some of the lizards' ovaries—glands that produce eggs and female hormones. She gave others a testosterone implant.

A second group of lizards served as controls. The control group underwent fake surgery. Woodley didn't remove or implant anything on these lizards.

None of the lizard surgeries harmed the animals. They were put to sleep so they didn't feel any pain. Surgery only lasted about 15 minutes, and all of the lizards survived.

Each patient went home the day after surgery. Each was marked with a harmless paint stripe for identification.

Woodley says the lizards acted skittish for several days. As a result, she waited three weeks before making new behavioral observations. During those three weeks, she visited the site every other day to help the lizards get used to her presence.

Woodley watched to see how the surgically altered females reacted to intruders. She used a fishing pole and dental floss to place intruders on rocks in the territories of other females.

Females without ovaries showed a decrease in aggression. Since ovaries are the main source of the hormone estradiol, Woodley believes that estradiol plays some role in aggression.

However, the female lizards with testosterone implants rated high in displays but low in charging intruders. The testosterone wore off over time. Woodley thinks the hormone may have caused females to exhibit male-like courtship displays.

Fake surgery did not seem to change the lizards' behavior. More than 70 percent of those females charged and bit intruders as usual.

Woodley's research is important because most scientists have assumed that aggressive competition is a characteristic of males. They have ignored the large number of situations where females aggressively compete.