ASU Research E-Magazine: Tale of Tails

South Mountain residents tailgate with the best of them when they visit Tempe. Matthew Flowers is an inquisitive host who enjoys video taping their carefree behavior for hours at a time. Neighbors are divided over the tailgating parties. Some find them quite amusing, but others get so annoyed that they physically harass the guests.

The tailgaters are chuckwallas, four-legged revelers from South Mountain, home to one of the world’s most dense populations of the large lizards. Flowers wants to determine if South Mountain chuckwallas are a brand new species of lizard. Unlike people who have seldom seen chuckwallas, Flowers, his wife Catherine, and their son Ryan live a version of “chuckwallas at our door.”

South Mountain Park in Phoenix is the largest municipal park in the world, covering 16,500 square acres. It features unique geology and supports diverse plant communities. Within the park’s boundaries, the concentration of chuckwallas is five times higher than the normal density found in Southwestern deserts. As many as 60 lizards live in every hectare.

Rocky hillsides are perfect habitat for chuckwallas, the second largest of 38 lizard species found in Arizona. Elsewhere in the Southwest, one can expect to find about a dozen chuckwallas per hectare.

During cool months, chuckwallas bask at the edge of rock crevices. Social behavior and displays resume in warmer weather. Mating displays take place from May to June. The displays include puffing, frantic pushups, and fights among the males.

The average life expectancy for a chuckwalla in the wild is 10-15 years, but some of the big lizards live 20 to 25 years. They grow to almost a foot in length and can weigh as much as 12 ounces.

“South Mountain Park contains a unique population of chuckwallas,” says Flowers, a third-year doctoral student in biology at Arizona State University. “When I first arrived, the question was whether or not the South Mountain population should be considered a new species of lizard. We don’t have enough data to say that chuckwallas are an endangered species. But they are a popular species for collectors, particularly Europeans.”

The Arizona Game and Fish Department currently protects the South Mountain chuckwallas from collectors. The department supports Flowers’ research effort. The ASU scientist observes and collects live specimens. He also does radio tracking.

Flowers says that chuckwalla color patterns offer an ideal trait for investigating female preferences. Males possess conspicuous orange color tail patterns in some populations. The typical tail color is white. South Mountain chuckwallas are unique. The entire tail on most males is bright orange.

Flowers converted his Tempe backyard into a chuckwalla enclosure. He wants to know if female chuckwallas prefer one color tail to another. To find out, he paints the tails of males, then tests specimens within a population or between populations. To a scientist, the work is the purest form of tailgating.

“I’m using field data to determine how natural selection happens within the South Mountain chuckwalla population,” Flowers explains. “I want to know what evolutionary mechanisms are acting in the population right now that result in this unique color pattern.”

Brian Sullivan is an evolutionary biologist and ASU West professor. He also is one of Flowers’ advisers. “Matthew’s study of geographic variation in chuckwalla mating behavior has generated widespread interest among evolutionary biologists,” Sullivan says. “He is providing new understanding of how environmental factors can shape differences in the behavior of males and females of a species.”

Flowers makes behavioral observations on South Mountain during all seasons, including the sweltering heat of an Arizona June.

“We do focal animal observations,” Flowers says of the work he conducts with biology student Seth Heald. “We pick an animal, watch it for half an hour, and record its behavior. I watched the males fight and quantified their color patterns.”

Chuckwallas are secretive reptiles. Gathering enough data through observation of individual animals takes time. The ASU researcher says that the battles between male chuckwallas can be an amazing spectacle.

“They lock jaws and roll around on the desert floor,” Flowers explains. “Chuckwallas also make interesting social displays. Fighting is the ultimate part of social interaction between the males. But they will try to solve issues before fighting. They puff up their bodies and go through other physical displays, including what appears to be a series of rapid pushups.”

Flowers logs his observations into a computer database. He uses the computer to compare behavior, such as the number of pushups done among individual chuckwallas over a certain time period.

Interactions with his study subjects can be more painful. Flowers might spend anywhere from five to 90 minutes trying to collect a single lizard. Chuckwallas retreat from danger by wedging into rock crevices and inflating their bodies. A well-entrenched lizard can prevail, forcing the frustrated scientist to retreat, bloody fingers his only reward.

Tailgate Party
Over a month’s time last summer, Flowers caught 18 male and 12 female chuckwallas. He took them home for tailgate party observations.

The researcher built two large piles of cinder blocks in his backyard. A board is placed between the piles to separate these simulated rock dens. A smaller pile of cinder blocks is set off to the side. A male chuckwalla is tethered to the top of each large pile of blocks.

Flowers uses a non-toxic, water-soluble paint to enhance the tail color of each male chuckwalla. He paints the tails either dull orange, bright orange, or white.

One by one, female chuckwallas are placed without restraint on the small pile of cinder blocks. They immediately skitter to safety inside the blocks. The idea is that the larger piles of blocks offer more security to females than the smaller pile. Flowers waits and watches to see if the females relocate to a larger pile based in part on the color of the males’ tails.

Like a goose hunter in a blind, but armed with a video camera instead of a shotgun, Flowers observes chuckwalla behavior from inside his home.

Flowers watched as 10 females relocated to the block pile topped by males with bright orange tails. The other two selected males with tails painted white. During a second set of tests, nine of the 12 females selected males with bright orange tails over males with dull orange tails.

“The fact that South Mountain females prefer orange tails over white tails suggests that they will only mate with males from their own population,” Flowers explains. “This evidence suggests that they might be a different species.” The fact that South Mountain females prefer bright orange tails over dull orange tails tells us more.

Flowers says that orange tails may indicate the quality of a male chuckwalla’s territory. The orange color is based on carotenoid pigments absorbed from plants consumed by the individual lizard.

Flowers presented his findings during a symposium in Seattle. Response from other researchers was enthusiastic. Examples of male color patterns being influenced by female preferences have been observed in fish and birds, but never before with lizards.

Chuckwallas also have good chemical senses. Little nuances determine which male a female will select for mating.

“There probably are many factors that affect mate choice by female chuckwallas,” Flowers says. “Male tail color is just one.”

“Matthew’s study is still in the early stages,” says Ron Rutowski, an ASU biology professor and another of Flowers’ advisors. “But I think that his work should contribute to changing the way we think about the causes of differences among populations in male color patterns, as well as the consequences and origins of geographic variations in female mate preferences.”—Dennis Durband