IN THE SONORAN DESERT of southwestern Arizona, the sand keeps a temporary record of all the comings and goings. In comparison to the giant tracks of the rare human staggering through, the flat-tailed horned lizard dashes about as lightly as a feather.

It was the skulls that caught April's attention. They hung from creosote bushes, their useless horns and gaping jaws a stark reminder of the dangers of soloing in the desert.

Intact or beheaded, recently hatched or decorated in combat, the condition of the body didn't matter to April or her husband, Utah State graduate student Kevin V. Young. The Youngs were capturing and measuring live specimens for Kevin's dissertation and a census on behalf of the Marine Corps Air Station in Yuma, Arizona.

After April spotted the first skull, the Youngs found many more skulls, and a basic ecology study to determine whether the flat-tailed horned lizard qualified as an endangered species metamorphosed into an evolutionary biology project to test an unproven theory. The habits of horned lizards are of interest not only to base commanders complying with federal statutes but to speculating evolutionary biologists. Even in less hostile territory than the Sonoran Desert, few biologists witness final encounters between predator and prey, and they can only theorize about the survival traits favored by natural selection.

The loggerhead shrike leaves behind gruesome evidence of its horned lizard diet. Swooping down on its target, the shrike aims the hook of its beak at its quarry, skewering it through the neck and filleting its soft tissue into digestible strips. The skulls are left dangling in larders as trophies for attracting mates. A large stash signals, "Hey, I'm a good provider!"

When Kevin Young came to Utah State for his master's degree in biology, he figured he and his wife were bound for southern Utah to study a common lizard there. That idea went by the wayside when Professor Butch Brodie, Jr., called Kevin into his office and said, "We just got this grant from the defense department to work with horned lizards. I don't do horned lizards. You want to do it?

"Of course, we said, yes."

Having grown up in Salt Lake City and spent many a summer family vacation in southern Utah, Kevin Young was fond of lizards and no stranger to heat. more