Hominid skull gets abrupt sex change.

About 50 years ago, the Sterkfontein site in South Africa yielded the nearly complete skull of a hominid, or member of the human evolutionary family. Given its relatively small size, the 2.5- million-year-old Australopithecus africanus individual was dubbed "Mrs. Ples," a nickname it still retains.

But a new method for identifying the sex of fossil hominids indicates that this skull is male and thus deserves the title "Mr. Ples." The same method tags another, larger A. africanus skull from Sterkfontein as female, asserts Susan R. Loth of Florida Atlantic University in Boca Raton.

Loth collaborated with South African researchers Maciej Henneberg of the University of the Witwatersrand Medical School in Parktown and J. Francis Thackeray of the Transvaal Museum in Pretoria. She presented her findings last week at the annual meeting of the American Association of Physical Anthropologists in Oakland, Calif.

"When it comes to sex, size isn't everything," Loth says. "The small male and large female A. africanus skulls we studied differ comparably in size to a small male Bushman and a large female Zulu of today."

Unless pelvic remains turn up -- a rare occurrence -- the sex designation of a fossil skull or jaw hinges on a comparison of bone size and thickness among other examples of the same species.

Loth's group looks for sex-specific facial traits. One such marker is the shape of the zygomatic arch, a narrow bone that runs horizontally on each side of the face from just below the eye to just above the ear.

When viewed from the rear, the zygomatic arch of male primates typically rises sharply near the ear and juts back into the skull, creating a trapezoidal shape, Loth contends. In females, the zygomatic arch gently curves into the skull, producing a triangular shape.

This feature correctly signaled sex in 168 out of 200 adult modern human skulls, 23 of 23 adult baboon skulls, and 68 of 73 adult chimpanzee and gorilla skulls, Loth says.

Zygomatic arch analysis has also identified an A. boisei skull as male and a Homo erectus skull as female. Both fossils come from East Africa.

Previous research directed by Loth found that, in modern humans, the lower jawbone angles sharply toward the rear of the skull as it ascends to the cheek in nearly all males, but it remains relatively straight in females.

Hormonally regulated growth of chewing muscles and their associated bones create sex-typical facial traits such as these, Loth asserts.

"I think Loth has discovered something very important," remarks Milford H. Wolpoff of the University of Michigan in Ann Arbor.

The new technique may offer insights into the debate over whether A. afarensis, which includes the famed fossil Lucy, consisted of large males and small females or actually comprised two separate species, Wolpoff notes.