(Agencies)
Updated: 2007-11-30 20:07

Two-million-year-old skulls found in South African caves hint that ancient men preferred troops of females to a monogamous relationship.

The skulls belonged to Paranthropus robustus hominids, mostly males, extinct human relatives who split diverged from our evolutionary path about 2.5 million years ago. Scientists said the preponderance of male skulls, located in what were likely lairs of hyenas or leopards, offered a clear view into our close relative's social culture.

"That's a major clue because it tells us males are at greater risk of predation," said Charles Lockwood, a paleoanthropologist at the University College London. "You see this kind of death trend with other primates, where dominant males cast out the competition. Most of the specimens in our sample were the equivalent of young adults, 18 to 19 years old."

Run off by larger male suitors, Lockwood thinks younger males lived a solitary life, effectively exposing themselves to predators outside of group safety that females had. That, Lockwood said, is evolutionary selection in action for bigger, impressive males living in with a resident group of females.

"Males being selected for were the equivalent of dominant silverback gorillas," Lockwood said, effectively dwarfing their female companions - and rivals. "The dominant males survived to pass on their genetic heritage while others risked a greater chance of death."

Recent estimates of P. robustus suggest mature females were probably light at 65 pounds (30 kilograms), while dominant males weighed up to 130 pounds (60 kilograms) after five or six years of growth. Lockwood said these estimates are certainly close, but future access to bones other than skulls "would be fantastic" for better estimates.

"Predators always ate the good parts," he said; hungry animals left the thick, hard-to-bust skulls behind, preferring to munch on narrower bones with nutritious marrow inside. "But it's possible that we'll eventually find a cave or pit were P. robustus fell in. That might preserve most of the bones."

Lockwood said comparisons of the skulls and skull fragments added more evidence of a gorilla-like mating society: The males kept ballooning in size beyond sexual maturity, unlike human males that taper off soon afterwards.