"Orangutans are the most extreme situation," Lockwood said.
"Male orangutans go through an amazing transformation. They put on a lot of weight and get flanges on the sides of their faces. They appear nearly twice their old size, though some of that is an illusion and the change in the skeleton is much more subtle."
Adult males can stand 5 feet (1.5 meters) tall and weigh more than 200 pounds (90 kilograms), while females reach about 3.5 feet (a meter) tall and weigh in around 110 pounds (50 kilograms).
Such extreme size differences aren't common in other primates such as chimpanzees or humans—species in which harem mating behavior is abnormal.
Among humans, a single dominant male ruling a bevy of mates may seem to be a lucky individual. But the reproductive strategy was actually quite hazardous, scientists say.
The P. robustus fossils, found at the Cradle of Humankind World Heritage site in South Africa, are predominantly those deposited by ancient predators such as leopards and hyenas.
Because the remains are mostly male, they appear to have become victims far more often than females.
"Basically, males had a high-risk, high-return lifestyle in this species," Lockwood said.
"They most likely left their birth groups at about the time they reached maturity, and it was a long time before they were mature enough to attract females and establish a new group.
"Some of them were killed by predators before they got the chance," he added.
"One reason why that's important is because there are so few females in the fossil sample, [so] it appears that they were relatively safe.
"A key inference is that there were stable groups of females that would have allowed males to pursue this [harem] strategy."
Food vs. Females
Richard Potts, director of the Human Origins Program at the Smithsonian Institution's National Museum of Natural History, notes that it's very difficult to reconstruct early human ancestors' sex and social life.
(Related news: "Human, Chimp Ancestors May Have Mated, DNA Suggests" [May 17, 2006].)
"Lockwood's study is very clever in using the fossil record to interpret the pattern of growth in males and females in P. robustus and connecting it to the pattern of male competition, which seems to parallel that of gorillas," Potts said.
He also cited earlier theories that suggested the species may have lived in harem-type groups.
These more speculative ideas centered on the environmental conditions of life on an open savanna.
Widely dispersed plants might have caused males to emphasize defending their proximity to females rather than basing their territories on food sources, the theories say.
"It's intriguing to me that Lockwood and colleagues have come up with anatomical evidence that converges on a similar interpretation about the social lives of Paranthropus," Potts said.
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