An ancient human relative may have had a love life much like the modern gorilla, with single dominant males keeping “harems” of females, research has suggested.

A study of 35 fossilised specimens of Paranthropus robustus – a hominin that lived between 1.5 million and 2 million years ago – has revealed large differences between male and female growth that shed light on its probable mating habits. Paranthropus males continued growing for much longer than do modern human beings, well into adulthood, and eventually reached sizes that made them very much larger than females, according to the analysis led by British and Italian scientists. Most would still have been growing long after their female contemporaries had started to breed.

Such disparities in size between males and females, known as sexual dimorphism, is usually associated throughout the animal kingdom with mating structures in which a single dominant male secures access to plenty of females, while smaller, subordinate males have few opportunities to breed. The gorilla has just such a sex life, with males growing for many years before they develop into fully mature “silverbacks” that start to dominate and acquire a troop of females.

The new findings, published in the journal Science, suggest that Paranthropus lived in very similar fashion. “When we examined fossils from 1.5 to 2 million years ago we found that in one of our close relatives the males continued to grow well into adulthood, just as they do in gorillas,” said Charles Lockwood, of University College London, who led the research.

“This resulted in a much bigger size difference between males and females than we see today. It’s common knowledge that boys mature later than girls, but in humans the difference is much less marked than in some primates.

“Male gorillas continue to grow long after their wisdom teeth have come through, and they don’t reach what is referred to as dominant silverback status until many years after the females have started to have offspring.

“Our research makes us think that, in this fossil species, one older male was probably dominant in a troop of females. This situation was risky for the males and they suffered high rates of predation as a result of their social structure and pattern of growth.”

The collection of fossils came from the Swartkrans, Drimolen and Kromdraii sites near Johannesburg, and many are thought to have been left by predators. Many more of the fossils belong to males than to females, and this may reflect their lifestyle.

Young males who were not yet sufficiently mature to challenge for dominance would likely have wandered on their own or in small groups, which would have made them more vulnerable, Dr Lockwood said.

“Basically, males had a high-risk, high-return lifestyle in this species. 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.”

Paranthropus robustus was not a direct ancestor of modern Man but lay on a separate branch of the human family tree that is now extinct. It walked upright on two legs, and grew to about 4ft 3in to 4ft 7in (1.3m to 1.4m).

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