Women evolved to have bigger hips than men because our primitive ancestors laid EGGS millions of years ago, study claims
- For years it was thought that women have wide hips to allow delivery of babies
- But female chimps also have wide hips, despite having smaller babies
- Experts say a shared ancestor developed wide hips to make it easier to lay eggs
Women have evolved to have wider hips than men because our primitive ancestors used to lay eggs, new research claims.
For years it was thought that the reason women have a wider pelvis is that human females have ‘child bearing hips’ to allow the passage of big-brained babies.
The heads of human babies are the widest part of their bodies, and the pelvis needs to be wider in ladies to accommodate this.
But the new findings – which will give new meaning to the phrase ‘being broody’ – say wide female hips go back much further than that.
Women have evolved to have wider hips than men because our primitive ancestors used to lay eggs, new research claims (stock image)
Barbara Fischer and colleagues from the University of Vienna, lay out their research in Nature Ecology and Evolution.
No bone in the human body is as different in size between men and women as the human pelvis – and as such it is the most reliable ways of differentiating between the sexes, the authors write.
But the reason there is a wider pelvis in women must have another explanation than childbirth, as even in creatures with tiny babies, such as the Virginia opossum, which gives birth to offspring just one thousandth the size of the mother, females also have wider hips.
Further evidence that big hips has an ancient evolutionary history comes from the study of chimps.
Chimpanzees, our nearest ape ancestors, have enviably trouble-free labours and pelvises that do not obstruct the birth of babies, but females still have wider hips than males, suggesting that big hips began many years earlier.
Analysis of reptiles also show that females have relatively wider hips too – and this may be key.
Wider pelvises in females make it easier to lay eggs.
The researchers suggest that wide hips is a trait with a long evolutionary history, and that the wider pelvis in females ‘existed already in early mammals’, adding that its purpose may have been ‘for laying large eggs relative to adult body size.’
Dr Fischer told the Daily Mail: ‘The female pelvis is indeed possibly wider because an evolutionary ancestor of ours laid large eggs.
‘In our paper, we show that the evolutionary pattern of a sexually dimorphic pelvis [differing between the sexes] has not been developed by modern humans but is inherited from our ancestors and it might indeed stem from early mammals or amniotes [other animals such as birds and reptiles], who laid large eggs or gave birth to large foetuses.’
In the study, the researchers measured the average female pelvis shape (top row) and the average female chimpanzee pelvis shape (bottom row) in anterior (leftmost), superior (centre) and lateral (rightmost) views
It is just as well that humans kept genes that allowed for wide hips, as when our brains in our ancient ape-like ancestors started rapidly expanding 2.5million years ago, the genetic ‘machinery’ was already in place to allow the hips to expand, the researchers said.
The researchers write ‘the genetic and developmental mechanisms to evolve a more spacious female pelvis were already in place, they did not need to evolve anew.’
Of mammals that exist nowadays, only platypuses and spiny anteaters, also known as echidnas, lay eggs.
Women with wide hips have more sexual partners and are ‘more likely to have one-night stands’
A study in 2014 revealed that women with wide hips are more likely to have one-night stands and more sexual partners in general.
This is essentially because they do indeed find childbirth less traumatic, the authors suggest.
The researchers define wide hips as those wider than 14.2 inches (36 centimetres) and small hips as those under 12.2 inches (31 centimetres wide).
This is measured by the distance between the upper outer edges of the iliac crest bones of the pelvis – the widest part of the hip bone.
The measurement is that across, not the circumference.
The 148 women in the study – aged between 18 and 26 – also had their hip circumference at the widest point measured and their waist circumference at its narrowest point.
All the women had at least one sexual partner previously. They also completed a questionnaire about their sexual histories, including the age at which they lost their virginity, the number of sexual partners they’d had, and information about emotionally significant sexual relationships they had had.
The researchers, from the University of Leeds, found women who were more inclined to have one-night stands had wider hips.
More specifically, the women for whom one-night stands accounted for three out of every four of their sexual relationships had hips at least 0.8inches (2cm) wider than those who had fewer one-night stands.