Why are humans the only species with curvy females?

IT TAKES a zoologist to study the weirdest animal of all: humans.

And one of the biggest mysteries concerns the woman of the species — how women came to have curves.

Why, wondered David Bainbridge, a Cambridge-trained veterinary surgeon and zoologist, are humans the only species with curvy women?

His ensuing investigation resulted in Curvology: The Origins and Power of Female Body Shape, a new book that examines why women are shaped the way they are and our cultural fixation therewith.

“Fat is part of femininity,” Bainbridge tells The Post on the phone from England. “In other species, you don’t get this weird thing where females lay down large amounts of fat before needing it.”

The average man is 14 per cent fat, the average woman 27 per cent.

That extra fat women begin to gain in puberty seems to prepare them for breastfeeding, which requires a huge number of calories. In modern times, this evolutionary development is often a mixed bag.

“When you’re a girl, you want to look more womanly, but that means putting on fat, and that’s a conflict because fat, we’re always told, is so awful,” Bainbridge says.

Women are also more obsessed with their bodies than men.

“It’s hardwired,” Bainbridge says. “They’re not looking at themselves because they’re worried about what men think. It’s largely because of how their body makes them feel about themselves and how other women see them. How you look is a very important part of your social status when you’re a woman.”

Here’s how Bainbridge breaks down the biology of the female of the species.

FACE

“All the things that we consider human — a round face, small jaws, teeth and brows — are more accentuated in women,” Bainbridge says. Men’s preference for those traits may have evolved at a time when several human species populated the Earth and it was more advantageous to breed with the most human.

BREASTS

Women are unique in the animal kingdom for their clearly defined bosoms. One theory suggests they’re the result of eons of sexual selection among men who’ve been conditioned to associate curves with health and fertility. And any size, apparently, will do. “Large breasts are preferred in some cultures, but as long as they look feminine, absolute size doesn’t matter,” Bainbridge says.

ARMS

“As a man, your arm is pretty straight,” Bainbridge says. “As a woman, it bows out towards the thumb direction in a 10- to 15-degree bend. Some have suggested that women have wider hips and so their arms need to swing past.”

FINGERS

Women’s ring fingers tend to be shorter than their index fingers, the result of low concentrations of male sex hormones during development. Men are often the opposite, although studies have shown men with longer ring fingers are nicer to women.

BELLY

Women have longer abdomens than men, providing more space for growing a baby.

“The belly is something that’s universally considered attractive,” Bainbridge says. “It’s related to reproduction and something men are hardwired to appreciate.”

THIGHS AND BUTT

Women with larger thighs have been shown to bear smarter kids, probably due to the brain-building lipids stored in butt and thigh fat.

WAIST

The measure of a woman’s attractiveness may well be her waist-to-hips ratio: Divide waist width by hip width, and the golden ratio is around .70. Some have suggested that the male obsession with a small waist stems from a desire to avoid hooking up with a pregnant woman. Other (contested) studies seem to show women with lower ratios are healthier and more intelligent, perhaps giving dudes an evolutionary reason for preferring them.

LEGS

Pin-ups aside, men actually prefer women with medium-length legs, not long ones. But no matter the size, what really counts is how straight they are — as that’s a sign of genetic health.