We humans are fascinated by sex. Some would even say obsessed. And it’s not difficult to understand why.
Not only are we one of the few species where both genders can find sex pleasurable – a gift we seem to share with some kinds of monkey – but we are the only species which, thanks to efficient contraception, has been able take the primary purpose out of the sexual equation.
For many people today, sex is more about recreation than reproduction, hence our endless worries over whether we’re doing it correctly and our preoccupation with such matters as frequency, duration, position.
A prickly inssue: Some say humans are obsessed by sex… but so are animals, says zoologist Dr George McGavin
I could, of course, go on. And a new exhibition at the Natural History Museum, Sexual Nature, certainly does.
However, this is not about human sexuality. Instead, its aim is to cast a spotlight on the variety of mating behaviours and extraordinary sexual goings-on in the wider animal kingdom.
If you want to discover what species has the longest penis relative to body size (the humble barnacle) or why the male angler fish should pick his bride carefully (his body becomes biologically merged with the far larger female and eventually atrophies until just a pair of testicles are left) or how hedgehogs do it (very carefully, obviously) this is the place to go.
Tell an audience about the mating habits of the honey bee and I know the men in the audience will all wince
As a trained zoologist, I have grown accustomed to how easy — fun even — it is for us to see the sex life of anything from bugs to bears through very human eyes.
Tell an audience about the mating habits of the honey bee, where the successful male is rewarded by having its abdomen and genitals ripped off by the queen bee, and I’ll know the men in the audience will all wince.
But they’ll all be smiling a little later when I recount the almost spookily human courtship ritual of balloon flies, where the male presents the female with a gift — normally a freshly-killed small insect — and while she’s happily chomping her way through it, has his wicked balloon-fly way with her.
Some species take to wrapping the gift in a sort of silk purse, making the parallels with a Valentine’s Day box of chocolates even more apparent.
But there are cads out there in an animal world. Leg-stroking, web-spinning and eye-stalk inflating are all perfectly respectable ways of getting your girl.
Once they’ve finished mating with one female, some balloon flies will promptly take their gift away and give what remains of it to the next female, a pattern the male will repeat until the gift is finished or he collapses in exhaustion.
The real rotters, however, are the males that produce a beautiful-looking silk-wrapped parcel — with nothing inside it.
By the time the female has discovered the deceit, the male has got what he came for and moved on.
Cruel world: What the queen bee (left) does to the male is excruciating, while everyone’s heard about the preying mantis (right) biting the head off the male
Of course, there are deceitful females, too, particularly among insects, where females are often much bigger than the male.
Best known, of course, is the praying mantis and the story about the female biting off the head of the male mid-copulation. It’s not apocryphal; it does happen — though more often in the laboratory than in the wild.
Perhaps the story is so well known because it strikes a chord in women; partly because many have known occasions when they’d have liked to pull a similar trick.
‘Sexual selection’ as Charles Darwin called it, is a mechanism that has evolved to ensure the best genetic material is passed on from one generation to the next
But there’s a serious side to the extraordinarily rich variety of sex that we find in the animal kingdom, where dragonflies have developed their own Swiss-Army-knife-style sperm removal tool which takes out the sperm of rivals; and where male hedgehogs plug their mate’s vagina with semen to foil any competitors.
The big question is why these variations in mating behaviour have evolved at all. Because there is a far less complicated alternative: we could all reproduce asexually (offspring arising from a single parent, without any fertilisation) as many plants and simple organisms do. It would be far easier if we could just produce endless, genetically-identical copies of ourselves: clones.
But, time and again, in just about any corner of the animal kingdom, evolution has favoured sexual reproduction over asexual reproduction.
There’s something about combining DNA from a female with the DNA of a male that produces such benefits that it makes all the time, energy and hassle involved worthwhile.
Scientists have been arguing about what these benefits are for years, but clearly they involve the creation of genetic variation — enabling at least some of the next generation to cope with a change in conditions.
It can also be a way to repair damaged DNA. If one gene has become faulty in the female, for instance, but not in the male, sexual reproduction offers the chance of repairing that damage.
In asexual reproduction, the damaged gene would be passed from one generation to the next, possibly resulting in the extinction of that genetic line very quickly.
Horses for courses: The male Angler fish (left) becomes merged with the larger female, while Bonobo monkeys (right) do it simply because it’s pleasurable
It seems that sex is best and the reason there are so many mechanisms for bringing it about is simple — they all seem to work. Sex is why a cockerel has his magnificent comb in order to attract the best possible mate, why a peacock has his gorgeous tail feathers, and why male deer lock their antlers in mating season.
(Some males never have to compete for a woman’s attention, a fact reflected in their anatomy. Guy the gorilla, a resident of London Zoo in the Seventies, weighed 37 stone but his penis would have only been an inch long because he wouldn’t need to use his genitals to compete for mates.)
All over the animal kingdom, there are displays or patterns of behaviour that effectively shout ‘choose me, I’m the best’ to a potential mate.
‘Sexual selection’ is what Charles Darwin called it, a mechanism that seems to have evolved to ensure that the best genetic material is passed on from one generation to the next.
Unless, of course, someone cheats — which is what a certain species of firefly does. When the female is ready to mate, she uses her glowing abdomen to flash a signal that attracts the right sort of male.
But these females have also learned the flashing signal that a smaller species uses, and, when she’s hungry, that’s the signal she flashes — when a small hopeful male turns up, she eats him.
By comparison, the male crane fly gets off pretty lightly, running only the risk of being dragged backwards through the air by its genitals, due to their habit of mating top-to-tail. If the female gets bored or hungry, that’s when the locked-on male knows he’s in trouble.
In contrast with the complexities of sex in the animal kingdom, we humans seem to get off pretty lightly. But there are obvious parallels.
We do ‘display’ through the clothes we wear and what we do with our hair; we engage in courtship rituals in the way we behave and dance and we all hope to end up with the best-looking mate we can.
Just as long as there are chocolates in that chocolate box and nobody gets dragged through the air by their genitals, most of us are pretty happy to leave it at that.