According to Cole Porter, “birds do it, bees do it, even educated fleas do it.” Ostensibly, “it” was “fall in love”, but I think we know what he was getting at.
All manner of creatures are referenced in his song, from oysters to electric eels, hippopotami to dragonflies. That’s appropriate because sexual reproduction is remarkably widespread. In fact, it’s the norm in almost every branch of the animal kingdom.
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But why? After all, there is an alternative: asexual reproduction. In many groups, including the vertebrates, there are species that reproduce by parthenogenesis, i.e. from unfertilised eggs. Indeed there are some species of fish, amphibians and reptiles composed entirely of females.
Parthenogenesis has even been observed among some birds. However, it’s extremely rare; which is good news for cocks.
Still, the fact that asexual reproduction is the exception and not the rule does require explanation.
Consider two females: one reproduces sexually, the other asexually. The latter would appear to be at a huge evolutionary advantage. She passes on all of her genes to her offspring instead of having to share parentage with a male. So, in terms of evolutionary ‘fitness’ sexual reproduction comes with an instant 50% penalty. This should be more than enough to guarantee the rapid extinction of a sexually reproductive lineage — unless, that is, sex comes with offsetting advantages.
The most obvious one is that dad sticks around to help protect and raise his kids. Such cooperation between the sexes is observed in many species. That’s especially true of the birds who tend towards monogamy and, in some cases, lifelong monogamy — as with swans and parrots. This is why there are so many avian metaphors for true love.
The mammals, though, are no better than they ought to be. On the whole, they’re a polygamous lot. Males, with their abundant supply of cheaply produced gametes have an obvious interest in sowing their seed as far-and-wide as they can get away with. Thus occupied, they’re not going to make the best of fathers — so why do females put up with them?
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To begin with, there’s the fundamental advantage of all sexual reproduction, which is genetic variation. Asexually produced offspring are genetical identical to the parent. Sexually produced offspring, however, are genetically distinct from both parents, and also from one another (with the exception of identical twins, triplets etc). Half the genes come from each parent, but the particular halves vary from sibling to sibling. Furthermore if she mates with several males, that’ll mean even more genetic diversity among a female’s offspring.
To understand why this is so advantageous think of reproduction as if it were a lottery. In this analogy, each kid is like a ticket — a gamble that it has the right combination of genes to survive into adulthood and reproduce. Obviously, if you’re going to invest in multiple tickets, then picking different numbers for each one increases your chances of winning. Reproducing asexually is like sticking to the same number no matter how many tickets you buy.
But what about species of low fecundity, i.e. where each female has a small number of offspring — perhaps no more than one or two. When you’re making one big investment rather than making lots of little bets, diversity isn’t nearly so important. So when it comes to sex, what’s in it for females in species of this kind? Philandering, fly-by-night males who don’t help out with the childcare would appear to be a serious liability. Indeed, the conflict of interest between the all-your-eggs-in-one-basket strategy of the female and the scatter-gun tendencies of the male is at a maximum.
Except that females do have an evolutionary interest in this set-up. Though they don’t have an immediate opportunity to spread their genes far-and-wide, they might over several generations through their male descendants. If their sons are reproductively successful, then a small number of children can turn into a large number of grandchildren and great-grandchildren all carrying copies of the matriarch’s genes — which in Darwinian terms is the main thing.
Females in such species will therefore seek mates with heritable traits that will, in time, make their sons attractive to females. This is a positive feedback loop that leads to the evolution of exaggerated features like the peacock’s tail. It serves no other purpose than display — and what it signals is “mate with me and your sons will have a magnificent tail too.” If you’ve ever wondered why the male in so many species is more brightly attired than the female, there’s your answer. However, I do wonder if show-off males also serve as decoys — diverting the attention of predators away from better-camouflaged females and their young.
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The evolution of males can be driven by other forces too. Male displays might be communicating more than the message “I’m gorgeous” — they might be saying “I’m healthy and well-fed, my feathers wouldn’t be so nice if I wasn’t capable of looking after myself.” Alternatively, sexually-selected male traits might not be about display at all, but some more direct way of winning the competition for females. Sheer size and strength, for instance, come in handy when you have to fight off other males. Which is why males in a species like the elephant seal are so much bigger than the females.
Then there’s the icky business of sperm competition — in which the struggle between males is more post-copulation than pre. In species where females are polygamous, the males evolve features that maximise the chances that their sperm will win the race to fertilise the egg. For instance, the intromittent organ (in mammals, the penis) may evolve into a shape designed to scoop-out the ejaculate of a previous suitor. It can get much weirder than that. One of my zoology professors once told of us of a species of gnat where the male organ breaks off inside the female thus blocking off access to subsequent visitors. Now, that’s what I call commitment.
A more common strategy is simply to up the volume of the ejaculate — flushing out the enemy, so to speak. That’s why testicle size to body mass ratio is an indicator of what kind of sex life a species is likely to have. The bigger the balls, the more promiscuous the species. Comparatively small balls tend indicate a monogamous lifestyle or a species where highly territorial males have exclusive access to the females they mate with.
OK, so at what point does zoology become anthropology? It’s a highly sensitive subject — and rightly so. Things that are merely behaviour in animals fall under a complete different moral category when they occur among human beings. That, unfortunately, hasn’t stopped some scientists from providing evolutionary ‘explanations’ for abusive and criminal acts.
Of course, on one level we are animals too and thus products of our evolution. And yet while we are influenced by our biology, we are not prisoners of it. Indeed the sheer variety of human sexual behaviour, which ranges from freely embraced celibacy to the most extreme promiscuity, is proof that alone of all species we have a choice — whether as individuals or as societies.
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There’s an old saying (attributed to Plato among others) that having a libido is like being shackled to a madman. That’s all too self-serving, especially of the madness of men. Biology is not an alibi.
In this respect, we are better guided by the Psalms than by Darwin:
“…what is man that You are mindful of him, or the son of man that You care for him? You made him a little lower than the angels; You crowned him with glory and honor.”
Whatever meaning you wish to find in these words, it is to the angels not the beasts that we should aspire.