Why should swans get divorced?
There has been a divorce.
So what? About a third of all British marriages end in divorce. Nothing unusual there, surely?
Yes – but this break-up has nothing to do with fickle humanity. It involves two swans and everyone knows that swans mate for life. It’s in many a folk-tale and confirmed by professional scientists. If one swan dies, its partner may mourn or at least remain celibate for several seasons – a big slice from the life of an animal that can expect to live in the wild for only 15 years or so. But now a pair has broken up.
The swans are Bewick’s – just a bit smaller than the familiar Mute Swans, and with a black and yellow beak. Unlike Mute Swans, Bewick’s migrate from Britain each April to feed and breed in Scandinavia and Arctic Russia, returning only in October to wile way the time and get themselves together in our somewhat milder winter. They tend to return to the same spot each year and about 300 of the 8000 that come to Britain home in on the Slimbridge Wetland Centre in Gloucestershire.
At Slimbridge, all the Bewick’s swans are known individually. So the curators know that the mated pairs normally fly out together, they return together. But this year the male Sarindi and the female Saruni broke the rules. Sarindi returned from his Arctic summer with a new mate – who, just to keep the confusion levels high, is called Sarind. The Slimbridge scientists feared the worse, because swans don’t normally take a new partner unless they are widowed. But a little while later, Saruni turned up as large as life – and she too had a new partner: Surune. So it was a break-up: only the second recorded divorce among Slimbridge’s Berwick’s in 40 years.
It all seems very sad – and disappointing, for swans are nature’s paragons, chivalric through and through: brave and powerful, faithful and beautiful. Shelley wrote rhapsodically to the skylark, “Bird though never wert!” – and could have written just as aptly of the swan.
How could such creatures have come into being? Surely, so many have said, they must be the work of a divine Creator, a supreme artist. Yet Charles Darwin told us that all living creatures are shaped above all by natural selection, subject only to the laws of physics, and honed above all for survival. There is, he said, “a grandeur in this view of life” – but it’s a stark kind of grandeur, leaving no room for sentiment.
Yet swans seem to defy the laws of physics. How could such an enormous creature – they are among the heaviest of flying birds –just sit on the surface, and sail like a galleon? Why that splendid, curved neck, like the prow of a Viking warship? Why do they glow so white – so visible? Why not disguise themselves like female mallards so they can hide among the reeds?
Ah, but natural selection seems to answer this too. Birds evolved first and foremost to fly and have gone to enormous lengths to reduce body weight. Even the birds that have long since given up flight, like emus and penguins, clearly evolved from flying ancestors. Inside the bodies of birds, and extending far into their bones, are spaces for air. Their feathers hold air, too, which helps to keep them warm. So swans float so high and mightily for the same reason as the bath-time plastic duck. Their bodies are full of air.
They have long necks because of the way they feed. Swans are closely related to geese and ducks and all of them live all over the world, or almost so; and anywhere you go you may find all three carving out the waterside habitats between them. Many geese, like the Canada Goose, Bean Goose, and Canada Goose may nibble the grass, sometimes far from water. So too do widgeons – and so, sometimes, may swans. Others, like the Shoveller duck, feed from the surface, sweeping the surface with their broad beaks. Some, like the Mallard, “dabble”: reaching down from the surface for whatever they can find. Some, like the Pochard, dive to the bottom to feed on shoots and roots. Swans reach down to feed on waterweeds like dabbling ducks – but they can reach much further. They can do much of what the Pochard does, without the trouble of diving.
Natural selection can even explain their faithfulness. It’s all a matter of strategy. Life in the wild is always precarious and many wild birds are widowed routinely – so it just doesn’t pay to put too much store by any one spouse. So we find that many birds, from Mallards to many a songbird may seem to have just one partner but in practice mate with everyone they can find – or at least, with anyone that’s attractive. Often, too, female birds – again including Mallards and many a songbird – lay an egg or two in a neighbour’s nest.
Modern DNA studies – the same kind used in lawsuits to prove paternity – can show exactly which babies in any particular nest belong to which parent. Cliff Swallows, which live in the United States, seem monogamous at first sight: certainly they nest in pairs, sharing parental duties between them. But in one colony, a DNA study found that nearly 40 per cent of all the nestlings were the fruits of what scientists dourly call “extra-pair copulations”. Yet by songbird standards that’s modest. Australia’s Superb Fairy Wrens are as sweet on the surface as their name suggests. But in one study, 98 per cent of their nests contained at least one baby that had not been sired by the female’s regular mate; and 75 per cent of all the chicks had been fathered by some other fellow far from the scene.
It is idle to moralize. For creatures for whom life is always on the edge it pays to spread the genes. It’s good for any one male – or any one female – to have eggs in several nests; and since the birds that practice adultery are also likely to be bringing up other people’s babies, it all evens out in the end. Respectable, English middle-class families in the late 18th and early 19th centuries similarly spread their offspring around the family, as reflected in the novels of Jane Austen and Charlotte Bronte (although they were not, as a rule, so adulterous).
But for swans, unfaithfulness just wouldn’t pay. Swans are highly territorial – they know what stretch of the river is theirs, and so does everybody else. The pairs gang up on intruders – and they get better at it year by year. Demonstrably, pairs that have been together for several years raise more young than pairs that are newly formed. So while natural selection encourages Cliff Swallows and Fairy Wrens to put themselves about, it says to swans – “Stick together!”
So why do swans divorce at all, albeit rarely? Because in any given season, a pair may fail to produce any eggs. If this happens more than once, then they simply decide to give it a try somewhere else. This, probably, is why Sarindi and Saruni broke up. Although humans divorce far more often they can also be far more romantic. Many a childless couple has lived happily ever after. But in nature after all there seems little room for poetry. Natural selection rules.
Yet Darwin was not the Victorian killjoy he may seem. Life isn’t just about survival. It is also about raising families, and that requires cooperativeness and selflessness. To raise a family, first find a mate – and that requires animals to be attractive, and not just physically, although outward signs certainly help. So Darwin in late middle age coined the expression, “beauty for beauty’s sake” – and this is what we see in swans. They are shaped as they are because this suits their way of life. But they are as visually stunning as they are because this is how other swans prefer them to be. We, mere onlookers, can just give thanks that there is such beauty to admire.
Give thanks to whom? Well, many have suggested that a creature like a swan – or any creature – could not have come about without a God to make it. Others claim that natural selection shows how life may come about without a God – and so the two sides go to war. But some simply point out that natural selection works. So why wouldn’t an intelligent creator make use of it?
It all makes perfect sense.
Colin Tudge, Wolvercote, January 25 2010