As the final stages of the red deer rut come to an end, Highland keepers are shaking their heads in sorrow. It looks almost certain that yet again, many of next year's calves will be bastards, the result of chance matings between native red hinds and sika stags, originating from the Far East.
The result of the matings between the native "hill" reds and the smaller, greyer sikas, is unpredictable, but tends to be a creature halfway between the two in size, with a mixture of the two species' behavioural characteristics. No one is quite sure of the full implications of the process, but prophets of doom warn of dire consequences.
Interbreeding presents both practical and emotional problems. On a pragmatic level, sika are more damaging to forestry: naturally preferring woodland to moor; browsing at a more damaging height, and, being much shyer, therefore difficult to control. In addition, because sportsmen pay handsomely for trophies, were sika genes to reduce antler size, it could affect a vital source of revenue in one of Europe's last wildernesses.
On the more emotional level, hybridisation risks threatening Scotland's distinct "hill" deer, which are smaller and more at home in open ground than other European reds. The problem stems from the sika's original introduction last century, but it was not until recently that the hidden dangers emerged. Now an increasing number of the Scottish red deer herd are being found to have "alien" DNA.
"The two species are distinct," explains Edinburgh University's Dr Josephine Pemberton, who is halfway through a three-year study of Scotland's deer. "Sika are much smaller, and are spotted in summer. In addition they behave and sound very different." At present, she says, Scottish reds can be divided roughly in two, with the A9 marking the border between pure and hybrid. In western areas such as Kintyre, crosses are common: to the east there is little or no hybridisation - so far. Yet she adds that after countless releases of imported animals by Victorian landowners trying to "improve" their sport, it is doubtful how pure hill deer are in reality.
"It's not just sika which have mixed with the herd," says Andy Rinning, director of the Deer Commission. "For over a century there have also been introductions of related species from Eastern Europe and America."
The dangers besetting Scotland's deer are not unique. "Crosses occur naturally in the wild and there is evidence to show that occasionally it may be important in species' development," explains Mike Bruford, head of the conservation genetics group at Regent's Park's Institute of Zoology. In addition, the injection of fresh genetic material can prove vital for flagging populations. For example, in 1900 Britain's red kite population was reduced to 15 individuals. This was recovering painfully slowly until the Twenties, when a solitary female was blown in by storms from Germany. The fresh genetic material revolutionised breeding success and today numbers are healthy.
But while scientists agree that natural instances of genetic mixing are perfectly acceptable, most feel that a distinction should be drawn between these and the results of man's intervention. A good example of the latter is the ruddy duck, originally imported from Carolina, which now threatens Spain's white-headed duck. "The ruddy is much more aggressive, and drakes drive off white-head males to mate with their females," says the RSPB's Chris Harbard. "The offspring look like neither parent, but they're fertile and carry on the hybridising process, so within a few generations we could lose the white-head altogether."
Mike Bruford agrees that hybridisation is an all too efficient way of losing a species. He explains that the worst problems come when one species has a numerical advantage over another and swamps it, leading to the rapid loss of pure genes.
In Ireland, in a matter of decades, County Wicklow's red deer have blended completely with imported sikas to produce a herd that is neither one nor the other. But, says Dr Pemberton, there is no evidence this will happen in Scotland: "Given the choice, hybrids tend to stick to their own dominant genes," she says. "So where possible sika 'types' breed either with pure sika or other lookalikes, rather than red deer."
In any event, it is now too late to stem the tide. "It is questionable whether we should waste time and effort trying to prevent the inevitable," she says. "What we can do is prevent damage in areas where we really can make a difference. For example, we should ban any deer releases in the genetically-isolated Outer Hebrides and all introductions of new species."