Don't ruin your children's excited anticipation on Thursday evening by telling them that reindeer can't possibly have red noses. Or that Rudolph in fact must be Ruth or Rosemary - not to redress any long established, macho, sleigh-pulling image, but because only female reindeer possess antlers at this time of year.
Indeed, when Santa arrives at your gate, his sleigh laden with gifts, he would be hard pressed to find an animal less well suited to speed him around on his arduous Christmas Eve mission than this rather clumsy creature.
Of all the plants and animals associated with the festival, the reindeer is the least realistic. The robin on your cards is as friendly a soul as he seems. Holly is to be found in woods and hedgerows everywhere. Likewise ivy. Even mistletoe, the oldest Christmas associate of them all, is still available, albeit imported from France because we have destroyed most of our home-grown variety.
But the teams of reindeer pulling a smiling Santa Claus on your cards simply do not fit the facts. True, reindeer can be used as draught animals. The Lapps in the very north of Scandinavia sometimes harness them to pull lightweight sledges. But reindeer are so well adapted to living in an intensely cold climate that they easily become too hot, panting and salivating in less than a kilometre at full gallop. The Lapps never race their animals, even in sub- zero temperatures. If they did, the creatures could die from heat
So their enormous delivery task on Thursday night is a non-starter. If Santa gave it a go, every town and village would be strewn with dead reindeer come Christmas morn.
In any case, reindeer rarely run fast. When they are made to, they look stiff and ungainly, more akin to a pregnant cow than a member of the deer family. The image of a reindeer team, complete with sleigh, galloping majestically over crisp snow owes a great deal to a vivid imagination. And - just to quell any lingering doubts - reindeer don't fly.
In a typical British winter, reindeer would find it uncomfortably warm. Their adaptations to a harsh climate are many and various. Their hooves are severely splayed, giving the animals a clubfooted appearance, but providing them with a broad pad - hairy on the underside for gripping - for walking on deep snow. Other deer have narrow, dainty feet.
Reindeer fur is a highly efficient insulator, so good that snow does not melt on the animals' heads or backs. In warmer weather they climb high into the mountains, not only to escape the warbles, nasal botflies, horse flies, mosquitoes and other insects that plague them, but to cool down by rolling in snow. Even reindeer noses are so designed that air breathed in is warmed, and exhaled air cooled, to conserve both heat and water vapour, which freezes instantly in very cold weather on human beards.
Reindeer noses stay dry. They are also completely covered in hair; bare skin would be a liability in the animals' sub-zero world. Not only would Rudolph have died of heat exhaustion, but his nose could not have turned even the palest pink.
Unique to deer, both male and female reindeer grow antlers. After the mating season in autumn, the males shed them, spending the winter devoid of their tree-like crowns. Only the females retain them, for reasons unknown, until after they have given birth in the spring. So on Christmas Eve, the only reindeer with antlers available to pull Santa would certainly not be called Rudolph.
It may sound indigestible but reindeer eat a diet of mosses, lichens and low-growing shrubby plants, often digging through feet of snow to find them. In summer their diet is more varied, including mushrooms. Digging in very cold, powdery snow is easy. In a winter like many of Britain's - damp with some freezing and thawing - reindeer would have a hard time of it. Snow that melts and then refreezes as ice is almost impenetrable by reindeer hooves.
A spartan diet suits the animals best. More concentrated foods such as cattle cake, even vegetables, give them diarrhoea. What a mince pie put out for them on the mantelpiece on Christmas Eve might cause is best left to the imagination.
Recent research at the University of Tromso in Norway shows that reindeer appetites vary considerably with the season. Of the seven recognised reindeer sub- species (the North American caribou is one), the most northerly variety - living on the Arctic archipelago of Svalbard, east of Greenland - eats well between July and August. It has to, because very little vegetation grows during the rest of the year.
The Norwegian populations further south are able to eat well for more of the year. But come winter, they, too, live largely off fat reserves. By Christmas, Norwegian reindeer have been fasting for a couple of months, a state not well suited for pulling a sleigh loaded with presents.
The now-established tradition - in Christian societies anyway - of reindeer at Christmas is only 170 years old. For this we can thank, or blame, Professor Clement Clarke Moore, a poet and classical scholar who lived in the small town of Troy in New York State. In 1822 he wrote a poem variously titled 'Twas the Night before Christmas or A Visit from St Nicholas and which he published in the Troy Sentinel.
A blend of personal make-believe and tradition-cum-myth from the Dutch Sinter Klaas, Teutonic and Norse stories, it was the origin of the jovial - then pipe-
smoking - Santa Claus on a sleigh drawn by miniature flying reindeer. The professor had almost certainly never set eyes on the animal. Rudolph - of red nose fame - first appeared in 1939 in a short story written by Robert May. Ten years later Johnny Marks used it as the theme for his song.
Estimating the animals' present world population is fraught with difficulty, mainly because most reindeer are to be found in the Arctic north of the former Soviet Union, where counts are virtually unknown. Estimates based on available information, compiled by the World Conservation Monitoring Centre, suggest there are more than 2 million. The north of North America has more than 1 million, followed by Sweden, with 300,000, northern Norway (40,000) and Greenland (20,000). Iceland and northern Finland have much smaller numbers.
Some reindeer are semi-domesticated, used as pack and draught animals. They supply meat and milk; their pelts are used for clothing, boots, tents and many other purposes, their tendons for stitching, their antlers for utensils. Many semi-nomadic peoples within or close to the Arctic Circle are dependent upon them.
When you take delivery of your Christmas presents this Friday, spare a thought for this unusual animal. For, even with a world population of more than 3 million, a reindeer's lot is not an entirely happy one.
The North American caribou is declining quite rapidly because of overhunting. Development and exploitation of northern lands for forestry, mining, hydroelectric power, oil pipelines, roads, even for airstrips and camping sites, are all reducing their habitat and disrupting essential seasonal movements. Air pollution and the legacy of Chernobyl - fallout from which continues to contaminate large areas of Arctic Europe - may be having an impact, too.
But as children dream of Santa Claus this Thursday night, the myth of the helpful reindeer is perpetuated. Long may it be so.