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So what is a fox?

If someone asked you to define a fox, how would you answer?

A small red and white, doglike wild animal?

Well, yes, but define is not the same as describe. If scientists are to distinguish between foxes and the rest of the estimated 30 million species currently existing on this planet, they must use rigorous techniques that can be equally applied to all of them.

The need for a structured method of defining species was recognised centuries ago. Over the centuries a number of techniques have developed, reflecting scientific progress in a variety of fields:

• Measurements of skulls, teeth and other body parts

• Behavioural studies - are the two "species" reproductively isolated; that is, do they interbreed?

• Genetic analysis

The Victorians, who loved collecting and analysing, tried to get all their discoveries - from dinosaur bones unearthed by Mary Anning to the collections of naturalists sent to accompany explorers - put into neat scientific species "boxes".

Is it really that simple?

Unfortunately - no.

Back in the 1940's, Mayr thought that successful breeding only occurred within a single species. Horses and donkeys are separate species, so their offspring (mules) are infertile.

But we know now that the boundaries between wild species can be distinctly blurry. In North America, a fierce debate erupted over the status of the eastern / red wolf, one of the world's rarest large mammals. Genetic analysis showed in 2000, after years of controversy, that it is a unique species, but it is capable of breeding with both coyotes and grey wolves. This presents particular conservation challenges - hybridisation has hugely increased in recent centuries because of habitat destruction and fracturing of wolf packs caused by hunting.

But when all is analysed...

It is as well to realise that species are frequently reclassified as new research comes along. More rarely, entirely new species are found.

But currently there are believed to be about twenty species of fox distributed throughout the world, with South America having the greatest number of species.

Foxes include insect-eating specialists (bat-eared fox), an Arctic specialist (the Arctic fox, left) and species of the high mountains, such as the Tibetan fox. Representatives of the fox family are found in almost every habitat imaginable.

Is calling all these different creatures "foxes" a bit clumsy? Not really, because the diversity of life is such that scientists must group species into a complex classification system in order to have any chance of understanding their relationships. It works like this:

• Animal (Kingdom) - clearly foxes are not plants or fungi.

• Chordata (Phylum) - this is the category of animals with backbones.

• Mammal (Class) - foxes are warm-blooded, have fur, and suckle their young.

• Carnivore (Order) - this refers to animals with canine teeth, not necessarily meat eaters.

• Canidae (Family) - on the basis of physiology and genetics, foxes are considered part of the dog family.

• Vulpes (Genus) - more specialised analysis of form and structure put foxes in this sub-family, rather than, for example, Canis (where wolves are found).

• Vulpes vulpes (Species) - this is the red fox's scientific name, distinguishing it from other similar species, such as the Corsac fox Vulpes corsac.

Foxes and their relatives

The fox species which often features on my blog is the red fox Vulpes vulpes. This is the largest of all the world's foxes.

Apart from the other species of fox, they are also distantly related to wolves, coyotes, and other species within the Family Canidae.

However, while red wolves and coyotes - very close relations - hybridise as described above, foxes do not breed with dogs, coyotes or indeed any of their larger relatives. Red foxes have 34 chromosomes and dogs have 78, a massive difference. Red foxes are sometimes artificially bred with Arctic foxes in captivity but this is very unlikely to occur in the wild.

Grey wolves Canis lupus are by far the largest species found in the Canidae. Corsac foxes (below right) are found at the other extreme, being about half the size of a red fox. The family shows great variety in diet, behaviour and habitat preferences. The giants of the family (grey and red wolves, African hunting dogs, dholes etc) are considered apex carnivores due to their positions in their respective ecosystems.

The relationship of foxes to their larger cousins varies. While swift foxes are killed by coyotes as competition, the elk (and historically bison) carcasses left behind after wolves have eaten their fill are an important food resource for many smaller meat-eaters, including various species of fox.

Conversely, a study in Alaska found that many wolf dens are enlarged fox earths! It is strange to think that many wolves are born in holes originally dug by foxes.

Many larger species in this family have serious conservation problems, generally linked to persecution by livestock farmers. The last species of the Canidae to become globally extinct was the Falkland Island wolf Dusicyon australis (a species of large fox), also known as the warrah. Intensive hunting by livestock farmers and the fur trade eliminated this fascinating species, with the last known individual dying in 1876.

Apart from direct persecution, this family is also affected by diseases spread by domestic dogs (especially rabies) and their wide-ranging territories make them very vulnerable to road mortality.