The Hidden World of the Fox

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Domestic dogs possess a tapetum lucidum too, but in another aspect their eyes are very different from those of a fox. In bright light, a dog’s pupils contract to a round shape, which is not surprisingly also the case for the ancestral grey wolf. But a fox’s pupils contract vertically, like those of a small cat. Is this an advantage? Absolutely. Canid eye lenses contain concentric zones of different focal lengths, and a vertical pupil can exploit all these zones even when at its narrowest in bright light. This improves the focusing of long and short wavelengths of light, reducing or elimin­ating chromatic blur/haze in bright conditions. In short, foxes are multifocal.

More differences from dogs are under the skin. Fox skulls can appear superficially similar to those of dogs – I recently had to confirm the identity of one unusually large fox skeleton by examining whether the pre-orbital processes in its muzzle were concave or convex – but it is always worth considering the sagittal ridge. This runs vertically across the top of the braincase, and in case you are now rubbing the top of your head, please be reassured that you do not possess one. It is found in some great apes, however, as well as many Carnivora species, and even some dinosaurs. Attached to the sagittal ridge are the tem­poralis muscles, which are used for biting. The bigger the ridge, the more powerful the snap of those jaws.

Many thousands of years ago, foxes shared North America with a huge canid called the dire wolf. We have learned of this species Sherlockian-style, piecing its life together from minute details of the bones it has left behind, mostly in the macabre La Brea Tar Pits of California. This lethal but fossil-rich site has gifted museums a grand sur­plus of dire wolf skeletons. One is now on display at the Natural History Museum in London, and I recommend pausing beside it should you be visiting South Kensing­ton, for the sagittal ridge jutting out from its skull cuts an eye-wateringly impressive flange. A mighty hunter that pursued bison and elephants, or a mighty scavenger that crunched the bones of carcasses left behind by giant cats; we will never know. Regardless, its jaws packed incredible power.

Skulls of a dire wolf (left) and a red fox.

Foxes don’t hunt mammoths. Their sagittal ridge is remarkably small; in fact, their bite is weaker than that of a dog of the same weight, a fact exploited by many hunts­men over the centuries. Some dog breeds bred to kill foxes, such as Jack Russells, are noticeably smaller than their targets.

But just how big is a fox? It is a complicated question because they play an optical illusion on the human brain: they appear larger when they are further away. Some of us suspect the same phenomenon occurs with pet cats, which suddenly become ‘black panthers’ and ‘Beasts of Bodmin’ when sighted across a foggy meadow. Regardless, despite media speculation, there are no foxes the size of Labradors wandering British streets.

Measured objectively, a fox often weighs less than a pet cat: the average weight of male foxes in England is 6.7 kg (14 lb), with vixens about a kilogram (2.2 lb) lighter. Scottish foxes are somewhat bigger, with males tip­ping the scales at an average of 7.3 kg (16 lb). Of course, this is not due to over-indulgence in haggis, but rather an example of Bergmann’s rule – the ecological principle that in a species with a broad geographic range, individuals in colder climates are likely to be larger.

Head and body length averages 62–65 cm (24.4–25.5 in.) in English foxes; for comparison, a typical domestic cat is around 46 cm (18 in.). The thickly furred brush is equal to about 70 per cent of the fox’s body length.