Hedgehog Erinaceus europaeus (Linnaeus)

The hedgehog is found over the whole of Western Europe, as far south as the Pyrenees and the Alps. Further south the animals in Spain, Italy and the Balkans are distinguished as several sub-species that differ from the phenotype and each other by their larger or smaller sizes and depth of colour.

Photograph by Mark Wheadon	Photograph by ny156uk

Well known to everyone are the spines of the hedgehog that it uses as protection from predators. Although the hedgehog is well able to use its teeth against animals its own size they are not of much use against larger predators. Equally ineffective are the scent glands which do not smell obnoxious or deter attacks. This creates a situation where its defence relies entirely upon its spines which are confined its the back. When threatened or alarmed the hedgehog curls up in such a way as to completely conceal the soft under parts. This action is helped by skin muscles that run from head to tail, draw the spiny edge close up to the edge of the way in. This defence is so effective that like many well protected animals it makes very little effort in concealing itself, and will scuffle about in dead leaves and undergrowth making enough noise tell any nearby animals of its presence.

The spines of the hedgehog are also very useful as a shock absorber if it falls to the ground from a height, which given that they are fairly agile climbers is a fact that will undoubtedly come in very handy to them. The spines are firmly planted in its thick skin and each spine has a narrow neck at its base which acts as a flexible joint so that the base of the spine is not driven into the animal when landing from a fall. Beneath the spines thin neck it expands hemispherically as an anchor, and so firm is the attachment, that the animal can be lifted up by one spine alone. If an attempt to pull the spine out is made then it will snap off at the thinner neck, leaving the base intact.

Each spine is a highly modified hair and grows from a modified follicle in the skin containing at its base a papilla from which the spine is produced. The sharp end of the spine is solid and consists of the outer cortex layer only, but the body of the spine is hollow and consist of a thick medulla surrounded by a thin layer of cortex in which there are about two dozen longitudinal grooves. The medulla contains a number of successive compartments filled with air and separated from each other by thin plates of material that serve as struts to keep the spine rigid.

There are no sebaceous or sweat glands in the spine clad skin but they are in plentiful supply in the hair covered underside.

A big disadvantage of spine covered skin is the inability of the hedgehog is unable to groom the coat on its back and as such is heavily infested with fleas, mites and ticks which remain unmolested among its spines.

Teeth

The first incisor in each jaw is a comparatively large tooth that projects forwards and is followed by five unicuspids in the upper jaw and three in the lower. In the upper jaw the first two are incisor, the third is a canine followed by two premolars, The lower jaw, however, has jus one incisor, a canine and a premolar. In both jaws these are followed by 4 molariform teeth, the first of which is a premolar with the remaining three being molars. The upper canine sometimes has two roots as opposed to the normal one of other mammals. There is also a tendency for the third upper incisor to be also two rooted, the root bearing a longitudal groove as in animals where the root is not completely two rooted.

	Hedgehog dental formulae	I	C	P	M	total
.	upper jaw	3	1	3	3	36
.	lower jaw	2	1	2	3

I=incisors; C=canine; P=premolars; M=molars

In the hedgehog the peak of the breeding season occurs in May and June and second litters are born in July, August and September. For the remainder of the time there is complete sexual inactivity and the reproductive organs remain very small during this time but increase in size rapidly in the spring. In the male, the seasonal increase in the size of the reproductive tract is very great, particularly in the accessory glands. The complete tract reaches about 10% of the total body weight. The tissue of the testis is functional from April to August, after which regression is very fast and finishes by the beginning of October. The period of gestation last for about 4 weeks. The young are weaned at about 4 weeks of age and a new set of egg cells will not be released during that period the sow will not get pregnant again until the first litter has weaned. The litter size ranges from 3-7 with an average of 5. Lactating hedgehogs are sometimes found in October which suggests that some of the second litters are born rather late in the season. The release form the ovary of the egg cells is spontaneous and does not depend on the occurrence of mating and infact up to 3 to 5 sterile heat cycles may occur at the beginning of the breeding season in which even when mating does take place a gestation will not occur.

It is unlikely that all females breed twice every year and it is certain that the young hedgehogs do not reach sexual maturity in the year that they are born, infact those born late in the season do not breed until late in the following one. The young are born blind and have soft spines which do not harden until they are about three weeks of age. They are about three quarters grown at the onset of hibernation.

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Hedgehog hibernation

Hibernation in the hedgehog is much less regular than other mammals, such as the dormouse, because they do not always lie up for the winter until the end of the year, which discounts a phototropic period trigger to induce hibernation. They may only be torpid for 3 months but even this may vary from one individual to another.

Hibernating animals are aroused by warmth but cannot withstand freezing. This means that they have to ensure a good nest that is well protected from the effects of frost. This is usually provided by locating the nest underground amongst tree roots or under accumulations of leaves and brush.

Hibernation in a warm blooded animal is a lot more stressful than in a cold blooded one, as the hedgehog has to relinquish its body temperature regulation and in affect become cold blooded.

Just like any other hibernating mammal, the hedgehog puts on a lot of fat, which not only serves to keep the metabolism going through, at a reduced rate but also helps to provide a stimulus to initiate hibernation. In addition to this there is also a deposit of dark fat and lymph tissue round the blood vessels in the neck, chest and other areas. It is large in autumn but decreases to very small proportions by summer, but at a much slower rate than the general fat reserves. It has been termed as the hibernating gland but its actual functions are as yet obscure. At the beginning of hibernation the general fat deposits are very large, not only under the skin but also in the mesenteries (the thin sheets of tissue that anchor the viscera to their places inside the abdomen). It is these deposits that are used up first so that little fat is removed from the hibernating gland until the end of march. The largest part of the gland lies under the fore arm and is triangular in shape with the apex extending forward closely investing the external jugular vein, a smaller part extends up between the shoulder blades and along the to the base of the skull, and another along the front of the neck to the thymus and thyroid glands.

The gland is lobulated and at the beginning of hibernation is orange brown in colour, structured from lymphoid, glandular and fatty tissues. The amount of water in the hibernating gland varies conversely with the amount of fat. The fat is replaced by water as it is used up even though the animal does not drink. This water logging of tissues is similar to that seen in cases of starvation. The amount of water loss by the body is reduced by a slow down in the kidneys, by a system of short cuts of the smaller blood vessels. This reduces the amount of blood flowing to the surface area which is where the most fluid leaves the blood in the formation of urine.

There is some doubt about the temperature of the body during hibernation for when rectal temperatures are taken then very low values are obtained, but when the temperature within the heart is measured then higher values approaching normal waking temperatures are recorded. Although not entirely unexpected for a temperature gradient where the extremities are almost at surround temperature and with the centre remaining comparatively warm. Add to this temperature gradient the fact that the animal may only sleep for a few days at a time within intervals of activity until winter is well advanced.

As the hedgehogs temperature drops the rate of respiration decreases and may become so slow and shallow that it becomes difficult to detect any sign of breathing at all.. At the same time the heart beat becomes very slow and the blood flow around the body is greatly retarded. The blood itself alters in composition with the amount of blood sugars reduced to less than half and the magnesium levels doubled compared to normal wakeful levels. A great concentration of white blood corpuscles have been found within the coats of the stomach and in some places near the larger vessels are so densely packed as to from a gland like mass. This migration occurs as the animals temperature decreases at the beginning of hibernation and goes to extreme of from 18,000 to 20,000 white corpuscle per millilitre of blood down to 1,000 to 3,000 in the same volume. This may be by way of ensuring against bacterial invasion from the gut as the animals temperature decreases in that there are not only large amounts in the stomach but also along the alimentary canal and in the tissue around the pancreatic and bile ducts. Parts of the pancreas however, the islets of Langerhans, in which the insulin is secreted, remain in a fully functional state. This prompt the thought that insulin plays a very important role in hibernation.

Artificial hibernation can be induced at will by placing the hedgehog in low temperatures (providing that no freezing occurs), and in this state will sleep for many days until the temperature is increased and they awake in a normal manner returning to a warm blooded state. Thus proving that hibernation, in hedgehogs at least, is triggered by the average temperature of their surroundings.

Although no longer considered to be fair game, the hedgehog was regularly eaten in England. The classic way of cooking it was to gut and stuff it with sage and onion stuffing, then to sew it up and plaster the outside with clay. It was then suspended over a fire with a length of twisted worsted (yarn spun from combed long-staple wool) as a roasting jack, and when the clay cracked it was cooked. The spines then come away with the clay when it is broken up but leave behind the smaller hairs. Another way was to singe the spines and hairs off in the fire after gutting, then to scrape them off with a knife before cooking it with out the clay. A third way was to gut and skin the animal, was it well and simmer with a seasoning in water for several hours. When it cooled down it set into a jelly and could be cut into slices like pressed meat.

The hedgehog regrettably may be of some economic importance to human interests on occasions due to the species susceptibility to infection with the foot and mouth virus. Diseased hedgehogs have been found outbreaks of foot and mouth and it is believed that infected animals may sometimes spread the disease to livestock. The disease may be fatal to the hedgehog but the animal is not thought to be a major vector in the spread of disease due to its relatively small territory size and the ease with which the virus is spread by other vectors, including man. Because of its susceptibility the hedgehog is a very useful laboratory animal for use in research into the disease, and comparatively large numbers have in the past been kept in captivity for this purpose.

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