Hedgehogs are insectivores, i.e. their diet mostly consists of insects. Although there may be some differences due to availability, in general their diet is similar regardless of species or location. British and German research has found the following foods in the diet of European hedgehogs:
A hedgehog will spend almost all of the hours between dusk and dawn searching for food. In this time it will travel around one kilometre, usually ending its search where it started - at its nest. It appears that the animal knows exactly where it is going and will regularly visit the same place if it finds a plentiful food supply there.
During the summer, hedgehogs spend the day sleeping in a light, flimsy nest constructed from grass and leaves. They will have a number of such nests, and often sleep in the same one before returning to a nest they have used previously. A nest may be slept in at different times by several hedgehogs; they don't seem to mind who originally built it. When the weather is warm, a hedgehog may not bother to build a nest at all but will simply lie under a pile of leaves or among long grass.
The main reason for the hedgehog being nocturnal is that its food is mostly nocturnal too. The creatures that it eats are small invertebrates that are active at night to avoid other predators, or those that must keep out of the heat of the sun to avoid water loss.
Like all animals, hedgehogs need energy to survive. In their case, it comes from beetles, worms slugs and other small creatures that they consume each night. In summer, these invertebrates are plentiful, but as the weather gets colder its food becomes scarcer. The hedgehog is then in danger of using up more energy in finding food than it gains from eating it. Hedgehogs are more prone to this problem than other small mammals as they lack the insulation provided by a fur coat.
Hibernation is not always a part of the hedgehog's life cycle. European hedgehogs found in New Zealand generally do not hibernate, and if they do it is only for a few weeks. In contrast, Scandanavian hedgehogs have an extended hibernation period as winters in the region are longer. Hedgehogs will only hibernate for the minimum necessary period as the process is hazardous. Nevertheless, hibernation is a valuable strategy that gives the hedgehog a chance to live through conditions through which it would have no chance of surviving.
The hedgehog's winter nest, or hibernaculum, is made of grass and especially of leaves, which are weatherproof and long-lasting. The hedgehog brings leaves to the nesting site in its mouth, a few at a time. It makes a pile, adding new leaves to the centre - and these are held in place by the surrounding twigs, brambles etc. It then burrows inside and turns round and round, packing the leaves flat and ending up with walls up to 10 centimetres thick. The hedgehog will make a new nest in this way every year.
The hedgehog's hibernation is more complex than just a lengthy sleep. It involves lowering the energy consumption of the animal by lowering the metabolic rate, slowing down the pumping of blood and lowering its breathing rate so that it may not take a breath for several minutes - making it appear dead. The only observable difference being that it will bristle its spines when touched. The hedgehog's body temperature can be lowered from the usual 35ºC to as low as 4ºC and the heart will only beat around 20 beats per minute. These reductions mean that the energy required to keep it alive is around one fiftieth of its normal energy expenditure. These reductions mean that the hedgehog requires very little energy, but it must survive the whole winter on its fat reserves. It is therefore vitally important that the hedgehog eats well before the winter season - building its weight up to at least 600g. It a hedgehog's weight is much below this threshold, it will probably not survive the freezing winter.
Hibernation is not continuous - a hedgehog usually wakes for a short time every seven to eleven days. Its body temperature returns to normal, and it usually remains alert inside its nest, although sometimes it may leave the nest and be active for several days or even move to another nest. It is unknown why the hedgehog might do this as there is no apparent benefit. Arousals are mostly spontaneous but can also be due to outside factors such as flooding, warm weather or disturbance of the nest by humans or animals.
By the beginning of March, the hedgehog's fat reserves which have sustained them through the winter will be almost exhausted. The first mild nights of March see the emergence of the first hungry hedgehogs. During the inter they will have lost a substantial art of their body weight and they must quickly replace it. If cold weather sets in again, they will return to their winter nests. By mid-April all hedgehogs should be out busily feeding on the insects that are once again abundant in gardens and the countryside.