There is a fairly wide variation in survival with geography. In a paper to the Journal of Zoology in 1990, for example, Hans Kristiannson noted that hedgehogs in New Zealand, where the climate is milder and they have no predators, tended to live about twice as long than those in Britain and Sweden, where winters are harsh and predators more abundant. Indeed, Kristiansson found that protracted very cold spells were associated with high winter mortality, writing that:
“Long cold winters seem to reduce the chances of survival because hedgehogs, which lose 40 to 50% of their autumn weight, lack the energy reserves to meet the extra demands of such winters.”
Similarly, on the island of South Uist, part of Scotland's Outer Hebrides archipelago, RSPB biologist Digger Jackson found a strong positive correlation between hedgehog abundance in a given year and the mean daily minimum temperature during the previous year, particularly between November and April. In other words, hedgehog populations were higher following warm years, particularly mild winters, and vice versa that cold winters reduce hedgehog populations the following year.
Despite being a broadly beneficial strategy that improves survival by drastically reducing metabolism during the coldest period of the year when starvation is all be inevitable, hibernation is by no means without risks. Indeed, several studies have shown that most hedgehog deaths happen during the winter. In Germany, for example, studies published during the mid-1980s estimated that 20-40% of adults and 70-80% of juveniles died in any given winter, while in Sweden, Hans Kristiansson found that juvenile winter mortality between 1972 and 1979 ranged from 6% to 94%, with adult mortality between 26% and 43% over the same period. Interestingly, Kristiansson's data suggest that the average (mean) winter mortality during his study was 33% per year in both adults and juveniles.
While in hibernation, hedgehogs are vulnerable to predators and unpredictable environmental events, such as flooding. How significant either of these factors are in terms of mortality remains unknown, but there are several accounts in the literature describing hedgehogs having been dug out of hibernacula by foxes and badgers. Hedgehog houses provided by humans in back gardens may go some way to reducing the likelihood that a hibernating animal will be predated, although there are some observations to suggest it is not just large mammals that pose a threat.
In a fascinating short communication published in the Journal of Zoology in 1987, Universität Konstanz biologist Hendrik Hoeck reported the findings of his study on hedgehog mortality in an enclosure near the university's campus in southern Germany during the winter of 1985/86. The enclosure was a 900 square metre (one-fifth of an acre) area encompassing deciduous woodland and open field fenced to exclude large mammals, allowing only small mammals (rats, mice, voles and shews) and birds free access. Thirteen hedgehogs were equipped with radio tags and their location and activity monitored throughout hibernation.
Hoeck observed that nine of the animals received an injury of some sort during hibernation, three of which were severe enough to result in death. One female in particular was found dead on the surface of the snow in early February 1986 and Hoeck described how patches of spines on the neck and back were short, as if they'd been cut and there was a deep round wound, about 2cm in diameter, on the neck. Two other injured individuals were recovered from their hibernacula and found to be in such poor condition they had to be euthanised. In the paper, Hoeck noted that he has observed “nibbled spines” in hedgehogs in south-west Germany during the spring, and other authors have told him of similar findings elsewhere in Germany.
Similarly, Walter Poduschka, writing in 1971, described a hedgehog just going into hibernation in his garden in Vienna being attacked and injured by a brown rat (Rattus norvegicus) and Paul Fowler described similar injuries in one of his captive animals that had been caused by another hedgehog. Subsequent analysis of the hibernacula in the enclosure showed no signs of damage by birds and no indication that any larger animals had broken in, leading Hoeck to conclude that the injuries had been caused either by rats/mice or other hedgehogs. In his paper, Hoeck concluded:
“Thus spine nibbling seems to be a common event in the wild, and it is possible that one or more rodent species may play an important role in the winter mortality of hedgehogs.”
In the case of juveniles, inexperience may also play a part in winter mortality and, in his London study, Pat Morris noted how most of the hedgehogs that died in their nests over winter were juveniles, probably too inexperienced to build a suitably insulated hibernaculum.
This seems like a rather obscure category, but hedgehogs are somewhat accident-prone and death through misadventure isn't uncommon. Such instances include hedgehogs drowning in garden ponds, becoming entangled in netting, electrocution by electric fences and falling into holes, particularly cattle grids, many of which now have hedgehog ramps built-in (see Declining QA). Indeed, the late Tiggywinkles founder Les Stocker wrote how hedgehogs are very good at falling into things, while Pat Morris, in a paper to the London Naturalist in 1966, described a hedgehog courting a bizarre death by licking sulphuric acid off a car battery in Orpington!
Hedgehogs are also fond of lying up in long grass and piles of leaves and branches—this makes them particularly susceptible to being injured by strimmers/mowers and burnt in bonfires. The advice is always to check for these animals before you start strimming/mowing or light your bonfire. Additional “collateral” human-mediated mortality comes from sources such as road traffic, litter (particularly elastic bands, which can fix around their waist and restrict them as they grow, and pots/cans that get stuck on their heads), the use of pesticides, metabolic bone disease that we think can result from over-consumption of foods such as mealworms and peanuts, and tidy gardens—see Interaction with Humans for a more detailed review of these.