What Denmark’s dead hedgehogs teach us about their lives – and how we can aid them

The European hedgehog is in decline across Europe. In the UK, the species is already considered endangered The population has declined by at least 46% in the past 13 years, to about 500,000 individuals.

I have examined which is the reason for their disappearance for a decade. This included several research projects at the University of Oxford Nature Conservation Research Unit focused on optimizing conservation strategies to protect wild hedgehogs.

One of these projects, the Danish Hedgehog Project, involved more than 400 volunteers collecting 697 dead hedgehogs from all over Denmark, where I do research. My colleagues and I then examined how long did these hedgehogs typically live and why did they die.

The method for determining the age of a dead hedgehog is similar to counting growth rings on trees. When hedgehogs hibernate, their calcium metabolism slows down, which can be seen as a line of stunted growth in their jawbones. This allowed us to determine how aged 388 of these hedgehogs were when they died.

We have found the world’s oldest scientifically confirmed European hedgehogs. The oldest, called Thorvald, was 16 years aged, surpassing the previous record by seven years. Thorvald died in 2016 after a dog attack, which is unfortunately a fairly common cause of death for hedgehogs, but along with other hedgehogs we have studied, he now contributes critical knowledge to the mysterious lives of these animals.

Status of hedgehogs in Denmark

We also collected some surprisingly aged hedgehogs, aged ten, eleven and thirteen years. However, on average the Danish hedgehogs we studied lived only about two years.

Male hedgehogs lived longer than females. Males lived an average of 2.1 years, while the average female lived just 1.6 years. This finding is scarce in mammals and is likely due to the fact that being a male hedgehog is simply easier.

Hedgehogs are not territorial, so males rarely fight, and females raise their offspring alone. The high fitness cost of raising offspring alone may partly explain why the risk of death in female hedgehogs increases with age, compared with the constant risk of death in male hedgehogs throughout their lives.

More than half (216) of the hedgehogs were killed while crossing roads. These deaths, 70% of which were males, peaked in July, during the mating season. Male hedgehogs tend to have larger home ranges than females, and because they expand their ranges during the mating season, they often cross roads. Tests Research I co-authored in 2019 showed that the range of male hedgehogs in suburban areas around Copenhagen, Denmark, increased five-fold during the mating season.

Of the animals that were not killed in road traffic, 22.2% (86) died in wildlife rehabilitation centres after the public found them unwell or injured, while a further 21.6% (84) died of natural causes in the wild.

Low genetic diversity

We also studied the effect of inbreeding on the lifespan of European hedgehogs. My previous research The Danish hedgehog population was found to have low genetic diversity, indicating a high degree of inbreeding. Low genetic diversity can reduce the fitness of an individual and may lead to several potentially fatal inherited diseases.

Inbreeding can occur when hedgehogs are constrained in their search for suitable mates. The likelihood of inbreeding increases when their habitats are fragmented by roads, buildings, fences and railways, and population declines limit the pool of potential mates.

Surprisingly, we found no association between the degree of inbreeding and age at death in our hedgehogs. This is captivating because there is a general lack of knowledge about the effects of inbreeding in wild animals.

how can you aid

Hedgehogs are becoming more numerous inhabited areas which are occupied by humans. However, our study reveals that humans are the main factors causing the decline of hedgehog populations.

Many hedgehogs only live long enough to have one or two breeding seasons. However, our discovery of Thorvald and several other aged hedgehogs suggests that their ability to avoid dangers such as cars and predators would improve if they managed to survive for at least two years.

There are several steps you can take to aid hedgehogs cope with the dangers they face. Hedgehog Streetenvironmental protection campaign jointly funded by People’s Foundation for Endangered Species (PTES) and British Hedgehog Protection Society (BHPS) offers some useful advice.

For example, removing barriers between our gardens to create hedgehog highways will allow hedgehogs to move freely between gardens in search of food, nesting sites and mates. This may reduce the need for hedgehogs to cross roads as often.

We take care of our gardens hedgehog friendly is another option. Piles of logs and leaves or specially designed boxes called hedgehog houses provide safe and sound and secure places for breeding and nesting. Providing plenty of greenery in the garden will also attract insects, snails, earthworms and slugs, which the hedgehogs will feed on.

It is critical to remove anything from your garden that could harm hedgehogs, however. This includes poisons, netting, garden tools, aggressive dogs, deep holes and steep edges around pools or ponds.

Any improvement in our knowledge of hedgehogs in the wild will also be critical. The Zoological Society of London studies hedgehog populations in Greater London through its London Pork Guard project. And BHPS finances Hedgehog-friendly campus an initiative that awards prizes to universities, colleges and primary schools for activities aimed at helping hedgehogs thrive in their areas.

The latest BHPS and PTES report on the state of hedgehogs in the UK indicates that the decline in the UK hedgehog population may be stabilising in urban areas. This may be due to the efforts of the public who have been inspired by these campaigns.

The fact that Thorvald lived to be 16 gives hope for the future of European hedgehogs. If we work together, we can save this charismatic species.