For a decade, I’ve been researching the causes of their disappearance. It involved several projects carried out at the University of Oxford Wildlife Conservation Research Unit, which focused on optimizing conservation methods to protect wild hedgehogs.
The “Danish Hedgehog Project,” where my research was based, involved over 400 volunteers collecting 697 dead hedgehogs all across Denmark. We then studied to determine how long the hedgehogs lived and why they died.
It is possible to age a hedgehog by counting the growth rings of a tree. The calcium metabolism of hedgehogs slows during hibernation, resulting in a line of stopped growth on their jawbones. We were able to estimate the age of 388 hedgehogs when they died.
Researchers can tell the age of a hedgehog by examining growth rings in its jaw. Thomas Bjorneboe Berg CC BY NC-ND
We found the world’s oldest scientifically-confirmed European hedgehogs. Thorvald was 16 years old and beat the previous record of seven years. Thorvald, who died in 2016, was attacked by a canine, which is a common cause of hedgehog death. He now adds to the knowledge of the mysterious lives these animals lead.
The status of the hedgehogs in Denmark
We also collected a few other surprisingly old hedgehogs aged 10, 11, and 13 years. The average lifespan of the Danish hedgehogs that we studied was only two years.
Male hedgehogs tend to live longer than their female counterparts. The average male lived 2.1 years compared to the average female’s 1.6. This is a rare finding in mammals, and it’s likely due to the fact that male hedgehogs live longer.
Hedgehogs do not have territoriality, so they rarely fight, and the females raise all their young. It may be due to the high fitness costs of raising children alone that female hedgehogs are at greater risk of dying as they age compared to male hedgehogs, who remain in constant danger of death throughout their lives.
Hedgehogs have been killed in over half of cases (216). The number of deaths peaked, with 70% being males, in July, during mating season. As male hedgehogs have a larger home range than females, they are more likely to cross roads during mating season. A study I co-authored with colleagues in 2019 showed that male hedgehogs’ home ranges increased by five times in the suburbs of Copenhagen, Denmark, during mating season.
The public found the animals sick or injured, and 22.2% of them (86) died at wildlife rehabilitation centers. Another 21.6% (84) perished in the wild from natural causes.
One of the dead hedgehogs was collected by the Danish Hedgehog Project. Tue Sorensen, CC BY-NC-ND
Low genetic diversity
Inbreeding also has an impact on the life expectancy of European hedgehogs. In my previous research, I found that the Danish hedgehog population has low genetic variation. This indicates a high level of inbreeding. Low genetic diversity may reduce an animal’s fitness and lead to a number of hereditary diseases that can be fatal.
Hedgehogs can inbreed if they are limited in their ability to find a mate. Inbreeding is more likely to occur when hedgehogs’ habitats are fragmented due to roads, fences, railways, and buildings.
We were surprised to find that there was no correlation between inbreeding levels and the age of death for our hedgehogs. Surprisingly, there is so little information on the effects of genetic inbreeding among wildlife.
Sophie Lund Rasmussen is studying a hedgehog jawbone. Thomas Degner, CC BY-NC-ND
What you can do
Hedgehogs are increasingly inhabiting areas occupied by people. Our study shows that humans are the main cause of the decline in hedgehogs.
Most hedgehogs only survive long enough to participate in one or even two breeding seasons. Thorvald and other older hedgehogs that we have discovered show us that they will be better able to avoid predators and cars if they survive at least two years.
You can help hedgehogs by taking a few simple steps. Hedgehog Street is a conservation initiative jointly funded by the People’s Trust for Endangered Species and the British Hedgehog Preservation Society.
By removing the barriers between gardens, we can create highways for hedgehogs, which will allow them to freely move between parks, searching for food, nesting sites, and mates. It may be possible to reduce the number of times hedgehogs need to cross roads.
Another option is to make our gardens hedgehog-friendly. The nesting and breeding sites are safe with log and leaf piles or specially designed boxes called Hedgehog Houses. By ensuring your garden is full of greenery, you will attract insects, earthworms, snails, and slugs that hedgehogs can feed on.
It’s also important to remove from your garden anything that could harm hedgehogs. Included in this are poisons, nettings, tools for gardening, aggressive dogs, and deep holes or steep edges around ponds or pools.
It is also important to improve our knowledge about hedgehogs. The Zoological Society of London monitors Greater London’s Hedgehog Populations through its London Hogwatch Project. The BHPS also funds the Hedgehog-Friendly Campus program, which awards universities, colleges, and primary schools for completing measures to help hedgehogs flourish on their campus.
Recent reports by the BHPS and PTES on the status of Britain’s hedgehogs indicate that the decline of UK hedgehog populations is stabilizing in urban areas. It could be because of the efforts made by the public who have been inspired to take action as a result of these campaigns.