Hedgehogs

The Western European Hedgehog (image from https://scottishwildlifetrust.org.uk/)

Small, spiny and super cute, hedgehogs are mammals belonging to the Erinaceidae family, part of the order Eulipotyphla, which contains a range of small mammals including the hedgehogs, the venomous solenodons, moles and shrews. The West European hedgehog (Erinaceus europaeus) is perhaps the most well known of the 17 species of hedgehog found across Europe, Asia and Africa, snuffling around hedgerows and gardens throughout the UK. Records of hedgehogs date as far back as the medieval ages in UK records, with the little mammals being named as such because of their tendency to frequent hedgerows and their pig like snout.

A hedgehog illustration in a medieval bestiary from France, estimated 1270. (image from https://brewminate.com/the-hardworking-homemaking-hedgehog-of-the-medieval-bestiary/)

They first evolved around 15 million years ago, making them one of the oldest mammal species on Earth, and fossil records and genetic analyses have revealed they have changed very little since the first hedgehogs were trundling around the feet of mammoths and sabre-toothed tigers. There has been no real reason for them to change; spines are a perfect protection against predators, hibernation means they don’t starve in the scare winter months, and their omnivorous diet and ability to adapt to a range of habitats helps them thrive. They can adapt to urban environments and utilise gardens and allotments, with research showing that without these urban green spaces, suitable hedgehog habitat would be decreased by up to 63% across the UK.

Having a ball; hedgehog behaviour and ecology

Hedgehogs are arguably most known for their spines or quills, which are made of keratin, the same thing human nails and hair are made from. Unlike Porcupine spines, these don’t detach from the body, and will only fall out when the hedgehog is transitioning from juvenile to adult, or when the individual is diseased or stressed. When threatened, hedgehogs roll into a tight ball, protecting their tucked in face, feet and stomach, forcing all the spines outwards towards any unfortunate predator that might fancy a bite. Running across the hedgehogs back is a band of excess skin and two large muscles that can control the positioning of the quills, acting like a drawstring bag. When hedgehogs are born, their spines are white until they mature, when they will turn a pale brown at the base. Some hedgehogs, however, possess a genetic mutation that gives them white spines even into adulthood, and they are known as leucistic hedgehogs.

A close up of hedgehog spines taken with a scanning electron microscope (image from https://theworldcloseup.com/)

Hedgehogs are mostly nocturnal, although they are sometimes seen during the day foraging for food. All wild can hibernate, although not all do, depending on environmental conditions like availability of food, temperature and the species in question. During hibernation, hedgehogs aren’t simply having a particularly long lie in; they actually drop their core temperature down to match their surroundings and enter torpor, a state of decreased physiological activity with low body temperature and next to no metabolic activity. Their heart rate drops from the usual 190 beats per minute down to as little as 20 per minute, and respiration almost stops entirely; the hedgehog will only breathe once every few minutes. They do occasionally wake up for brief periods, often for no apparent reason, although it can be due to disturbances or sudden increases in temperature, as the optimal temperature for hibernation is around 4 degrees Celsius. For this very reason, although it may be tempting to keep the cute little mammals inside a shed in the harsh winter months, it is important that hibernating hedgehogs are left outside during the winter months. If they are forced to hibernate at higher temperatures, it can make them use up their vital fat stores twice as fast, potentially leading to starvation. Hibernation can start any time between November and January depending on the climate, but towards the end of the Autumn hedgehogs will start house hunting for the perfect winter hideaway, called a hibernacula. Preferred sites include under hedges (it is in the name after all!) and the roots of trees, in piles of dead wood and leaves, compost heaps and even old rabbit warrens or under sheds and other garden buildings.

Hedgehogs and highways: traffic mortalities and conservation

Most species of hedgehog are currently listed as least concern on the IUCN redlist, and generally there are few direct threats to these species across their range, largely because of their adaptability. However, hedgehogs are one of the most common roadkill fatalities, particularly in the United Kingdom, and there is evidence that they are facing marked population declines. Males have the greatest likelihood of road traffic mortality, likely because they travel larger distances looking for a mate. Roads can also fragment and isolate populations, which can cause genetically distinct populations to arise on either side of major roads or transport links.

A study conducted between 1981 and 2020 by the People’s Trust for Endangered Species and the Hedgehog Preservation Society in the UK revealed that up to 75% of the hedgehog population has been lost from the UK in the last 20 years, although there was some encouraging evidence that urban populations have remained pretty stable. Although road traffic accidents were noted as a problem, the study noted that a large pressure now facing hedgehogs is habitat loss and fragmentation, which increases their chances of being hit by a car as they have to pass through human inhabited areas more frequently as they forage. Since urban populations seem to be stable after years of declines, the reasons behind this still need to be understood to prevent countryside extinctions. Loss of vital hedgerows is partly responsible for the decline, but as there has been a real effort to restore them, this can’t be the whole reason. Scientists suspect declines may be related to the management of farmlands and urban-rural interfaces, but a lot more research is needed before anyone can say for certain.

Although hedgehogs are a beloved sight across Europe, the hedgehog is a far more unwelcome sight in New Zealand, where they are considered an invasive species. There are currently more hedgehogs in New Zealand than there are in the UK! Brought to the shores of New Zealand in the 1870’s by British colonists who wanted a cute, furry reminder of home, hedgehogs have become a scourge on New Zealand’s native wildlife. Once held in captivity, soon people began to lose track of their spiky compatriots, and they spread remarkably quickly. Free from the predators that hold them in check in Europe, like foxes and badgers, New Zealand’s hedgehogs bumble through habitats unchecked. Aside from a few species of bats, New Zealand has no native land mammals, and its diverse bird population has adapted to this, with many species completely flightless, laying their eggs on the ground and completely naïve to any predators. For such a small, cute mammal, hedgehogs are having a devastating impact on New Zealand’s native animals, hoovering down bird and lizard eggs and crunching on the iconic weta, a huge flightless cricket found only in New Zealand. By 2050, authorities are somewhat ambitiously aiming to eliminate all invasive predators from both islands, but this is proving nigh on impossible, especially with animals as small as the hedgehog. There is also something of a PR issue with this campaign; generally the public don’t find the mass killing of small, furry mammals palatable, a phenomenon researchers have dubbed ‘The Beatrix Potter’ effect. That psychological barrier seems even stronger with the lovable hedgehog. There have been discussions about shipping all the hedgehogs back to the UK to boost the failing populations there, but this is simply logistically and financially impossible sadly. The best researchers can do is humanely kill the unfortunate hedgehogs for the sake of New Zealand’s unique native species. It is after all, our mess to clean up.

Conservation optimism

The bounce-back of urban hedgehog populations is a huge win for community driven conservation. Wildlife loving gardeners have implemented a range of conservation measures, including hedgehog houses and highways, which is as simple as cutting a CD sized shape in the bottom of a fence so hedgehogs can travel through gardens without ever having to go near a road. Better public understanding and awareness of hedgehogs has been almost entirely responsible for the increasing urban populations seen today.

A hedgehog tunnel installed by a helpful gardener (image from https://www.rspb.org.uk/)

If you are a wildlife lover and want to support wild hedgehogs, here are some top tips from hedgehog experts:

If you are building a bonfire in your garden check the woodpile for any hibernating hedgehogs.
Leave out food and water: ensure it is good quality meat-based dog or cat food, and don’t leave out milk; it is a common misconception that hedgehogs like milk, when in reality it gives them diarrhoea. You can create a simple hedgehog feeding station by cutting a hole in a large plastic box to protect the food from the elements and any other hungry mammals from stealing the food, like maybe your neighbours cheeky cat!
Properly discard of your litter to stop hedgehogs swallowing plastic or getting trapped in rubbish and starving.
Avoid the use of chemical pesticides and slug pellets as they can directly poison hedgehogs but also affect their food chain.
Cut a small hole in the bottom of your fence to allow hedgehogs in and out of your garden, allowing them to avoid dangerous roads.
If you have a pond on your property, provide a slope for hedgehogs to climb, as although they are good swimmers they can easily become trapped.
Check vegetation and hedgerows for hedgehogs before cutting or trimming any plants.
Take care on the roads at night; good advice for life in general, but especially helpful for our little hog friends.

Mitigations against road traffic accidents are either in use or being researched across hedgehogs’ ranges. Fencing has been used around major roads to prevent hedgehogs crossing, although a study in Europe showed that this can be even more of a barrier and can cause even more genetic isolation and is more damaging to hedgehog populations than the road itself. Under road tunnels or hedgehog highways are also considered, although how often these tunnels are utilised depends on local factors like the presence of predators, differences in habitat and the design of the tunnel itself. Traffic calming measures, like speed bumps and speed limits, have been put in place and these can have an impact on hedgehog populations as they often cross quieter roads to reach key habitat. However, the effectiveness of these measures relies entirely on whether drivers will adhere to them, which is difficult to govern, and it is still very difficult for drivers to see such small mammals on the road, even at slower speeds. Improving habitat and habitat connectivity could have the highest impact, as better resources would mean hedgehogs would travel less for food and mates, limiting their need to cross roads in the first place. Even changing the structure and configuration of roads has been shown to help reduce hedgehog mortalities, which can help governments plan better roads for wildlife in future developments.

The British Hedgehog Preservation Society (BHPS) is a UK charity founded 1982 that works to protect wild hedgehog populations through advocacy, education and carrying out the practical steps needed to preserve habitat. They also offer advice and support to people who find sick or injured hedgehogs and maintains a list of rehabilitators in the UK. You can check out their work and the projects they are involved with at https://www.britishhedgehogs.org.uk

Partnered with the BHPS is the People’s Trust for Endangered Species who are co-ordinating a range of conservation work across the UK. They provide a hedgehog management course for land management professionals, give advice to farmers and other landowners and conduct research into hedgehog habitat and the threats facing them currently. You can check out their hedgehog conservation work at https://ptes.org/campaigns/hedgehogs

It isn’t just the Western European Hedgehog that gets all the attention, although many conservation charities are focused around them, in large part because they are such an iconic British mammal. The IUCN have a small mammal specialist group of top mammal scientists and conservationists that work to research and conserve rodents, moles, shrews, solenodons, tree shrews and hedgehogs. You can check out their work at https://small-mammals.org/

Awesome videos!

References

App, M. Strohbach, M.W. Schneider, A.K. and Schröder, B. (2022) ‘Making the case for gardens: estimating the contribution of urban gardens to habitat provision and connectivity based on hedgehogs (Erinaceus europaeus). Landscape and Urban Planning

Bergers, P.J.M. and Nieuwenhuizen, W. (1999) ‘Viability of hedgehog populations in central Netherlands.’ Lutra.

Brockie, R.E. (1975) ‘Distribution and abundance of the hedgehog (Erinaceua europaeus) in New Zealand, 1869-1973. New Zealand Journal of Ecology.

De Vries, J.G. (1999) ‘Hedgehogs on the road: from research to practice.’ Lutra.

https://www.britishhedgehogs.org.uk/

https://www.doc.govt.nz/nature/pests-and-threats/animal-pests/hedgehogs

New Zealand trying to eradicate hedgehog ‘killing machines’ (nypost.com)

https://ptes.org/campaigns/hedgehogs/

Jones, C. Norbury, G. and Bell, T. (2013) ‘Impacts of introduced European hedgehogs on endemic skinks and weta in tussock grassland.’ Wildlife Research.

Moore, L.J. Petrovan, S.O. Baker, P.J. Bates, A.J. Hicks H.L. Perkins, S.E. and Yarnell, R.W. (2020) ‘Impacts and potential mitigation of road mortality for hedgehogs in Europe.’ Animals

Pipek, P. Pyšek, P. Bacher, S. Bolfíková, B.C. and Hulme, P.E. (2020) ‘Independent introductions of hedgehogs to the North and South Island of New Zealand.’ New Zealand Journal of Ecology.