Spikes in the data: The importance of measuring stress in hedgehogs and how a new method could enhance welfare

Stress is a natural response to danger, helping animals react quickly when they feel threatened. Usually the response is short-lived, and once the threat passes stress levels return to normal.

Environmental changes that reduce or fragment habitats, declining food availability, and human activity can all affect wildlife. If a stress response is prolonged, chronic stress can occur, reducing immunity, negatively affecting reproduction and litter success, and increasing mortality rates.

For species like the European hedgehog, which is already declining, identifying what causes stress could provide vital clues about the challenges they face and support conservation efforts.

As an ‘indicator species’, hedgehogs can provide insights into the ecosystem's overall health. They are also the most rescued mammal, with estimates suggesting around 40,000 hedgehogs in captivity in the UK at any time.

Understanding hedgehog responses to stress could also help to improve treatment and welfare during rehabilitation. For wild animals, captivity, handling, examination, and veterinary treatment can all increase stress in an already compromised animal. Identifying ways to minimise this could aid recovery, and measuring stress would enable post-release monitoring to assess how hedgehogs adapt once returned to the wild.

How do you measure stress in a wild animal?

In our new research we have validated a method for measuring stress in European hedgehogs by analysing hormones in their faeces. This provides researchers with a reliable, non-invasive way to investigate how hedgehogs respond to different stressors, both in the wild and in rehabilitation.

When an animal feels threatened, the hypothalamic-pituitary-adrenal (HPA) axis triggers the release of hormones that help it respond to the danger – the flight or fight response. Once the threat has passed, glucocorticoids – cortisol and / or corticosterone – are released to help the body return to normal. These are later excreted in urine and faeces. When animals experience chronic stress, they maintain high levels of glucocorticoids in their system.

To measure glucocorticoid levels and determine what constitutes an elevated reading, the assay, or chemical test, being used must first be validated. This can be done in two ways. First, hormones are injected into an animal, then blood samples are taken at regular intervals to monitor changes. This is invasive, however, and requires a veterinary surgeon to take the samples. Alternatively, faecal samples can be collected before and after a stressful event.

Either method must show that the assay detects relevant changes in glucocorticoids, i.e., those caused by the perceived threat and not by other factors. The assay must also be sensitive enough to detect small changes in glucocorticoids, so comparing assays can help determine which is most sensitive. When an assay is both relevant and sufficiently sensitive to detect changes, it has been validated for the species.

What did the research show?

Our study tested three non-invasive assays commonly used in other mammals. For hedgehogs admitted to rehabilitation, faecal glucocorticoid levels peaked on the day after admission and reduced seven days later. Admission to rehabilitation involves several events known to be stressful for other animals, namely capture, transport, examination, and veterinary treatment, so glucocorticoids were expected to peak following admission. All three assays that we tested could detect relevant changes in glucocorticoid levels following a stressor. The most sensitive assay showed the largest difference between the lowest and highest readings, so we chose it to test further samples.

The study found that hedgehogs admitted to rehabilitation which went on to die had significantly higher glucocorticoid levels in their faeces than those that survived. Hedgehogs infected with a gastrointestinal parasite, Capillaria, also had significantly higher levels of glucocorticoids than uninfected hedgehogs.

After seven days in rehabilitation, glucocorticoid levels returned to similar levels seen in wild uninfected hedgehogs.

Assessing the impact of specific threats

Now that we have  a validated method for testing stress in hedgehogs, further research can examine the effects of different stressors, or threats.  

For example, changes in habitat and the effects of human activities - such as music festivals, housing developments, or road proximity – could be investigated to determine whether they affect stress in hedgehog populations.

Our method could also improve welfare for hedgehogs in rehabilitation. This would allow assessment of the impact of certain treatments, longer stays in captivity, and specific injuries and diseases to monitor hedgehog welfare both while in care and after release.

This paper is the first to validate an assay for use in European hedgehogs and provides a reliable method for future studies.