Armoured in a coat of apparently impenetrable spines, hedgehogs should be almost invincible. Yet, the rapid loss of their hedgerow homes and changes in agricultural practice may be forcing these iconic creatures out of their rural homes into more urban settings. And badgers, one of the hedgehog's main predators and competitors for food, might also be restricting the small mammal's ability to forage and roam. Explaining that the decline in the hedgehog population appears to be most severe in rural locations, Carly Pettett from the University of Oxford, UK, and her colleagues, Paul Johnson, Tom Moorhouse and David Macdonald, began tracking the enigmatic mammals and measuring their daily energy expenditure to learn more about the impact of modern environmental factors and predation on hedgehog populations.
Selecting four sites in Yorkshire and North Norfolk, UK – including one site in each county that was occupied by badgers – Pettett and her field assistants roamed the countryside at night in search of the spiky mammals. Pettett recalls, ‘The hedgehogs were initially hard to track down’, but adds, ‘Luckily, some of the inhabitants of the villages who regularly fed the hedgehogs gave us access to their gardens and allowed us to wait close by the food until the hedgehogs appeared’.
The team attached tiny radio transmitters to the animals to track their activity, and returned 3 weeks later to measure how much energy the animals used while roaming around their home patch. Injecting the hedgehogs with 0.6 ml of heavy water (made from the 18O isotope and deuterium [2H]), the team collected a minute blood sample before releasing the animals to continue rambling. Four days later, Pettett and her assistants relocated each hedgehog and collected a second blood sample, before despatching the samples to John Speakman and Catherine Hambly at the University of Aberdeen, UK, for analysis. Measuring the amount of 18O and deuterium remaining in the hedgehogs’ bodies, Speakman and Hambly then calculated the amount of energy consumed by each animal – based on the amount of 18O that had been lost from the animals' bodies as carbon dioxide – over the 4 day period.
Pooling together the energy expenditure data with details of the animals’ routes, Pettett could see that energy consumption varied enormously between individuals, with some consuming as little as 227 kJ day−1, while others consumed almost six times as much (1272 kJ day−1). And when she compared the distances covered by the small mammals, the males wandered over ∼1.5 km while the females covered a smaller range of ∼1 km. However, the females tended to consume more energy than the males and the hedgehogs that roamed farthest from buildings had higher energy consumption levels than those that remained near human habitation. However, when Pettett measured the energy expenditure of the hedgehogs that shared their territory with predatory badgers, she was surprised to see that the hedgehogs consumed 30% less energy than the animals that were free to roam without risk.
‘Our findings support the suggestion by other scientists that arable land represents a “landscape of fear” for hedgehogs’, says Pettett, suggesting that hedgehogs may reduce the amount of time that they spend foraging when badgers are on the prowl and may even reduce their body temperatures to conserve resources when unable to forage. She also suspects that hedgehogs that roam further from human structures have to work harder to find nutritious meals in arable land where worms and insects are scarce. Pettett says, ‘These results are of conservation concern for hedgehogs’, and recommends that farmers increase the area given over to hedgerows while reducing the use of pesticides to provide hedgehogs with greater dining opportunities while providing refuge from hungry badgers.