In his 1981 book, The Mammalian Radiations, John Eisenberg tells of the inherent problems involved in classifying animals into groups on the basis of what they eat. Eisenberg writes:
“Mammals, like all other living organisms, have a perverse tendency to defy exact classification.”
Indeed, in our garden we have observed variation in the hedgehogs' response to gravy bones (a type of small dog biscuit) - some ignore them, some sniff and reject them, while others eat them. This implies individuals have food preferences, albeit perhaps transitory ones, which further confirms one should be cautious in ascribing dietary preferences in this species. This hasn't stopped some trying, though, and in the late 1960s Maurice Burton and Konrad Herter argued that the blunt teeth, stronger jaws and shorter body-to-gut ratio (1:6) of hedgehogs compared with other insectivores implied an omnivorous diet. In Hedgehogs, however, Nigel Reeve notes that hogs possess simple stomachs and a non-complex colon with a poorly defined ileocolonic junction (where small intestine becomes large intestine), suggesting they're not built for a diverse diet.
To be an omnivore (from the Latin omne meaning all and vorare to devour) in the most rigorous sense, an animal must be physiologically adapted to consume and digest animal and plant tissues. In support of such a classification, in The New Hedgehog Book, Pat Morris describes how hedgehogs have more than a metre (over three feet) of guts and a very large stomach for an animal of their size that contains potent digestive juices capable of handling a varied diet. Despite this, Reeve notes that the lack of a caecum in hedgehogs means that food passes through the gut rapidly, within 12 to 16 hours, making it unlikely that much of the fermentation necessary for plant digestion occurs.
Thus, although hedgehogs have been found with plant remains in their stomachs, and some having been observed eating plants, they are not technically omnivorous. Rather, they are insectivorous (insect-eating) predators that feed on a wide spectrum of invertebrate prey and supplement their diet with small vertebrates, carrion and fruit. Most dietary studies have concluded that plant material is a comparatively unimportant component of their diet.
Those who feed and care for hedgehogs are usually quick to point out how captive individuals can become rather choosy about what they will eat, while wild individuals display a less discriminating palate. Pat Morris affirms this in his New Hedgehog Book, remarking how captive hedgehogs may become very fussy about what they will and won't eat, although this may not always the case. Indeed, in his presentation to the Third International Hedgehog Workshop, Ray Jackson of the Lower Moss Wood Wildlife Hospital in Cheshire quotes Manchester Metropolitan University's Gillian Key, who wrote of their hedgehog food choice test:
“... the results will be no surprise to anyone who has ever kept hedgehogs. Nine out of ten animals did not mind what they ate (nose in the first bowl encountered and munch)—the tenth preferred to roll in it”.
Several studies have been conducted in a bid to establish what hedgehogs eat, but most have analysed stomach contents or remains found in droppings. These methods are widely used and can provide some interesting insights as to the dietary preference of animals, but they also have their problems. Most significantly, they tend to underestimate certain components of the diet; the relatively inedible items like bone, fur, feathers and beetle elytra survive digestion, while softer material like flesh and muscle is rapidly broken down and unlikely to be found in faeces. Hence, care must be exercised when drawing conclusions from faecal studies.
Only one study, Andrew Wroot's 1984 Ph.D. thesis, has attempted to classify prey in terms of energetic contribution to the diet. The result was to challenge the idea that hedgehogs feed opportunistically and unselectively. Indeed, Wroot's data suggest quite the opposite; that hogs feed to maximize their energy intake, the “energy maximization” arm of the oft-cited Optimal Foraging Theory. There are, however, some exceptions to this concept. In particular, earthworms are often not highly selected, despite yielding roughly nine-times more energy than other major prey items, and seem to be taken when more preferable prey is scarce.
Dietary studies by other authors have documented apparent switching of prey to take advantage of seasonal abundance. Caterpillars, for example, seem to be taken more frequently during their peak abundance during April and May, while Nigel Reeve's golf course studies found that leatherjackets were important all year around, but earthworms and carabid beetles were taken most frequently during June and July. In the end, as Reeve notes, one should remember that energy intake is only part of the role filled by food; provision of nutrients is equally as important.
In her studies of captive hedgehogs, E.J. Dimelow found evidence to suggest that there may be ontogenetic changes in the diet. In other words, a hedgehog's diet changes as it grows up. She observed that younger hedgehogs would try, unsuccessfully, to tackle thick-shelled snails and take prey that adults found unpalatable, such as woodlice. Similarly, data from a dietetic study by Derek Yalden suggest that juveniles may need to learn to tackle trickier prey, initially taking more vertebrate carrion while adults took more earthworms, carabids and slugs. In his 1988 paper to the Journal of Zoology, Chris Dickman found that young hedgehogs consumed a wider variety of prey than adults, which demonstrated a tendency to specialise. Dickman concluded that:
“Young animals may lack the skill to detect and track carabids by their odour, and achieve a low capture success rate for the soil-dwelling larvae because they dig more slowly and inefficiently than older animals.”
Hedgehogs don't cache (store) food and tend to eat what they find where they find it. That said, I have received reports of hedgehogs dragging larger prey items such as toads and pigeons across roads, and one of a hedgehog carrying a large worm across a patio. Additionally, in early October 2021 a hedgehog picked up a whole gravy bone and carried it off up the garden. This was unusual behaviour as the hogs ordinarily stand and chew at the gravy bone in situ.