A: The short answer is that slugs and snails do feature on the diet of hedgehogs. However, the frequency seems to vary locally and in accordance with season and, consequently, the availability of other prey items. Before we look at some of the dietetic data for hedgehogs, let us take a moment to consider what slugs and snails are, and the role that they play in Earth’s ecosystems.
I have known many a diligent gardener to turn off their TV and go into the garden on a warm spring evening to tread on slugs and snails; this being (in their opinion) the most efficient way to get rid of the plant-eating pests. Here in the UK, there is also a reasonably common practice among the more morally conscientious individuals to pluck snails off their prize vegetables and lob them over the fence into a neighbour’s garden or farmer’s field – how much good this does is debatable given that snails painted prior to being ejected generally seem to come back! I think it’s safe to say that slugs and snails generally suffer from bad PR; biologically they are pretty cool (if you like that sort of thing!) and they play an important ecological role.
Slugs and snails (collectively termed Gastropods, which is Latin for “belly-footed” and refers to the broad, tapered foot on which they glide) are, physiologically-speaking, the same creatures; slugs are effectively snails for whom the shell has been reduced to vestigial status, or lost altogether. I think it is safe to assume that most people are reasonably familiar with the terrestrial snails found in gardens and parks. However, land snails comprise a small percentage of the global snail population; the greatest radiation (number of species) and biomass (sheer weight of snail tissue) is to be found in the water (largely in the seas and oceans). Indeed, the Gastropoda is the largest class of the Mollusca phylum -- mollusc is the British spelling, mollusk the American equivalent --, which contains a host of soft-bodied critters including, slugs, snails, clams, limpets, squid and octopuses. Taxonomically, the gastropods comprise about 13 orders, with a staggering number of families and bewildering 60,000 to 75,000 proposed species. Britain is home to about 30 species of slug and 120 species of snail, around 90 of which are terrestrial.
It may not appear so upon cursory inspection, but gastropods have a ‘purpose’ (I use this in an unconscious sense) entirely separate from annoying keen gardeners. Leaving aside, for the moment, their fascinating and potentially medically-important biology -- studies are underway at the University of Washington to see if their mucus biochemistry can be linked to abnormal mucus production in humans -- gastropods are ecologically important too. Slugs are decomposers: they feed on dead and decaying matter, including plants, leaves, fungi, vegetable matter and carrion – some, including ‘shelled slugs’ of the genus Testacella and ‘wolf’ snails of the genus Euglandina, are carnivorous. The decomposition service provided by gastropods serves to recycle organic matter, helping to create and enrich the soil. Indeed, gastropod activity adds humus (partially decomposed organic material) to clay particles, forming soil crumbs – as any keen gardener knows, soil crumbs are important for water drainage and air circulation within soils. Ultimately, were it not for the activities of slugs and snails (and other decomposers, such as earthworms) the recycling of organic material would cease and nutrients essential to the ecosystems would remain locked up inside the dead plants and animals.
So, slugs form an integral part of the ecosystem, but this doesn’t help pacify those whose vegetable garden or herbaceous border has been decimated by nightly gastropod grazing. Slug and snail damage is particularly problematic because it generally happens under the cover of darkness -- gastropods are very susceptible to dehydration, so they come out at dusk when it’s cool and moist -- and the numbers of gastropods in a single garden can be considerable. Consequently, it is not surprising that gardeners take action against these molluscs: slug pellets along with some more environmentally-friendly methods (such as raised beds, crushed eggshells around plants and beer traps) are commonplace in gardens across the UK – some problems associated with slug pellets are discussed elsewhere on this site.
Man-made chemicals aside, pretty much everything is eaten by something – even where there may not be direct predation, there is parasitism. Slugs and snails are no exception to this and there are a number of species that will eat gastropods: slow-worms; various species of ground beetles, spiders and harvestmen; frogs and toads; and several species of bird (the Song thrush is probably the best known for eating snails, but robins, corvids, starlings and blackbirds also make the list of slug-eaters) are known to consume gastropods. Interestingly, some domestic fowl are also partial to gastropods most notably chickens and ducks. I have no experience with chickens, but when my parents owned two free range call ducks, the garden was pretty-much slug free, although this was rather compensated for by the mess the ducks themselves made while trampling through the flowerbeds! Getting back to the question posed, hedgehogs are also on the list of gastropod predators; indeed, in his excellent “Partners in slime” article to the BBC Wildlife Magazine in May 2003, the Royal Horticultural Society’s Dr. Phil Gates describes hedgehogs as “Avid slug-munchers…”.
Dietary studies on hedgehogs from the UK and New Zealand have found that slugs and snails can be quite common dietary components, although the numbers taken seem to vary geographically, and only certain species of snail are taken. In his study on the diet of hedgehogs in New Zealand, published in the New Zealand Journal of Science during 1959, Robert Brockie found that 40% of stomachs contained slug remains, while 36% contained snail remains. In a paper to the Proceedings of the New Zealand Ecological Society during 1973, P. A. Campbell presented dietary analysis of hedgehogs collected on pasture land in New Zealand, finding the remains of Grey field slugs (Agriolimax sp.) in 32% of stomachs and 30% of droppings, representing 4-5% of the diet. Slightly lower values were found by Hans Kruuk in his analysis of hedgehog faeces recovered at a gull colony (published in the Journal of Zoology during 1964); snail remains (of the genus Cepaea) contributed 3% of the total weight in 21% of recovered faeces.
In a study of the intestinal contents of hedgehogs collected from Schleswig-Holstein, the northern-most county of Germany (reported by Nigel Reeve in Hedgehogs), W. Grosshans found slug and snail remains in 26% and 32% of samples, respectively. Similar figures were presented by Derek Yalden from this study of 137 hedgehog stomachs collected from an East Anglian estate. Prof. Yalden’s data (published in the journal Acta Theriologica during 1976) revealed slugs in 31 (23%) stomachs, contributing slightly over 4% of the wet weight; snails were rare, found in only five (0.6%) stomachs. Yalden notes that the numbers of slugs and snails he found in stomach contents were lower than those reported by previous authors (Brockie and Campbell, above), writing: “East Anglian hedgehogs eat very few slugs compared with those from the rest of England and the difference is highly significant.” Indeed, the findings of Andrew Wroot during his Ph.D thesis, some eight years later, would seem to support Prof. Yalden’s conclusion; Dr. Wroot notes the presence of slug remains in 51% of samples, while snails were substantially rarer, occurring in only 5% of samples.
Perhaps of more interest than the number of stomachs or droppings that contain gastropod remains are the figures for the amount of energy that slugs and snails provide the hedgehog with. After all, as Nigel Reeve points out in Hedgehogs, gastropods are easier to digest than insect prey and could thus contribute more (in relative terms) to the hedgehog’s diet than the above figures suggest. In Hedgehogs, Dr. Reeve presents a table summarising the percentage of dietary energy obtained from gastropods according to the studies by Campbell, Yalden, Grosshams and Wroot. The values show considerable variation, from the 1.3% found by Grosshams, through the 3.1% calculated by Prof. Yalden and the similarly matched 5.3% and 5.6% calculated from Campbell’s and Wroot’s data.
It is interesting to note that the propensity for tackling slugs and snails is apparently related to the age of the hedgehog (although, according to Prof. Yalden’s data, not sex) and the abundance of other potential prey items. During her studies on the food preferences of hedgehogs, E. J. Dimelow found that younger (and hence, less experienced) animals were more prone to tackle larger, thick-shelled snails. (Even among adult hedgehogs, most are unable to tackle larger garden snails -- like the Edible, or Roman, snail, Helix pomatia -- because their jaws are too weak to penetrate the thick shells.) More specifically, Dimelow’s data -- from captive animals (published in the Proceedings of the Royal Society of London during 1963) -- suggest that hedgehogs can only deal with thin-shelled snails (e.g. species such as the White- and Brown-lipped snails, of the genus Cepaea), up to about 18mm (about ¾ inch) in diameter. In his 1976 study, Prof. Yalden noted that adults (three years and over) were more likely to tackle gastropods than juveniles (one or two years old) – slug remains were identified from 35% of adult stomachs, but only 13% of juvenile samples.
There is another possible theory to account for the difference in the numbers of snails taken compared to the number of slugs. It strikes me that this may simply reflect access. For example, my experience of slugs and snails tends to suggest that they exploit different ‘height niches’, with snails crawling up stems to attack aerial appendages, while slugs tend to focus their attention closer to the ground. I am certainly not implying that this is always, or exclusively, the case (snails may attack ground-based plants, while slugs may climb); this is purely based on my own observations. Thus, I am of the opinion that (in some gardens, at least) snails may be either out or reach of, or more easily missed by hedgehogs than their shell-less counterparts.
We have seen that gastropods may contribute almost 6% of the dietary energy of a hedgehog, but something else must make up the other 94% and this is the interesting part: the vast majority of animals consumed by hedgehogs are annelids (predominately oligochaetes – i.e. earthworms) and arthropods (predominately insects).
Derek Yalden’s study shows that although gastropods (and slugs more so than snails) may be fairly common in the diet, they pale by comparison to the numbers of insects eaten: Prof. Yalden recorded ground beetles (Coleopterids) and earwigs (Dermapterids) in 101 (74%) and 79 (58%) of stomachs, respectively. So, why do hedgehogs eat fewer gastropods than other invertebrates? Well, there are two main points to consider here: slugs and snails are consumed with greater frequency, but somehow underrepresented in dietary studies; or hedgehogs find gastropods distasteful, difficult to handle, or otherwise objectionable.
The idea that slugs may contribute more to a hedgehog’s diet than the statistics imply is not difficult to believe; I have already quoted Nigel Reeve, who wrote of the difference in ‘digestibility’ between gastropods and arthropods in his book Hedgehogs. Indeed, gastropods are generally only identifiable in stomach contents from their radula (a horny, tooth-bearing strip on the tongue, used for rasping at food) or shell fragments. Additionally, Robert Brockie found slugs in an average of 40% of the droppings of the New Zealand population he studied; however, when taken individually, the data showed that 56% of droppings recovered during the summer had gastropod remains in them. Similarly, Derek Yalden observed that more slugs were consumed during September and October than in other months. Thus, seasonality of consumption and superior digestibility may (either individually, or combined) lead to an underrepresentation of gastropods in the dietary studies.
An alternative to underrepresentation is that gastropod consumption may genuinely be significantly lower than other invertebrates because they are not preferable food items (i.e. in some capacity, the hedgehogs find them objectionable). Several people who care for sick/injured hedgehogs or simply put food out for their resident (?) hog(s) have noted – perhaps unsurprisingly -- how well-fed hedgehogs tend to eat fewer garden pests; I have heard accounts of hedgehogs feeding on a plate of dog food, ignoring the slugs on and around it. So, why might hedgehogs find gastropods less desirable than other prey species? In my mind, there are two possibilities: the invertebrates are difficult to handle; or they are dangerous to eat.
What might make a slug or a snail difficult to handle? Well, there are two main factors: their size; or their mucus. We have seen from Dr. Dimelow’s studies that hedgehogs are less able to deal with large, thicker-shelled snails, which immediately limits their options. However, without the protection conveyed by a shell, slugs are fairer game and size does not seem to be a considerable deterrent; Nigel Reeve notes how hedgehogs will accept slugs up to 15cm (6 inches) long, although the tougher-skinned species (e.g. the Great black slug, Arion spp.) are often rejected. Mucus, however, does appear distasteful to hedgehogs. During her studies, Dr. Dimelow observed hedgehogs to wipe the slime off large slugs (e.g. Great grey slugs, Limax spp.) with their front paws before eating it bit by bit. In their respective books, Drs Reeve and Morris note how hedgehogs have been observed rolling slugs on the ground in order to remove the mucus.
Slugs produce two types of mucus: a thick gloop, which contains fibres that are used for climbing and, in some species, suspension during reproduction; and a thinner slime that is used for locomotion. The two types of mucus are highly hygroscopic (that is, they readily absorb moisture from the air), which helps prevent the desiccation to which gastropods are extremely susceptible. Additionally, as we have seen above, the mucus may also serve to make the slug distasteful to predators. Indeed, in a 1979 paper to the Canadian Journal of Zoology, the authors report how, when attacked, the Field slug (Deroceras reticulatum) began secreting a thick white defence mucus. I’m not aware that any studies have been conducted looking specifically at the effect of mucus production on the relish with which hedgehogs will accept slugs, but similar studies of insects suggest that mucus can prove distasteful or otherwise unappealing to potential predators.
In a fascinating paper to the journal Biocontrol Science and Technology during 2002, biologists Jacqueline Mair and Gordon Port (based at the University of Newcastle upon Tyne) report on the influence of mucus production on the degree to which Carabid beetles attacked slugs. The scientists offered healthy and ‘stressed’ field slugs (the latter of which have impaired mucus production capacities) to two species of carabid beetles: the Black clock ground beetle (Pterostichus madidus) and the woodland ground beetle (Nebria brevicollis). While it is fairly safe to conclude that the beetles more readily consumed small (cf. medium or large) slugs and that neither species consumed them with great frequency, the data do show that they took significantly more ‘stressed’ than healthy slugs. However, where the ingenuity of this study really strides home is in the second part of the experiment. Drs Mair and Port took some blowfly larvae (Calliphora spp.) -- which both species of ground beetle are known to readily consume -- and coated some in slug mucus (tests), before offering them to the beetles. In this test, both species of beetles were noticeably more struck by the mucus-free (control) goodies. Within the first five minutes, the P. madidus specimens (both sexes combined) had eaten seven slime-free larvae and only a single test one; after 24 hours, they had eaten 17 of the control larvae and only three of the test ones. The results were similar, although less striking for N. brevicollis, who had consumed three control and one test larvae; after 24 hours this had increased slightly to five and two, respectively.
From their data, Drs Mair and Port conclude that the defensive mucus produced by healthy slugs hampered attacks by the beetle. The authors wrote: “Results indicate that these generalist beetle species are unable to overcome the defence mucus production of healthy slugs. Slugs sub-lethally poisoned by molluscicides [something we discuss elsewhere] may be a more suitable prey item due to a reduction in defence mucus production.”
A final circumstance that may lead to hedgehogs finding gastropods undesirable is a lethality one. Gastropods are hosts for lungworm and fluke parasites, which can infect hedgehogs if the slug is consumed. Indeed, lungworm infections are remarkably common among hedgehogs in the UK, and can represent a significant source of mortality. Moreover, some have suggested that slugs poisoned by slug pellets may, in turn, poison hedgehogs that eat them (see: Are Hedgehogs Declining in the UK?). Thus, if feeding on slugs is either learned or genetically predisposed (as opposed to simply being a generic ‘eat whatever runs slower than you’ rule) it is not unrealistic to think that slug predation could become rarer, at least locally: if genes for eating slugs exist, and prove lethal (through lungworm infection or poisoning), they will decline in the gene pool; if sickness rapidly follows slug consumption, learned avoidance cannot not be ruled out.
In conclusion, the answer to the question, do hedgehogs eat slugs, is an undisputed “Yes!”. However, the number of slugs they eat (and thus the benefit they provide the average gardener) varies according to a host of factors, including the hedgehog’s age, the season (with more eaten in autumn), the location (East Anglian stomach contents yielding fewer slug remains than those from elsewhere in the UK), the species of mollusc, and possibly whether or not supplementary food is provided (not all observations support the idea that hedgehogs fed by householders eat fewer wild prey). Consequently, it is unlikely that the hedgehogs visiting your garden are single-handedly going to rid you of your plant-chomping gastropod pests. Nonetheless, hedgehogs form an important part of the ‘natural army’ (consisting of birds, amphibians, slow-worms and various beetles) that will work together -- and I mean this in a purely coincidental sense -- to keep the number of slugs and snails down.
Fri Jan 06, 2012 12:35 pm
