Mrs Tiggywinkle, serial killer?

hedgehogEvery now and again a story pops up in the New Zealand media about hedgehogs: either some well-meaning member of the public has rescued one or one has been caught eating something it “shouldn’t.” Then follow comments and letters expressing often strong opinions about whether hedgehogs are “good” or “bad,” or carry disease, or control garden pests, etc. So where does the truth lie? Perhaps it’s time for a brief run-through of what we know (and don’t know) about hedgehogs in New Zealand.

Hedgehogs were introduced from Britain in the latter part of the 19th Century, primarily by acclimatisation societies as reminders of home, but also because of a perception (which still exists, but has never been demonstrated scientifically) that because they eat insects they are a natural biocontrol agent of garden pests. On arrival in New Zealand they were confronted by a relatively benign landscape free of natural predators and with a smorgasbord of their favourite foods. Unsurprisingly, they spread rapidly across the country and are now abundant in all but the wettest and coldest places. Hedgehogs in New Zealand also seem to carry fewer parasites than their British relatives although ringworm and particularly nasty mange are still common. They can catch bovine Tb, but are considered a “spillover” host of the disease rather than a “maintenance” host like possums. This means that Tb is highly unlikely to be sustained in a hedgehog population and passed on to other species.

Individual hedgehogs forage widely and, although they will have a home area with numerous nests, they do not maintain exclusive territories. They are also unusual in being the only mammal in New Zealand that hibernates when temperatures get too cold. Hedgehogs’ main foods are invertebrates: in the Mackenzie basin, hedgehog guts were found to contain rare endemic native beetles and wētā, with one gut containing 283 wētā legs.  A recent study at Macraes Flat, Otago documented a dramatic decline in ground wētā numbers as hedgehog numbers increased. In Central Otago, hedgehogs consume rare chafer beetles.

Small native lizards are also a popular hedgehog snack, with about one in every eight hedgehog droppings from Central Otago found to contain lizard remains.  Adult females appear to be the main culprits.  An analysis of gut contents from 615 hedgehogs trapped in the Mackenzie Basin showed that native lizard remains (mainly skinks) were present in three times as many adult female guts as adult males. An even more pronounced difference was found in a smaller scale DOC study from Macraes Flat.

Although hedgehogs’ diet consists mainly of invertebrates, they readily eat other foods, including the eggs and chicks of ground-nesting birds.  DOC video monitoring from 1994-99 revealed that hedgehogs accounted for about 20% of all recorded predation of banded dotterel and black-fronted tern nests in a braided riverbed system in the Mackenzie Basin. In the 2000-01 season this rose to 78%. Two of every three failed N.Z. dotterel nests at Tawharanui, north of Auckland, were due to hedgehog predation. A parallel situation has also occurred on the Uist Islands off the west coast of Scotland. A handful of hedgehogs were introduced there in the 1970s and subsequently thrived to the extent that a massive removal effort has had to be undertaken to protect the islands’ populations of breeding wader which were suffering terribly from hedgehog predation. There are also many accounts of hedgehogs attacking domestic hens, chicks and eggs. Hedgehogs don’t possess the sharp “killing” teeth of other predators like cats and stoats, so, when attacking a chick or adult bird, they tend to bite and gnaw away until the bird is exhausted, causing it a long and painful death.

A hedgehog doesn’t eat as much in a single feeding bout as a larger cat or ferret, but they fill their stomachs twice in a night’s foraging. Their impacts are magnified by the fact that they are very abundant in N. Z., possibly more so than in Britain where their numbers have declined dramatically in recent years. Although there are no reliable estimates of hedgehog densities in N.Z. habitats, they are almost always the most frequently trapped species in predator control programmes: over three years of DOC predator control at one site, 2294 hedgehogs were trapped compared to 635 cats, 423 ferrets and 242 stoats.

In spite of the endearing “Mrs Tiggywinkle” image created of hedgehogs by predominantly British popular culture, they should be treated no differently to stoats, possums and feral cats here in New Zealand. This is not to suggest we should attack any hedgehog snuffling around our garden at the first opportunity; they are remarkably difficult to dispatch quickly and humanely and the death of an individual hedgehog is likely to achieve negligible conservation benefit. On the other hand, a coordinated and sustained pest management programme using effective traps such as the DOC 200 is likely to make a difference. In and around conservation areas, hedgehog control should be a priority along with the ‘usual suspects.’ Particular care should be taken in controlling hedgehogs where there are ground-nesting birds; small or fragmented populations of lizards or threatened invertebrates and where managed sites border un-trapped grassy areas where hedgehogs are likely to be most abundant.

Chris Jones

Article by Chris Jones, Wildlife Biologist with Landcare Research.