Rats!

OVER the past year or so David Riddell has been trapping rats in his Gordonton garden, and slowly getting to grips with the tricky business of rat identification. He shares what he’s learned with N8N readers.

In the last year or so I’ve taken up a new hobby – rat catching.  I’ve always done a bit off and on, when suspicious-looking earthworks appear under the compost bin, or snug little nests are uncovered in the woodpile, but I’m now getting a lot more systematic about it, checking traps almost every day, and keeping proper records.  It’s very gratifying watching the tallies mount up, and it must be doing good for the local birdlife.

Two of the varmits

There are two types of rat around here: they go by lots of different names but in New Zealand they’re usually called ship rats and Norway rats, and they’re quite different in their behaviours, impacts on wildlife and interactions with people.  (A third rat, the kiore, was introduced by Maori, and is now mainly found on offshore islands, though a few persist in remote parts of the southern South Island.)

Ship rats (Rattus rattus) are now the most common rat in New Zealand, though they were the last to become established. In the late 18th and early 19th century the commonest rat on ships was the Norway rat (Rattus norvegicus), which despite its name probably originated in China; it largely displaced the ship rat from much of Europe as it expanded its range from about the 16th century onwards. As a result ship rats, which were the species responsible for spreading the plague, are now very rare in Britain.

In most of the world ship rats are mostly found in disturbed habitats close to human habitation, but in New Zealand they are found almost everywhere. They’re active climbers, so take a lot of eggs and chicks from bird nests, and are more likely to make themselves at home in the roofs of old houses. They don’t swim or burrow much.

Norway rats are more more patchily distributed, but common around water, so here in the Waikato there are a lot. They don’t climb much, but are active swimmers and burrowers, and they’re reported to be more wary around traps. I’ve been catching a lot more ship rats than Norway rats, but whether that’s because of differences in abundance or wariness I’m not sure.

Norways and ship rats

There are a number of characters that distinguish the two types, though it’s not always straightforward. Norway rats are supposed to have tails which are shorter than the body, for example, while ship rats have tails that are longer than the body. But sometimes you find animals where the tail and the body are pretty much the same length. The tail of a Norway rat is sometimes paler underneath, but can be uniformly coloured like a ship rat’s.

Supposedly you can tell a ship rat because its ears are so big that if you fold one forward it will cover the eye. Quite apart from the issue of not wanting to handle a dead rat any more than necessary, there are some individuals where this doesn’t apply, and it doesn’t help at all when the bar of the rat trap has come down on the back of the head.

Once you’ve looked at a few rats, though, it gets so that telling one from the other isn’t too difficult, and by looking at all the characters described below you can usually tell what you’ve got. So for those of you who want to know what you’ve been getting in your traps, here’s a quick guide (WARNING: pictures of dead rats ahead!)

Colour, Size and Tail

Norway rats are also known as brown rats, and ship rats as black rats. Norway rats really are usually an uneven brown colour, but ship rats are also usually grey-brown, and only occasionally black. They actually come in three main colour varieties: grey-brown with a white belly (by the far the most common around here), grey-brown with a grey belly, or black with a grey belly. A Norway rat’s belly is an uneven white/grey, while a ship rat’s belly is more uniform in colour.

You can also see from the above picture that Norway rats are generally bigger than ship rats (though obviously Norway rats start out small!), and have a proportionately shorter tail. A ship rat can be up to 230mm long (not including the tail), and up to 215g; anything larger than this is probably a Norway rat – they can weigh more than twice as much as a ship rat.

A Norway rat’s tail is often (but not always) paler underneath, while a ship rat’s tail is usually uniform in colour.

Finally, here’s a close-up of a ship rat’s back, showing the overall colour, and also the long black “guard hairs”. Norway rats also have some black hairs in their coats, but they’re shorter.

Ears and eyes
Ship rats have much larger ears than Norway rats. The rule of thumb that you can fold the ear over the eye doesn’t always work, but the difference is obvious once you’ve seen a few of each. Norway rats’ ears are also hairier, though you have to look pretty closely to see this. Ship rats’ eyes are also proportionately larger than Norway rats’.

Feet
The top side of a ship rat’s hind foot is usually slightly darker than the sides and toes, while a Norway rat’s hind foot is normally uniformly pale.

The more rats you trap, the more familiar you get with them.  Whether that’s something you want to do is another matter!

• David Riddell has worked for many years as an ecologist, including a stint with Department of Conservation trapping stoats and other pests in Southland and western Fiordland but has until recently never quite nailed down which rat is which.  He now lives in Gordonton and always deals with the mice that N8N’s cat Marley brings in. 

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