Know your rodents by Dr James Russell

Hi everybody Nick here. Love them or hate them Rats are all around us and one man that knows them intimately and may help us to understand these pests is Dr James Russell.

Welcome to Enhancing the Halo James.  Thanks Nick

Know your Rat

A brown rat or a black rat? [a brown black rat Rattus rattus]

 Rodents have been with us for most of human history, since we first started planting crops, living in cities, and sailing around the world. For most of that time we’ve been trying to catch them, and they have been getting better at avoiding us. They carry diseases, eat crops and kill wildlife. New Zealand has been successful at removing rodents from over 100 of our offshore islands (in fact we’re #1 in the world), however, even if you don’t live on a remote island, you can still play your part by controlling rats in your backyard or local reserve. The good news is, its easy. Rats live just about anywhere and eat just about anything, so all you need to do is set a trap somewhere where it won’t be disturbed (e.g. under a tree or by a building) and then have some patience. It may take rats a few days to familiarise with the new object in their territory, but eventually they will go for it. It also helps to change bait regularly, as just like humans rats like to have variety in their diet.


Three species of rats are found in New Zealand. A rat may look like a rat, but there are subtle and important differences among them. All three species in fact originated from mainland Asia where they since spread out to the world. The first rat species to arrive in New Zealand was Rattus exulans, otherwise known as the Pacific rat, Polynesian rat, kiore, or even the ‘native’ rat. It took the direct route and came down from Asia through Micronesia and travelled with Polynesians throughout the Pacific before arriving in New Zealand around 1200-1300 (earlier reports of arrival are probably incorrect). Although the smallest of the rats, and generally found eating vegetation, this belies the large number of bird and reptile extinctions it has been associated with. By the time Europeans arrived in New Zealand, many bird species had already gone extinct, and the rat species was presumed ‘native’ since it was already here.


In 1769 James Cook found New Zealand…and brought Rattus norvegicus with him, also known as the brown rat, sewer rat, wharf rat or Norway rat. These common names give away its history as it took the indirect route travelling from Asia to Europe with traders, before being named the Norway rat by early taxonomists (scientists who classify and name species on the evolutionary tree) in Scandinavia, hanging out on wharves, and finally hitch-hiking a ride with Captain Cook to New Zealand.  About a century later from 1860-1870s Rattus rattus also arrived from Europe through multiple introductions across New Zealand. This rat is also known as the black rat, ship rat and bush rat, also giving away it predilections to live in New Zealand bush and hitch-hike on ships. The common name ‘black’ is a bit of a red herring though, the ship rat comes in 3 colour varieties; brown, grey and black, and we have all three in New Zealand. The black morph is in fact the rarest of them (though common in some localities). I remember confusion once when a colleague told me brown rats were on an island (Rattus norvegicus) only to have big brown ‘black’ rats (Rattus rattus) brought in to the laboratory.


Ultimately, its better not to try to distinguish the rats by colour. Size is the better distinguishing characteristic. The Norway rat is probably easiest. It’s the biggest of them (although aren’t all rats big when you see them rummaging through your compost?). In New Zealand its clocked up weights of over half a kilogram (mainly on farms), and I’ve heard of them being caught in possum traps rather than rat traps. They don’t quite get as big as this Stuff article (actually turned out to be a possum) but still, they are the real deal, of the kind found in the sewers of London and the subways of New York. The ship rat is famous for its giant ‘mickey mouse’ ears and tail much longer than its body, which distinguishes it from the Norway rat. The kiore is the hardest to identify, having even been mis-identified by some of New Zealand’s finest ecologists. To the untrained eye it is simply a juvenile brown morph ship rat, but the clincher is a unique black diamond stripe on the back paws. If you’re not looking for this though, you won’t see it. The average person probably won’t come across kiore though, as they suffered greatly with the invasion of rats from Europe, and are now restricted to a few offshore islands and remote parts of Fiordland. There’s even an island reserve now for kiore in Northland to protect them as an important cultural and genetic resource.

We commonly use tracking cards in our monitoring programmes and rat footprints definitely all look the same there, but our research group at the University of Auckland is using derivatives of facial and finger print recognition technology to distinguish among the prints of the three species with over 80% accuracy.

fig6Other work we are involved in is developing ways to maximise the probability of rats stumbling upon control devices, and then interacting with them. We are also developing database software called CatchIT to help community groups store and analyse their data from trap lines and bait stations – watch this space as the software goes live later in the year! We’re also compiling a database of rat DNA samples from across New Zealand to use genetic methods to determine the pathways of invasion and re-colonisation. All this work is moving towards the goal of being able to control rats over larger and larger scales, to return more habitat back to its original inhabitants. But every time we get better at controlling the rats, once again they get better at avoiding us.


Mice are a completely different story, being very different from rats in both their physiology and behaviour We tend to have overlooked mice as a pest in New Zealand, while in Australia they have mouse plagues of biblical proportions, and on Gough Island in the South Atlantic they have giant mice the size of rats (thankfully, this hasn’t occurred on our own Antipodes Island). Once you have your rat population under control, you’re likely to have a mouse problem, indicated by bait going missing from traps without them being set off. Time to buy some mouse traps.

Happy ratting – its an age old tradition