Identification of Rats and Mice in New Zealand

New Zealand Journal of Agriculture, Volume 98, Issue 4, 15 April 1959, Page 365

Identification of Rats and Mice in New Zealand

By

J. S. WATSON,

Animal Ecology Section, Department of Scientific and Industrial Research, Wellington

MANY people are confused about the different kinds of rats in this country and have difficulty in recognising them. This is partly because there are many popular names, often quite misleading, for the same animal and partly because rats vary a great deal in colour, so that two animals which at first glance look different may belong to the same species.

THERE is some practical value in being able to recognise the various rat species, because they have different habits and are not equally susceptible to the same poisons. To get rid of an infestation of rats the species present must be known so that the best methods to use against it can be decided. There are three species of rats—the kiore, the Norway rat, and the ship —and the house mouse in New Zealand. Kiore (Ratfus exulans) This rat, popularly known here as the Maori rat, Polynesian rat, or. native rat, is found in south-east Asia, the East Indies, the Philippines, New Guinea, and on most of the Pacific Islands, but not in Australia. It was carried from one island to another round the Pacific probably as a stowaway in the canoes of the Polynesians and according to Maori tradition it was brought to New Zealand in one of the canoes of the Great Fleet, which arrived about 600 years ago. However, it is more likely that it had already come with the Moriori some hundreds of years earlier, as it was with them on the Chatham Islands before the Maoris arrived there. The kiore was widely spread throughout New Zealand in pre-

European times and the Maoris, who considered it an important food, had special traps for catching it. Soon after the first Europeans arrived it disappeared from most of the North Island. The two other rat species established themselves at about the same time, and it was generally supposed that they drove the kiore out. It lasted longer in the South Island and periodically became very numerous, particularly in the Nelson and Blenheim districts. These sudden increases in numbers probably coincided with the years when the beech trees were producing masses, of seed. However, the last of these outbreaks came in 1889 and there have been very few records of the kiore since then.

Today this rat is found on some of the off-lying islands, particularly in the north, where neither of the other two species is present, but it is also on Kapiti Island with the Norway rat and probably also the ship rat, and in some isolated localities in Fiordland and Stewart Island. The map shows the places where it has been found in the last 10 years. . The kiore is so locally distributed that it is generally of no economic importance, though on some of the Pacific islands it is abundant and damages native crops. In New Zealand it usually feeds on berries, seeds, and occasion all y insects. Appearance and Habits The kiore is the smallest of the three rat species and rarely weighs more than 4 oz. The length of the head and body of a full-grown animal is , about 6 in. and the tail

is about the same length, though it may be slightly shorter or. longer than the head and body. The ears are relatively large and thin. It has a greyish brown back and is pale grey underneath. Many people believe that the small blue-grey rat sometimes found in the bush is the kiore, whereas this is really the young of the black form of the ship rat. The kiore lives chiefly on the ground, nesting in hollow logs and

under rocks or other cover. The usual number in a litter is 5, though up to 8 have been found, and the main breeding season is probably in late summer. Norway Rat [Rattus norvegicus) The Norway rat, so called because ~ it was first described from specimens ' collected in Norway, came originally from the plains of Central Asia. It gradually spread westward and reached Britain a little more than 200 years ago, where it eventually occupied the whole country and replaced the ship rat, which is now found only in the ports. It came to New Zealand with the whalers and sealers, whose bases,. littered with decaying meat and offal, provided it with ideal conditions. Charles Darwin found it at the Bay of Islands at the time of his visit in H.M.S. Beagle in 1835. It is now distributed throughout the country, more particularly in towns and around houses, but may be found in remote places, . especially along creeks. It is established on subAntarctic Campbell Island and subtropical Raoul Island in the Kermadecs. It is popularly known as the common rat, brown rat, grey rat, sewer rat, or water rat. Appearance and Habits The Norway rat is the largest of the three rat species. Full-grown animals may weigh as much as 1| lb. and 1 lb. is quite usual. The length of the head and body may be up to 11 in. and the tail is always shorter than the head and body. The ears are relatively short, thick, and covered with small hairs; if bent forward they do not . reach as far as the eye. Most Norway rats are greyish brown on the back and pale grey on the under side. In London, Berlin, Paris, and some other large European towns about one in 75 of these rats is black, usually with a white spot on the chest. This rare melanic form has not so far been found in New Zealand, though it may well turn up when more rats have been carefully examined. The domesticated white rat is the albino form of this species. Norway rats are burrowing animals and live and nest in holes in the ground often under the floors of buildings. As they do not mind cold and damp, sewers provide them with an ideal habitat with ready-made tunnels where they are undisturbed and where there is a constant supply of food scraps washed down from kitchen sinks. They have been found living and breeding in cold stores maintained at a constant temperature of 20 degrees F. below freezing. They readily enter water and often make their holes along the banks of

rivers and creeks, where they feed on freshwater mussels and crayfish which they collect from the stream bed. As a result they are sometimes referred to as water rats,. though the animal known by this name in England (the water vole) is not found in New Zealand. Norway rats are attracted wherever food is easily obtained, particularly to rubbish dumps, pigsties, chicken runs, and the like and will often make their way into buildings through faulty drains connecting the sewers or through holes in the wall near ground level. These rats do not have any well marked breeding season, but may produce their litters at any time of the year, the average number in a litter being about 8, though up to 12 is not unusual. Ship Rat [Rattus rattus) This rat, also commonly known as the black rat, bush rat, house rat, or roof rat, probably came originally from south-east Asia and gradually spread westward. It reached Britain about 800 years ago probably in the ships carrying the soldiers returning from the Crusading wars in the Middle East.

Before this time there were no rats in Britain. It was the common rat in Europe until the end of the 18th century and as it readily adapts itself to life on ships, which before the use of fumigation were often heavily infested, it has been carried to most parts of the world. It is now generally distributed, particularly in seaport towns, and is very numerous in tropical and subtropical countries. Where bubonic plague occurs this species is usually the main carrier, though both the other . two species can and sometimes do carry the disease. Most ships reaching New Zealand in the early days would have had some of these rats on board, and if the first ones did not come with Captain Cook, they would have arrived soon afterward. Today the ship rat is generally spread throughout New . Zealand. It is found in houses both in town and country, and is the common rat in the bush. Appearance and Habits This rat when full grown weighs up to i lb. The length of the head and body is about 9 in., and the tail is almost always longer than the head and body. The ears are thin, hairless, and rather large, so that if pushed forward, they will cover the eyes. Rats of this species are very variable in colour, and there are three distinct colour forms. One has a brown back and a white or creamy under side, the second has a black or bluish black back and grey underside, and the third has a brown back and grey under side. The first of these is the commonest in this country, but about one in five is the black form, though the proportion varies from place to place. The black was. the common form in Britain before the arrival of the Norway rat, which explains why this species is sometimes referred to as the black rat. The third form is rather rare in New Zealand and has so far been recorded only from Stewart Island, Westland, and the main ports. These three varieties, though quite different in colour, are similar in other respects such as shape and habits. They all belong to the same species, as they freely interbreed and their young may be any of the three forms. The different rat species do not naturally interbreed and attempts to cross them in laboratories have failed. The ship rat is essentially an arboreal animal. In the forest it makes its nest among the branches of bushes, in clumps of kiekie and other plants growing on trees, or among the dead leaves of cabbage trees. The nest, which is somewhat like a sparrow’s, is a loosely built ball of dead leaves and grass, in the middle of which the young are born. In houses it nests inside hollow walls, in attics, in old cupboards, or in similar undisturbed corners. It breeds throughout the year, but more prolifically in spring and

autumn. About 6 young are born in a litter. Ship rats seldom live with Norway rats, and though both may sometimes occupy the same building, the ship rat will usually be on the top floor or under the roof and the Norway rat in the basement and on the lower floors. Though the Norway rat has replaced the ship rat in England, it does not seem to be doing so here. There is apparently plenty of room for both and there are infestations of each species in most New Zealand towns. House or Field Mouse (Mus musculus) The house mouse has been in Britain, Europe, and Northern Asia since prehistoric times. As it is so small and likes to live in close association with man, it is easily transported in boxes and bales of goods. In this way it has been carried . round the world and is now well established in most countries. It was reported at the mission station in the Bay of Islands in the early 1830 s and appeared in Dunedin 2 years after the city was founded. It can now be found wherever there are houses and also far out in the bush and even up to grassy mountain tops. Mice are also present on many of the off-lying and subantarctic islands. There is only the one kind of mouse in New Zealand, but as so many of them live out of doors well away from houses, many people wrongly imagine . that there are two different species, a house mouse and a field mouse. Appearance and Habits A full grown mouse seldom weighs more than 1 oz. The length of its head and body is about 3 in. and the tail is usually slightly longer. Its colour is an almost uniform dark grey-brown above . and buffy-grey below. The white mice which are often kept as pets by children are albino house mice. While mice may live in holes in the ground or in walls, they can climb easily and will make their nest wherever there is suitable cover within reach, of food. A mouse’s daily range of movement may be quite small where all its requirements can be met close at hand and the area covered may be as small as 50 sq. ft. This makes a mouse infestation in a large grain or seed store particularly difficult

to eliminate, as mice may live in the middle of a large stack of bagged grain without ever coming to the edge, where they can be dealt with. Mice do not have a clear-cut breeding season, but may produce young at any time of the year. About 6 are born in each litter and each adult female may produce about 5 litters per year.. The size and number of litters produced depend on the amount of food available. Identification The table below sets out the main characters that are a help in recognising the different rat species. Few of them are sufficiently precise by themselves to show the identity of a rat,

and some are little more than rough guides that can usually, but not invariably, be depended on. They are listed approximately in the order of their reliability. As many of these characters as possible should be looked at when trying to identify a specimen. The number of teats is probably the best character, but is the most limited in its use, for they can be seen easily only on nursing females. The teats are in pairs on the chest and lower belly (Fig. 2) and the number of pairs is different in each of the three species. The size of a rat, most conveniently indicated by its weight, has a definite, though limited, value. Large rats weighing more than J- lb. can safely be identified as Norway rats and any rat of more than 4 oz. cannot be a kiore.. The size and shape of the ears distinguish the Norway rat from the ship rat and kiore (Fig. 3), but not the ship rat from the kiore. The length of the tail is also useful for- separating the Norway from the ship rat, but is no help with the kiore. Some care has to be taken when using this character, as rats quite often lose a bit of their tail, and a few ship rats (about 1 in 100) may have a tail slightly shorter than the head and body.

The colour of the fur on the under side is a help in identifying the kiore. The ship rat, which it more nearly resembles, particularly in the size of the ears, has the under side coloured white or grey and the fur is usually almost uniform in colour, whereas in the kiore white hairs overlie the dark grey underfur.

In New Zealand the kiore is so rare that few people are likely to come across it. The Norway and the ship rat are the two species living around buildings that need to be distinguished and the easiest way of doing this is by the size of the ear and the length of the tail. Mice can be easily recognised by their small size and short, close fur. Occasionally they may be confused with very young rats just out of the nest, but their small feet should make their identity clear — hind foot of a house mouse is never more than g in. long; the smallest rat always has a hind foot longer than this. Signs and Traces Even if a specimen of a rat cannot be caught for identification, it is often possible to tell which species is present from the signs and traces that the rats leave behind them. As has already been pointed out, Norway rats live in burrows, so rat holes in the ground are sufficient by themselves to indicate the presence of this species. The way that rats enter a building may also be an indication of the species. Rats coming in through faulty drains would be Norway rats, while

those coming in through holes in the roof would be ship rats. The latter are much more agile and can climb in places that are inaccessible to the other species. They will run along beams supporting the ceiling and where their path is blocked by the joists they swing under them and back on to the beam and in so doing they leave a

dirty smear on the side of the beam. These curved smears under each joist are characteristic signs of ship rat infestation, as the Norway rat cannot perform this acrobatic feat. These smears along the beams will remain visible for many years and therefore are not necessarily an indication of rats in the building at the moment, but if the rats are using the beams as runways, these smears will be slightly tacky to the touch. Droppings Rats can also be identified by their droppings, and though it is not always possible to identify one dropping with certainty, there are usually enough characteristic ones about to be fairly certain which rats produced them. Fig. 4 shows typical droppings of the Norway rat, ship rat, and house mouse. Kiore droppings cannot be distinguished from those of the ship rat. Norway rat droppings are usually described as spindle shaped, long oval with pointed ends, and they are often deposited in groups. Those of the ship rat are thinner and often slightly curved or sausage shaped and are scattered where the rats have been

running. Mouse droppings are very much smaller, looking at first glance like spilt dry tea leaves. Reactions to Poisons and Traps The subject of rodent control in New Zealand was dealt with by Mclntosh and Adams in “The New Zealand Journal of Agriculture”, March 1955, and only differences in the various species which affect control will be dealt with here. There are some marked differences in the susceptibility of the rat species to various poisons. Two substances that have been much used in various commercial brands of rat poison are red squill and antu. These poisons can be very effective when properly used against Norway rats, but both the ship rat and the house mouse are relatively resistant to them so that they are inefficient to use against these species. Before either' of these poisons is bought it is as well to make sure that an infestation of Norway rats has to be dealt with. Not much is known about the kiore’s reactions to poisons, but it has been reported that warfarin, which is effective against the other two. species, is much less so against the kiore. “ The various species also react differently toward baits and traps. Norway rats are especially wary of any new object that is placed within their living area, so when treatment with a quick acting poison is carried out, feeding with unpoisoned bait for some days beforehand is essential. With trapping best results are obtained by leaving traps baited but unset for a few days; even so, it is very difficult to catch all the rats and some usually learn to avoid the traps. Though ship rats are not as wary as Norway rats, poisoning should still be preceded by pre-baiting, but it is often possible to clear out a small colony by trapping. House mice can quite easily be eliminated by trapping or by poisoning, but as the mice move over such short distances, it is especially important to put out plenty of traps or poison baits so that some are within the range of every mouse. Pre-baiting is usually unnecessary when poisoning mice.

Character Norway rat Ship rat Kiore Number of teats on female .. • .. 12 , 10 8 Maximum weight li lb. i lb. J lb. Ears Small, do not reach Large, cover eyes As ship rat eyes when pulled when pulled forward Tail forward Shorter than head and Longer than head and Shorter or longer than body body head and body Fur on under side .. 12 lb. Small, do not reach eyes when pulled forward Shorter than head and body White hair over grey Uniform colour, either As Norway rat underfur 10 i lb. Large, cover eyes when pulled forward Longer than head and body Uniform colour, either all white or grey 8 1 lb. As ship rat Shorter or longer than head and body As Norway rat

CHARACTERS USEFUL FOR IDENTIFYING THE DIFFERENT RATS