T A N E 29, 1983

RODENTS O F T H E G R E A T E R A U C K L A N D R E G I O N

by John L. Craig Department of Zoology, University of Auckland, Private Bag, Auckland

S U M M A R Y

Four rodent species are known in the Greater Auckland region. One rat (Rattus exulans) is known from only one offshore island in the region whereas the other three (R. norvegicus, R. rattus and Mus musculus) are known from islands and are also common on the mainland. Generally there is a paucity of information on these rodents in the Auckland area. The results of known studies and details of their biology are provided. More work is required.

INTRODUCTION

New Zealand has four species belonging to the Order Rodentia - the rodents. These four species include three rats and the house mouse, Mus musculus. The three rats are the Norwegian rat (Rattus norvegicus), the ship, roof or black rat (Rattus rattus), and the kiore or Polynesian rat (Rattus exulans). AU four species occur within the area of Greater Auckland. Taken as the area under the jurisdiction of the Auckland Regional Authority, this includes a wide range of mainland habitats plus the islands of the inner Hauraki Gulf.

The kiore is the least common of the four species being known to occur only on Tiritiri Matangi Island within the Auckland area. The other three rodents are widely distributed on the mainland but have more patchy distributions on the offshore islands. These distributions are partly historical, partly reflect the habitat requirements and partly relate to interactions between the species.

DESCRIPTION

The mature adults of the four rodents are readily separated (Fig. 1), but in mixed age populations individuals can be difficult to identify. Mus is by far the smallest of the four rodents and rarely weighs more than 23 g. The head plus body length is 80-85 mm and the tail is of similar length (Baden 1979). Typically they are a dark grey-brown above and light grey below.

Rattus exulans is the smallest of the rats and usually weighs about 60-90 g, but some males can weigh 130 g. The head plus body length ranges from 130-180 mm in adults with slightly shorter tail. Like the

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mouse, the upper parts are brown while the under parts are grey. Females have 8 nipples - 4 abdominal and 4 thoracic. The ears are large and will cover the eyes when pulled forward.

Rattus rattus is larger than R. exulans with a body weight of 120-160 g. It has the same large ears and pointed snout as R. exulans but can usually be distinguished at any age by having a tail considerably longer than the head plus body, which are 170-180 mm long. However, some

Rattus Rattus Rattus Mus norvegicus rattus exulans musculus

Fig. 1. The number and arrangement of nipples and relative body proportions of the four rodents found in Auckland. (Adapted from Watson 1959).

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individuals have shorter tails which makes them difficult to distinguish from R. exulans. Another distinguishing feature for females is the presence of 6 abdominal and 4 thoracic nipples. Young animals may closely resemble adult R. exulans while the very young R. rattus can be distinguished from mice by the disproportionately large ears of the baby rats. Rattus rattus has three colour phases: (i) brown back with white underparts; (ii) black back with grey underparts, and (iii) brown back with grey underparts. This has led to the confusing use of the names of black rat for this species whereas only 10-30% of the rats in an area are usually black (Daniel 1977)

R. norvegicus is our largest rat with a weight varying widely, 200-450 g, and a head plus body length averaging 190 mm (Beveridge and Daniel 1965). The more rounded body shape, small ears and the presence of a tail considerably shorter than the combined head and body length readily distinguish these rats from the other two species. The presence of 6 abdominal and 6 thoracic nipples is also characteristic for females. The fur is brown on the back with a white and grey belly. Albino R. norvegicus is the common rat used in laboratories and schools.

R. exulans and R. rattus are the more difficult species to distinguish especially if immature or males. The absence of the former from the Auckland mainland means that most Aucklanders will have little difficulty identifying any rodent they find.

DISTRIBUTION A N D A B U N D A N C E

(a) Mus The mouse has been known in Europe and Asia from prehistoric times

and was transported by humans to most parts of the world. Its occurrence in New Zealand closely follows patterns of European colonisation.

In Auckland it is commonly found in association with human habitation but also occurs in pastoral and forest habitats. Baden (1979) studied mice in both Woodhill pine forests and the native forest of the Hunuas. In both areas, mice went through annual cycles of abundance with population peaks occurring in April—May while relatively low densities were found between August and March. Using percentage snap trap success as a measure of abundance, Baden obtained higher densities in the pine forest (23-35%) than in the native forest (18%). Correction for trap interference (Nelson and Clarke 1973) increases these values to 45-70% in pine forest and 30% in the native forest. Extrapolation from work in other areas of New Zealand would suggest densities of 20-30/ha in pine and 13/ha in native forests. Baden's results show the greatest density (70% corrected snap trap success) in mature pine with an under storey of lupin and the lowest (15% corrected snap trap success) in mature pines without lupin. Abundance in three year old

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pine (45% CSTS) was higher than in the native forest of Hunua (30% CSTS). No other studies on mice are known from the Auckland area or from suburban areas within New Zealand. Although it is suspected that mice densities may be high in some urban areas, this is yet to be investigated.

(b)R. exulans A native to Asia, this rat has been transported throughout the Pacific

Islands by humans. Some workers (e. g. Watson 1956) believe that it was an accidental immigrant to New Zealand coming as a stowaway on Maori canoes. The importance of R. exulans as a food item suggests that deliberate introduction by Maoris may have been more likely.

R. exulans was once widespread throughout New Zealand, but after the arrival of the larger rats and mice, R. exulans is believed to have declined rapidly. By about 1850 it was considered extinct in the North Island although a specimen from Auckland was sent to the British Museum by Sir George Grey in 1849 (Watson 1956). Tiritiri Island is the only known refuge for R. exulans in the Auckland district although it is common on many of the outer islands of the Hauraki Gulf. R. exulans on Tiritiri has been intensively studied by researchers from the University of Auckland (Moller 1977, Craig and Moller 1978, Bunn 1979, Nicholas 1982). It is the only rodent on the island and occurs in all habitats - forest, bracken, grass, and around houses. Like mice in Auckland, annual cycles of abundance occur, varying according to habitat and year. Peak densities occur in late summer - autumn and, in grassed areas, can reach 240 rats/ha: the equivalent of 16 rats per suburban section. After attaining such high densities the population falls to levels of 20-50/ha. Densities are slightly lower in forested areas and extremely low under bracken (Pteridium aquilinum) (Table 1).

Table 1. Minimum density estimates for Rattus exulans on Tiritiri Island during population "highs" and "lows".

(c)R. rattus The ship rat or roof rat is a native of Asia that has been spread around

the world by ships. Watson (1959) considered that Captain Cook brought this rat to New

Zealand but Atkinson (1973) has argued that it more likely arrived in the latter half of last century. R. rattus is now widespread throughout the main islands and occurs on some offshore islands. It is common throughout Auckland being the more abundant rodent in forest and

Grass High population 140/ha Low population 40/ha

Bush 80/ha 20/ha

Bracken 60/ha 2/ha

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areas adjacent to forest or scrub. As well as inhabited islands, ship rats also occur on Goat Island off Leigh. There have been few studies on population densities within Auckland. Baden (1979) had very low capture rates in the Hunuas (23 captures/2 398 trap nights). On Goat Island in May, Craig (1977) obtained a corrected snap trap success of 20% which equates with 12-20 rats/ha. No other studies are known within the Auckland area but results from other areas in New Zealand (Daniel 1977) suggest relatively low densities occur in most areas except in years of high seed availability.

(d)R. norvegicus The Norway rat is actually a native of the U . S. S. R. and did not reach

Europe until the eighteenth century (Atkinson 1973). It has many names, e. g. the brown rat, house rat, water, or grey rat. It was probably the first rodent into New Zealand after R. exulans coming off the ships of the whalers and sealers. It was widespread and common last century. With the introduction of mustelids and R. rattus, however, it is now more commonly associated with human activities. It is still widespread in built-up areas within Greater Auckland and is the more common rodent in wet areas such as Pollen Island, farm drains, etc. Other than on inhabited islands, Norway rats are known from the Noises Group only. These rats are usually found at low densities except when associated with human wastes (e. g. dumps). For example, Baden (1979) trapped only 2 rats in 2 398 trap nights in the Hunuas and densities on the Noises were similarly low (0-18% snap trap success) (Moors 1979). However, occasional reports of high densities do occur, (e. g. Daniel 1978)

H A B I T A T S A N D INTERACTIONS

All four rodents are nocturnal and hence must be studied by trapping. All feed primarily on seeds, nuts and fruit but also take other vegetation, invertebrates, small vertebrates and carrion. This extreme overlap in diet suggests competition for food and this has been argued as the explanation for the observed patterns of co-occurrence (Taylor 1975).

Mus musculus, R. rattus and R. norvegicus commonly occur together. R. exulans is not known to exist anywhere in New Zealand in association with all the other three rodents, but it exists with all remaining combinations of these other rodents. Because of its small size Mas tends to take smaller food items and to live predominantly on the ground. It appears to thrive in more open habitats such as grasslands. R. exulans similarly appears to do better in open environments like grassland but is more of a generalist than the other rodents. It is found in all habitats, it burrows but also climbs as readily as it uses the ground. R. rattus is a

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ready climber and is rarely found in open habitats. R. norvegicus does not climb but takes readily to water and typically lives in burrows.

Hence the typical arrangement is for R. norvegicus and Mus to live at ground level taking different sized food items with R. rattus living in trees or roofs. This is the common situation on the mainland. R. exulans being the smallest of the rats can take smaller food items when the larger rats are present, or they can live on the ground or in open habitats when with R. rattus alone, or they can live in trees when with R. norvegicus alone. When R. exulans occurs in habitats with only Mus it can take the larger food items and live more in trees but when all the other rodents are present these seem to exploit all foods and habitats to the exclusion of J R. exulans (Taylor 1975).

B R E E D I N G

Rodents appear to be seasonal breeders in New Zealand but all species extend the length of the breeding season when food is abundant (Daniel 1977, Fitzgerald 1977, Bunn 1979). Whether this occurs in an urban environment is not known.

Litter sizes and the number of litters born per year vary (Table 2) but all species have high reproductive capabilities allowing them to increase rapidly in favourable times.

The seasonal breeding typically results in annual cycles of abundance accompanied by large changes in age structure. Few rodents live for more than a year and a high proportion of the young of a season do not live long enough to breed in the next.

Table 2. Reproductive characteristics of rodents

Species Litter Size No. of Litters/year

Annual product­ivity (young per

year)

Authority

Mus 5. 5 range 2-8

2-3 27 Baden, 1979

Rattus exulans 7 (Tiritiri) 4-5 (other islands)

2- 3 3- 5

17 Author

Rattus rattus 6 range 3-8

Max. 5 Max. 22 to 30 Daniel, 1977

Rattus norvegicus 8 range 5-10

Max. 5 Max. 30 to 40 Daniel, 1977

T H E F U T U R E

The introduction of rodents has had devastating effects on fauna and to a lesser extent on flora. Their role in the decline of our native birds is well known (Atkinson 1978). Less well understood has been their

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elimination of many of our large invertebrates (Ramsay 1978) and tuatara and lizards (Atkinson 1978). Many of our endangered fauna now survive on offshore island sanctuaries which lack mammalian predators or have R. exulans only. The continued existence of endangered fauna on these sanctuaries relies on the prevention of accidental introduction of any rodents but especially the arboreal R. rattus.

Rodents are also of considerable economic importance. Contamination of foodstuffs is common as is the spoilage and damage to stored grains. Furthermore, R. norvegicus and Mus are a serious pest to grain crops, especially maize in the Auckland region.

Rodents in all areas also spread disease to both humans and livestock. Rodents are a common source of Salmonella, ringworm, Clostridium, Leptospirosis, tuberculosis, typhoid, paratyphoid and Trichonosis (Health Officer, pers. comm. ).

Rodents in urban areas are controlled on a large scale by the use of poison baits although snap traps and domestic cats are important as well. In other areas, the natural predators such as mustelids (weasels, ferrets and stoats), cats and moreporks, help to reduce numbers. Cold winters and lack of food are the only other factors that control numbers by restricting breeding.

Our knowledge of rodents within our urban environments and especially within the Auckland area is limited. Regular use of standard snap trap lines along with careful autopsy can provide a wealth of much needed information with a minimum of effort and outlay. Hitchmough (1980) provides an outline of the information most frequently taken during autopsy. Further details are given in Moller (1977) and Bunn (1979).

A C K N O W L E D G E M E N T S

The text has been improved by my parents, and by Anne Stewart and Brian Foster. M y research has been supported by the Auckland University Research Committee, the Lands and Survey Department, and the Hauraki Gulf Maritime Park Board. I am grateful to all of these.

R E F E R E N C E S

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Atkinson, I. A . E . 1978: Evidence for effects of rodents on the vertebrate wildlife of New Zealand islands. In Dingwall, P R . ; Atkinson, I. A . E . & Hay, C. (Eds). "The Ecology and Control of Rodents in New Zealand Native Reserves. " Department of Lands and Survey Information Series No. 4: 7-32.

Baden, D. 1979: The ecology of mice (Mus musculus L . ) in two forests near Auckland. M . Sc, thesis, University of Auckland. 175 p.

Beveridge, A . E . & Daniel, M . J . 1965: Observations on a high population of brown rats (Rattus novegicus Berkenhont, 1767) on Mokoia Island, Lake Rotorua. New Zealand Journal of ScienceS: 174-189.

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Bunn, T. J . 1979: The effects of food supply on the breeding behaviour and population ecology of kiore on Tiritiri Matangi Island. M . Sc, thesis, University of Auckland. 197 p.

Craig, J . L . 1977: The rats on Goat Island and their effect on the vegetation. Report to Hauraki Gulf Maritime Park Board.

Craig, J . L . & Moller, H . 1978: The ecology of kiore (Rattus exulans) on Tiritiri Matangi Island. In Dingwall, P. R.; Atkinson, I. A . E . & Hay, C. (Eds. ). "The Ecology and Control of Rodents in New Zealand Native Reserves. " Department of Lands and Survey Information Series No. 4: 175-178.

Daniel, M . J . 1978: Population ecology of ship and Norway rats in New Zealand. In Dingwall, P R . ; Atkinson, I. A . E . & Hay, C. (Eds. ). "The Ecology and Control of Rodents in New Zealand Native Reserves. " Department of Lands and Survey Information Series No. 4: 145-152.

Fitzgerald, B. M . 1978: Population ecology of mice in New Zealand. In Dingwall, P. R.; Atkinson, I. A . E . & Hay, C. (Eds). "The Ecology and Control of Rodents in New Zealand Native Reserves. " Department of Lands and Survey Information Series No. 4: 163-173.

Hitchmough, R. A . 1980: Kiore (Rattus exulans) on Motukawanui Island, Cavalli group, northern New Zealand. Tane 26: 161-167.

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Moors, P. J . 1979: Norway rats on islands in Hauraki Gulf. Wildlife-a review 10: 39-45. Nelson, L . Jr. & Clark, F. W. 1973: Correction for sprung traps in catch/effort calculations

of trapping results. Journal of Mammalogy 54: 295-298. Nicholas, M . 1982: Spatial organization of Rattus exulans on Tiritiri Matangi Island.

M . Sc, thesis, University of Auckland. 108 p. Ramsay, G. W. 1978. A review of the effects of rodents on New Zealand invertebrate fauna.

In Dingwall, P. R.; Atkinson, I. A . E . & Hay, C. (Eds. ). "The Ecology and Control of Rodents in New Zealand Native Reserves. " Department of Lands and Survey Information Series No. 4: 89-98.

Taylor, R. H . 1975: What limits kiore (Rattus exulans) distribution in New Zealand? New Zealand Journal of Zoology 2: 473-477.

Watson, J . S. 1956: The present distribution of Rattus exulans (Peale) in New Zealand. New Zealand Journal of Science & Technology 37: 560-570.

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