**Abstract**

The distribution of the Norway rat *Rattus norvegicus* extends from the subarctic to the subtropics in Japan; yet it is limited by several factors. I discuss appropriate diet, water balance, and temperature as limiting factors based on surveys in the subarctic zone (Yururi-Moyururi, uninhabited islands in Hokkaido), the temperate zone (a business district in Yokohama and an uninhabited islet, Kaiho-2 in Tokyo Bay), and the subtropics (the Hahajima Islands in the Ogasawara Archipelago) in Japan. In Yururi-Moyururi, the rats recruited new generations in their population not only in the summer but also under snow cover, probably by preying on carcasses of their own species. In Yokohama, peaks of recruitment of their new generations were found in the winter and the summer, though the season with peaks changed every year. In Kaiho-2, rats stopped recruiting in the winter because of dehydration, and over the winter the group lost body mass as a result of body fat consumption. In Hahajima, rats lost body mass and preyed mainly on plant matter because of chronic dehydration. I conclude that protein-rich diets and water balance, but not temperature, are basic factors in the distribution of the Norway rat.

**Keywords:** *Rattus norvegicus*, geographical distribution, limiting factor, protein-rich diet, water balance

### **1. Introduction**

The Norway rat *Rattus norvegicus* Berkenhout is one of the commensal rodents, along with the roof rat *R. rattus* Linnaeus, the Polynesian rat *R. exulans* Peale, and the house mouse *Mus musculus* Linnaeus. These rodents expanded their distribution worldwide by taking advantage of human activities [1–3]. However, they have limitations in their geographical distributions. Brooks and Rowe [2] point out that Norway rats are fundamentally fitted to the temperate zone, and they are less prosperous in tropical and subtropical climate zones, whereas roof rats thrive in tropical and subtropical climate zones. The question arises as to whether Norway rats are fitted to the temperate zone due to the mild temperature. Tomich [4] points out that mild temperature is secondary to appropriate diet as a factor in determining the Norway rat distribution. Although they are omnivorous, Norway rats require a diet containing a certain amount of animal matter or that is protein-rich [5–7].

Many species of seabirds nesting on or near the ground or in burrows are vulnerable to predation by Norway rats because of the terrestrial behavior of the rats [1, 8]. Rats on an island in the Aleutians were supposed to prey on seabirds and to restrict the productivity of shorebirds and land birds by preying on the birds' food [9]. For Norway rats, such subarctic and subantarctic zones are severe environments in the cold season; when the rats' reproductive activities are depressed, their ears, legs, and tails are frostbitten, and their mortality rate is higher [10, 11]. However, Yabe et al. [12] discovered Norway rats breeding under snow cover on uninhabited subarctic islands in Japan. This fact suggests that they breed even during the cold season or under snow cover when an appropriate diet is available.

Also, the tropical and subtropical climate zones seem to be severe environments for Norway rats. Norway rats in the tropical climate zone are distributed in patches in limited areas such as seaports, irrigated villages, and large cities [2, 13–15]. Yabe et al. [7] found that the body mass of Norway rats on islands in the subtropical climate zone was smaller than those in the other habitats in the subarctic climate zone and the temperate climate zone in Japan because of a protein deficiency. Norway rats on the islands in the subtropical climate zone preferred plant matter to animal matter. On the other hand, Norway rats on an artificial islet in the temperate climate zone stopped breeding and lost body mass in the dry winter even though they preyed on some animal matter [16, 17]. Therefore, it seems that the appropriate diet changes depending on the habitat, and protein-rich diets do not always help Norway rats to thrive. Then commenting on the review by Yabe [18], I discuss the factors that cause the appropriate diet for Norway rats to shift based on their habitat and thus limit their geographical distributions.

## **2. Breeding in the subarctic zone**

#### **2.1 Breeding under snow cover**

Yururi (168 ha, 43° 12′ N, 145° 35′ E) and Moyururi (31 ha, 43° 13′ N, 145° 36′ E) (referred to as Yururi-Moyururi hereafter) are uninhabited islands situated 2.5 and 3.7 km off the Nemuro Peninsula of Hokkaido, respectively (**Figure 1**). They are in the subarctic climate zone and have a mean annual temperature of 6.3°C. Both islands are flat and covered with low vegetation such as alpine plants and the bamboo grass *Sasa nipponica* Makino and Sibata. According to the local people, Norway rats intruded into these islands in the 1960s or 1970s from a boat used for fishing or light house construction.

Generally, the reproductive activities of Norway rats in the subarctic and subantarctic zones seem to be restricted in the summer. Schiller [11] found that the breeding season of Norway rats in a business district and in dumping sites in Nome in Alaska occurred exclusively in the summer. Pye and Bonner [10] also found that the breeding season of Norway rats in a coastal area on South Georgia Island in the subantarctic climate zone was in the summer from December to February. The most active breeding season for Norway rats on Yururi-Moyururi also seemed to be in the summer. Here, 63 (86.3%) of the 73 rats caught in late July and early August 2013 were born from June to July. However, 10 (13.7%) of them were born from December to March, the heavy snow season (**Table 1**) [12].

Data collected by a metrological station at Nemuro, a city close to these islands, show that the amounts of snowfall were 52, 41, 52, and 29 cm in December 2012 and January, February, and March 2013, respectively. No rats entered these islands in these years because boats are restricted from approaching these islands, and there were no wrecked vessels after these islands were appointed to be a sanctuary for birds in 2011. The distance from the Nemuro Peninsula to Yururi-Moyururi is over 1 km that is pointed out by Russell et al. [20] as a possible distance for Norway rats to swim. Therefore, the 10 rats on Yururi-Moyururi must have been born during

**23**

**Figure 1.**

*Effect of Diet and Water Availability on* Rattus norvegicus *(Rodentia: Muridae) Distribution*

the heavy snow season. Snow cover protects Norway rats from cold temperatures. The temperature at the ground level under 50 cm of snow cover, for example, is kept above −5°C, even when the air temperature is below −30°C [21]. Inukai [22] also showed that the temperature at the ground level under 1 m of snow cover was from 0 to −2.8°C when the air temperature ranged from −6 to −13°C in Sapporo, Hokkaido. Furthermore, deep snow cover stabilizes the temperature under the snow [23], and thus, snow cover likely provides comfortable breeding conditions for Norway rats. Maeda [24] also found evidence of the breeding of Norway rats under snow cover just after the melting of the snow in a forested area in Sapporo.

*Map of Yururi and Moyururi Islands, Yokohama, Kaiho-2, the Ogasawara Archipelago, and Hahajima Island [7].*

*DOI: http://dx.doi.org/10.5772/intechopen.92307*

*Effect of Diet and Water Availability on* Rattus norvegicus *(Rodentia: Muridae) Distribution DOI: http://dx.doi.org/10.5772/intechopen.92307*

**Figure 1.** *Map of Yururi and Moyururi Islands, Yokohama, Kaiho-2, the Ogasawara Archipelago, and Hahajima Island [7].*

the heavy snow season. Snow cover protects Norway rats from cold temperatures. The temperature at the ground level under 50 cm of snow cover, for example, is kept above −5°C, even when the air temperature is below −30°C [21]. Inukai [22] also showed that the temperature at the ground level under 1 m of snow cover was from 0 to −2.8°C when the air temperature ranged from −6 to −13°C in Sapporo, Hokkaido. Furthermore, deep snow cover stabilizes the temperature under the snow [23], and thus, snow cover likely provides comfortable breeding conditions for Norway rats. Maeda [24] also found evidence of the breeding of Norway rats under snow cover just after the melting of the snow in a forested area in Sapporo.

*Rodents*

to restrict the productivity of shorebirds and land birds by preying on the birds' food [9]. For Norway rats, such subarctic and subantarctic zones are severe environments in the cold season; when the rats' reproductive activities are depressed, their ears, legs, and tails are frostbitten, and their mortality rate is higher [10, 11]. However, Yabe et al. [12] discovered Norway rats breeding under snow cover on uninhabited subarctic islands in Japan. This fact suggests that they breed even during the cold season or under snow cover when an appropriate diet is available.

Also, the tropical and subtropical climate zones seem to be severe environments for Norway rats. Norway rats in the tropical climate zone are distributed in patches in limited areas such as seaports, irrigated villages, and large cities [2, 13–15]. Yabe et al. [7] found that the body mass of Norway rats on islands in the subtropical climate zone was smaller than those in the other habitats in the subarctic climate zone and the temperate climate zone in Japan because of a protein deficiency. Norway rats on the islands in the subtropical climate zone preferred plant matter to animal matter. On the other hand, Norway rats on an artificial islet in the temperate climate zone stopped breeding and lost body mass in the dry winter even though they preyed on some animal matter [16, 17]. Therefore, it seems that the appropriate diet changes depending on the habitat, and protein-rich diets do not always help Norway rats to thrive. Then commenting on the review by Yabe [18], I discuss the factors that cause the appropriate diet for Norway rats to shift based on their habitat

Yururi (168 ha, 43° 12′ N, 145° 35′ E) and Moyururi (31 ha, 43° 13′ N, 145° 36′ E) (referred to as Yururi-Moyururi hereafter) are uninhabited islands situated 2.5 and 3.7 km off the Nemuro Peninsula of Hokkaido, respectively (**Figure 1**). They are in the subarctic climate zone and have a mean annual temperature of 6.3°C. Both islands are flat and covered with low vegetation such as alpine plants and the bamboo grass *Sasa nipponica* Makino and Sibata. According to the local people, Norway rats intruded into these islands in the 1960s or 1970s from a boat used for fishing or light house

Generally, the reproductive activities of Norway rats in the subarctic and subantarctic zones seem to be restricted in the summer. Schiller [11] found that the breeding season of Norway rats in a business district and in dumping sites in Nome in Alaska occurred exclusively in the summer. Pye and Bonner [10] also found that the breeding season of Norway rats in a coastal area on South Georgia Island in the subantarctic climate zone was in the summer from December to February. The most active breeding season for Norway rats on Yururi-Moyururi also seemed to be in the summer. Here, 63 (86.3%) of the 73 rats caught in late July and early August 2013 were born from June to July. However, 10 (13.7%) of them were born from

Data collected by a metrological station at Nemuro, a city close to these islands, show that the amounts of snowfall were 52, 41, 52, and 29 cm in December 2012 and January, February, and March 2013, respectively. No rats entered these islands in these years because boats are restricted from approaching these islands, and there were no wrecked vessels after these islands were appointed to be a sanctuary for birds in 2011. The distance from the Nemuro Peninsula to Yururi-Moyururi is over 1 km that is pointed out by Russell et al. [20] as a possible distance for Norway rats to swim. Therefore, the 10 rats on Yururi-Moyururi must have been born during

December to March, the heavy snow season (**Table 1**) [12].

and thus limit their geographical distributions.

**2. Breeding in the subarctic zone**

**2.1 Breeding under snow cover**

construction.

**22**

#### *Rodents*


*\* 1.2–1.7 months old.*

*Seven females less than 2 months old were pregnant. Ages were estimated using a formula [12, 19] based on eye-lens weight. Birth month was calculated by subtracting the age from the date when the rat was caught. New data on pregnant females were added [12].*

#### **Table 1.**

*Age composition of Norway rats caught in late July to early August 2013 in Yururi-Moyururi.*

### **2.2 Appropriate diet under snow cover**

Norway rats on Yururi-Moyururi thrived and reproduced under snow cover without depending on human beings for their diet. In the case of Maeda [24], Norway rats ate mainly bamboo seeds and rodents such as gray red-backed voles *Myodes rufocanus* Sundevall, the population of which exploded after the bamboograss flowering. The voles usually make their nests in tunnels under ground, but in the snow season they make their nests and breed in the space under bamboo grass covered by snow [25, 26]. Therefore, it is likely that Norway rats could easily find and prey on such voles. However, there were no rodents or other small mammals except Norway rats on Yururi-Moyururi (T. Hashimoto, pers. comm.).

Birds of prey such as common buzzards *Buteo Buteo japonicus* Temminck and Schlegel are known to live on Yururi-Moyururi [27, 28]. It is likely that shallow snow and dead grass cover these islands at the end of autumn or beginning of winter. Rats running across such white snow are vulnerable to birds of prey [29], and rats running on dead grass may also be. Among birds of prey, common buzzards are known to feed on Norway rats [30], and they probably leave behind body parts of the prey as in the case of roof rats (**Figure 2**). The dense population of rats that were born during the summer will provide the necrophagous rats with many rat remnants as a food supply to winter and breed under snow cover. All the rats in Yururi-Moyururi were less than 9 months old (**Table 2**). This suggests that their life spans were shorter than in any other habitats such as the Hahajima Islands, a business district in Yokohama and an islet (Kaiho-2) in Tokyo Bay. Norway rats that were 13 months old or older were common in the latter three habitats (**Table 2**). Predation by birds of prey was probably one of the causes of their short life span. Snow cover in the heavy snow season protected Norway rats from such predators and helped them to breed during the winter.

### **2.3 Appropriate diets for breeding in summer**

Meehan [31] reported that Norway rats become sexually mature at 2–3 months old, but it has also been found that they can become mature at less than 2 months old

**25**

**Table 2.**

*Moyururi, Yokohama and Kaiho-2.*

**Figure 2.**

*Effect of Diet and Water Availability on* Rattus norvegicus *(Rodentia: Muridae) Distribution*

[32–34]. On Yururi-Moyururi, seven young rats less than 2 months old were pregnant in the summer (**Table 1**). Why did Norway rats on Yururi-Moyururi tend to mature at a young age and breed actively in the summer? It is possible that a protein-rich diet

*Percentage of the number of Norway rats that were 13 months old or more in Hahajima Islands, Yururi-*

*Remains of roof rats* Rattus rattus *left by common buzzards* Buteo buteo japonicus *in the Ogasawara* 

**Age in months**

**Locality <13 ≥13 Total % of ≥13 Reference** Hahajima Islands 32 42 74 56.8 [7] Yururi-Moyururi 73 0 73 0.0 [7, 12] Yokohama 97 20 117 17.1 [7, 45] Kaiho-2 210 5 215 2.3 [16] *The numbers of both sexes were combined. All the rats in Yururi-Moyururi were less than 9 months old. See Table 1.*

*Archipelago (A: by T. Yabe; B: by F. Nomura, provided by PREC Inst. Inc.).*

*DOI: http://dx.doi.org/10.5772/intechopen.92307*

*Effect of Diet and Water Availability on* Rattus norvegicus *(Rodentia: Muridae) Distribution DOI: http://dx.doi.org/10.5772/intechopen.92307*

#### **Figure 2.**

*Rodents*

*\**

**Table 1.**

*1.2–1.7 months old.*

*pregnant females were added [12].*

**2.2 Appropriate diet under snow cover**

**2.3 Appropriate diets for breeding in summer**

Norway rats on Yururi-Moyururi thrived and reproduced under snow cover without depending on human beings for their diet. In the case of Maeda [24], Norway rats ate mainly bamboo seeds and rodents such as gray red-backed voles *Myodes rufocanus* Sundevall, the population of which exploded after the bamboograss flowering. The voles usually make their nests in tunnels under ground, but in the snow season they make their nests and breed in the space under bamboo grass covered by snow [25, 26]. Therefore, it is likely that Norway rats could easily find and prey on such voles. However, there were no rodents or other small mammals

*Seven females less than 2 months old were pregnant. Ages were estimated using a formula [12, 19] based on eye-lens weight. Birth month was calculated by subtracting the age from the date when the rat was caught. New data on* 

**Age in months Birth month Number of rats**

1 (1.0–1.9) July 28 24 52 7\* 2 (2.0–2.9)s June 10 1 11 1 3 (3.0–3.9) May 0 0 0 0 4 (4.0–4.9) April 0 0 0 0 5 (5.0–5.9) March 3 0 3 0 6 (6.0–6.9) February 1 3 4 2 7 (7.0–7.9) January 0 2 2 1 8 (8.0–8.9) December 1 0 1 0 Total 43 30 73 11

**Male Female Total Pregnant**

Birds of prey such as common buzzards *Buteo Buteo japonicus* Temminck and Schlegel are known to live on Yururi-Moyururi [27, 28]. It is likely that shallow snow and dead grass cover these islands at the end of autumn or beginning of winter. Rats running across such white snow are vulnerable to birds of prey [29], and rats running on dead grass may also be. Among birds of prey, common buzzards are known to feed on Norway rats [30], and they probably leave behind body parts of the prey as in the case of roof rats (**Figure 2**). The dense population of rats that were born during the summer will provide the necrophagous rats with many rat remnants as a food supply to winter and breed under snow cover. All the rats in Yururi-Moyururi were less than 9 months old (**Table 2**). This suggests that their life spans were shorter than in any other habitats such as the Hahajima Islands, a business district in Yokohama and an islet (Kaiho-2) in Tokyo Bay. Norway rats that were 13 months old or older were common in the latter three habitats (**Table 2**). Predation by birds of prey was probably one of the causes of their short life span. Snow cover in the heavy snow season protected Norway rats from such predators and helped them to breed during the winter.

Meehan [31] reported that Norway rats become sexually mature at 2–3 months old, but it has also been found that they can become mature at less than 2 months old

except Norway rats on Yururi-Moyururi (T. Hashimoto, pers. comm.).

*Age composition of Norway rats caught in late July to early August 2013 in Yururi-Moyururi.*

**24**

*Remains of roof rats* Rattus rattus *left by common buzzards* Buteo buteo japonicus *in the Ogasawara Archipelago (A: by T. Yabe; B: by F. Nomura, provided by PREC Inst. Inc.).*


*The numbers of both sexes were combined. All the rats in Yururi-Moyururi were less than 9 months old. See Table 1.*

#### **Table 2.**

*Percentage of the number of Norway rats that were 13 months old or more in Hahajima Islands, Yururi-Moyururi, Yokohama and Kaiho-2.*

[32–34]. On Yururi-Moyururi, seven young rats less than 2 months old were pregnant in the summer (**Table 1**). Why did Norway rats on Yururi-Moyururi tend to mature at a young age and breed actively in the summer? It is possible that a protein-rich diet

helped the rats to mature at a young age, as suggested by McCoy [5], who pointed out that a high-protein diet produces excellent reproductive conditions. Animal matter occupied 72.4 ± 39.8% (n = 38) by volume of the stomach contents of the rats in July–August 2013 in Yururi-Moyururi, and of this 11.9 ± 30.1% of rhinoceros auklets *Cerorhinca monocerata* Pallas [12]. From May to August, seabirds such as *Fratercula cirrhata* Pallas, *Cepphus carbo* Pallas, *Uria aalge* Pontoppidan, *Larus crassirostris* Vieillot, *Phalacrocorax urile* Gmelin, *P. capillatus* Temminck and Schlegel, and *P. pelagicus* Pallas also stay on Yururi-Moyururi to breed [35]. Norway rats probably prey on adults, nestlings, and eggs of these seabirds, which would supply the rats with sufficient nutrition to mature at a young age and engage in active breeding. Therefore, it is likely that Norway rats on Yururi-Moyururi depend on a diet of seabirds for their reproductive activities in the summer and a diet of carcasses of their own species under snow cover in the winter.

Norway rats preyed on adult *C. monocerata* irrespective of the body weight of the rats. The mean body weight of the predators, 187.7 ± 75.8 g (n = 16), was not significantly different (*P* = 0.09) from that of non-predators, 147.2 ± 54.2 g (n = 25) [36]. On the other hand, only larger roof rats on the Chichijima Islands in the Ogasawara Archipelago preyed on Bulwer's petrels *Bulweria bulwerii* Jardine and Selby, where the mean body weight of the predators, 201.6 ± 27.5 g (n = 22), was significantly larger (*P* = 3.0 × 10<sup>−</sup><sup>4</sup> ) than that of non-predators, 167.5 ± 35.4 g (n = 17) [36, 37]. Norway rats preyed on adults of *C. monocerata* (520 g [38]) that were larger than themselves, whereas roof rats preyed on adults of *B. bulwerii* (78–130 g [39]) that were smaller than themselves. These findings show that Norway rats are more aggressive predators of animal matter than roof rats [36].

As for the water supply for the rats, peat bogs are a source of water in Yururi but there are no peat bogs in Moyururi. However, the area around the Nemuro Peninsula is covered by dense sea fog for 101.4 days a year, and over 16 days per month between June and August [40]. Therefore, dew from dense sea fog is probably one of the water sources for Norway rats. I hypothesize that a process was established by which Norway rats have an appropriate diet and engage in water supply for survival and a bimodal cycle of reproduction in the summer and under the snow cover on Yururi-Moyururi.
