**3. Breeding in temperate climate zone**

### **3.1 Breeding independent of season**

Davis [41] reported that generally, the pregnancy rate in Norway rats is low in cold and hot seasons, and as a result, the rate shows a bimodal curve, with the highest peaks in the spring and autumn. The breeding season is usually estimated from pregnancy rates in adult females (percentages of visible pregnancies). However, recruitments of new generations in the population are more essential than pregnancy rates in population analysis [41, 42]. We can estimate the trend in the fluctuations of reproductive activities or recruitments based on age compositions even using surveys conducted once a year. Moors [43] discussed the age composition based on the age index estimated from the upper molars in Norway rat populations in Noises Island in New Zealand and concluded that recruitments were more active in the summer than in the winter. However, this age index revealed indefinite ages. Pucek and Lowe [44] recommended the eye-lens weight as the best criterion among the known indices for determining the age of small mammals. Then, Yabe et al. [45] analyzed age compositions based on the eye-lens criterion in Norway rat populations in February or March 2014–2016 in a 21-ha business district in Yokohama in

**27**

**Figure 3.**

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

the temperate climate zone (**Figure 1**). In this case, Norway rats showed recruitment peaks that were not always in the spring and autumn but also in the summer or winter, and the peaks changed every year (**Figure 3**). These results in Yokohama suggest that reproductive activities are controlled by factors other than temperature such as the food supply and environmental sanitation. In this business district in Yokohama, environmental sanitation activities conducted by volunteers control the

Kaiho-2 (Fort No. 2) in Tokyo Bay (**Figure 1**; 4 ha, 35°18′ N, 139°44′ E) is an uninhabited islet in the temperate climate zone. This islet is covered with concrete, bricks, sand, sandy soil, grasses, herbs, and shrubs. Norway rats probably intruded into the islet in the early twentieth century, when a fort was constructed there. I discovered from the age compositions of the rats that their reproductive activities were interrupted around December or January [16, 17]. On average, between 1981 and 2010, in November, December, January, and February, the minimum temperatures were 9.6, 4.9, 2.3, and 2.6°C, and the amounts of precipitation were 107.0, 54.8, 58.9, and 67.5 mm, respectively, at Yokohama, a city close to Kaiho-2 [47]. Therefore, Kaiho-2 was dry in December and January compared with November and February. The water supply for Norway rats was probably insufficient around December and January because the amount of precipitation was low, the majority of succulent plants died, dew and standing water were limited, and the sandy soil lost moisture. Norway rats on the islet consumed protein-rich diets such as the mussel *Mytilus galloprovincialis* Lamarck and other marine invertebrates, which amounted to more than 50% of their stomach contents by volume, even in the winter [6]. However, such an invertebrate or protein-rich diet demands a large turnover of water [48]. Furthermore, most marine invertebrates including mussels are osmoconformers to the surrounding sea water [49]. Therefore, the interruption of reproductive activities during the winter was probably due to dehydration, but

*Distributions of birth month and age in months in Norway rats caught in February or March 2014, 2015, and* 

*2016 in Yokohama. Rats over 12 months old are excluded. Modified after [45].*

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

**3.2 Interruption of breeding by dehydration**

not to low temperature or food shortages.

Norway rat population [46].

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

the temperate climate zone (**Figure 1**). In this case, Norway rats showed recruitment peaks that were not always in the spring and autumn but also in the summer or winter, and the peaks changed every year (**Figure 3**). These results in Yokohama suggest that reproductive activities are controlled by factors other than temperature such as the food supply and environmental sanitation. In this business district in Yokohama, environmental sanitation activities conducted by volunteers control the Norway rat population [46].

### **3.2 Interruption of breeding by dehydration**

*Rodents*

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

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

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

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

Davis [41] reported that generally, the pregnancy rate in Norway rats is low in cold and hot seasons, and as a result, the rate shows a bimodal curve, with the highest peaks in the spring and autumn. The breeding season is usually estimated from pregnancy rates in adult females (percentages of visible pregnancies). However, recruitments of new generations in the population are more essential than pregnancy rates in population analysis [41, 42]. We can estimate the trend in the fluctuations of reproductive activities or recruitments based on age compositions even using surveys conducted once a year. Moors [43] discussed the age composition based on the age index estimated from the upper molars in Norway rat populations in Noises Island in New Zealand and concluded that recruitments were more active in the summer than in the winter. However, this age index revealed indefinite ages. Pucek and Lowe [44] recommended the eye-lens weight as the best criterion among the known indices for determining the age of small mammals. Then, Yabe et al. [45] analyzed age compositions based on the eye-lens criterion in Norway rat populations in February or March 2014–2016 in a 21-ha business district in Yokohama in

) than that of non-predators, 167.5 ± 35.4 g (n = 17) [36, 37].

their own species under snow cover in the winter.

aggressive predators of animal matter than roof rats [36].

larger (*P* = 3.0 × 10<sup>−</sup><sup>4</sup>

the snow cover on Yururi-Moyururi.

**3.1 Breeding independent of season**

**3. Breeding in temperate climate zone**

**26**

Kaiho-2 (Fort No. 2) in Tokyo Bay (**Figure 1**; 4 ha, 35°18′ N, 139°44′ E) is an uninhabited islet in the temperate climate zone. This islet is covered with concrete, bricks, sand, sandy soil, grasses, herbs, and shrubs. Norway rats probably intruded into the islet in the early twentieth century, when a fort was constructed there. I discovered from the age compositions of the rats that their reproductive activities were interrupted around December or January [16, 17]. On average, between 1981 and 2010, in November, December, January, and February, the minimum temperatures were 9.6, 4.9, 2.3, and 2.6°C, and the amounts of precipitation were 107.0, 54.8, 58.9, and 67.5 mm, respectively, at Yokohama, a city close to Kaiho-2 [47]. Therefore, Kaiho-2 was dry in December and January compared with November and February. The water supply for Norway rats was probably insufficient around December and January because the amount of precipitation was low, the majority of succulent plants died, dew and standing water were limited, and the sandy soil lost moisture. Norway rats on the islet consumed protein-rich diets such as the mussel *Mytilus galloprovincialis* Lamarck and other marine invertebrates, which amounted to more than 50% of their stomach contents by volume, even in the winter [6]. However, such an invertebrate or protein-rich diet demands a large turnover of water [48]. Furthermore, most marine invertebrates including mussels are osmoconformers to the surrounding sea water [49]. Therefore, the interruption of reproductive activities during the winter was probably due to dehydration, but not to low temperature or food shortages.

#### **Figure 3.**

*Distributions of birth month and age in months in Norway rats caught in February or March 2014, 2015, and 2016 in Yokohama. Rats over 12 months old are excluded. Modified after [45].*


#### **Table 3.**

*Comparison of body weights of 3- and 6-month-old Norway rats living in the Hahajima Islands, Yururi-Moyururi, Yokohama, and Kaiho-2 (non-wintered and wintered groups) [7].*


*\* Excluding pregnant females.*

*Fat indexes were significantly different (t-test, p < 0.05) if they are followed by different letters. FI = 1.01FI' + 0.01, where FI' = fat free dry weight/dry weight [51].*

#### **Table 4.**

*Comparison of fat index (FI, mean ±* SD*) between wintered and non-wintered Norway rats on Kaiho-2 and a forested island (Shikine-jima) [17].*

Collier and Levitsky [50] showed that albino *R. norvegicus* rats lose their body mass to maintain water balance when the water supply is insufficient. Moors [43] suggested that shortages of protein-rich diets and fresh water restrict the sexual maturity of females, litter sizes, and the growth of juveniles in Norway rats on Noises Island in New Zealand. It is likely that a similar situation occurred in Norway rats on Kaiho-2. The age composition of Norway rats on this islet showed a gap between the generations borne before and after the season around December and January, when breeding was interrupted. As a result, their population was divided into a wintered group and a non-wintered group based on the gap. The body mass of the wintered group was lower than that of the non-wintered group (**Table 3**). I compared a body fat index determined by the method of Yabe [51] among the wintered group, the non-wintered group, and pregnant females. Also, I compared the index between Kaih-2 and Shikine-jima (a 390-ha forested island in the Izu Archipelago, 34°19′ N, 139°12′ E) (**Table 4**). As a result, I found that the small body mass in the wintered group in Kaiho-2 was due to body fat loss [17]. The body fat indexes showed that pregnant females kept a high level of body fat irrespective of whether they were in the wintered or non-wintered group, or on Kaiho-2 or Shikine-jima. Pregnant females deposit body fat for reproduction, probably because they require more energy than nonreproducing females as was pointed out by Robbins [52]. The lost

**29**

their food habit.

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

body fat in the wintered group was not recovered after the dehydration period, and the non-wintered group kept a high level of body fat [16, 17]. This fat deposition procedure is different from that in mammals, which deposit body fat as a prelude to

The Ogasawara Archipelago (Bonin Islands, Ogasawara Islands) is composed of the Mukojima Islands, the Chichijima Islands, the Hahajima Islands, and the Kazan (Volcano) Islands in the subtropics (**Figure 1**). Norway rats are thought to have intruded into the Ogasawara Archipelago between 1660 and 1862, but now they are living only in the Hahajima Islands and the Kazan Islands [53–55]. On the other hand, roof rats are prosperous and are distributed in most islands in the archipelago [56, 57], although they intruded there in the 1910s or 1920s, later than the Norway rats [54, 58]. It remains to be clarified why Norway rats are restricted to only a few

The body mass of Norway rats on the Hahajima Islands is about half the weight of Norway rats on Yururi-Moyururi, Yokohama, and Kaiho-2 (**Table 3** and **Figure 4**). The low mass of the Hahajima rats was due to environmental factors rather than genetic factors such as Bergman's rule and the founder's effect. This was proved by the fact that the head and body length, tail length, and length of the upper molar row were not significantly different between the rats from Hahajima and those from other localities [7]. Therefore, the skeletons were the same but the body masses were

Norway rats on the Hahajima Islands tended to feed on plant matter such as fruits and seeds (95.2 ± 21.8%, n = 21, by volume percentage in stomach contents) and no seashore animals were found even in rats living close to the seashore [7]. This is an abnormal food habit in the Norway rat, which prefers animal matter [6]. As I previously mentioned, preying on plant matter helps maintain water balance because the consumption of animal matter or of a protein-rich diet requires more water intake. However, this change in food habits may lead to a protein deficiency and body weight loss in the rats. To meet their energy requirements, mammals consume their gastrointestinal contents first, but finally they utilize their body fat and protein, which leads to long-term weight loss [52]. Moors [43] suggests that a shortage of protein-rich diets and fresh water limited the reproductive activities of Norway rats on Noises Island in New Zealand. It is likely that on the Hahajima Islands as well, protein deficiency and dehydration decrease the weight and inactivated the reproduction of Norway rats. I suppose that Norway rats on the Hahajima Islands are less aggressive predators than rats living in the other habitats because of

The Ogasawara Archipelago is probably an uncomfortable habitat for Norway rats due to chronic dehydration, which restricts their distribution. In the Hahajima Islands, there are streams and ponds on the main island but not on the surrounding islands. However, Norway rats were found even on the surrounding islands and in areas far from such water sources [7]. Therefore, dehydration in Norway rats on the Hahajima Islands was not due to a lack of such water sources. The mean annual precipitation in the Chichijima Islands from 1971 to 2000 was 1280 mm, and the mean potential evaporation (the amount of evaporation that would occur when enough water is given) was 1380 mm [27]. The former is less than the latter, and as a result, the soil tends to be dry. This indicates a potential cause of dehydration in Norway rats. However, the Hahajima Islands, with a mountain 462 m in height, is foggy and more humid than the Chichijima Islands, with a mountain 326 m in height, and the

times when the energy intake will be less than the energy expenditure [52].

**4. Dehydration and low body mass in subtropics**

different between the Hahajima rats and the others.

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

islands in the archipelago.

body fat in the wintered group was not recovered after the dehydration period, and the non-wintered group kept a high level of body fat [16, 17]. This fat deposition procedure is different from that in mammals, which deposit body fat as a prelude to times when the energy intake will be less than the energy expenditure [52].
