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

*Rodents*

**28**

*\**

**Table 4.**

**Table 3.**

*Excluding pregnant females.*

*forested island (Shikine-jima) [17].*

*where FI' = fat free dry weight/dry weight [51].*

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

*Comparison of fat index (FI, mean ±* SD*) between wintered and non-wintered Norway rats on Kaiho-2 and a* 

*Fat indexes were significantly different (t-test, p < 0.05) if they are followed by different letters. FI = 1.01FI' + 0.01,* 

Kaiho-2 0.10 ± 0.04a 0.16 ± 0.06b 0.22 ± 0.06c n = 28 63 9 Shikine-jima 0.11 ± 0.06a 0.12 ± 0.06a 0.19 ± 0.07c n = 37 40 4

*Comparison of body weights of 3- and 6-month-old Norway rats living in the Hahajima Islands, Yururi-*

**Locality Sex 3Months 6Months** Hahajima Islands Male 77.0 112.6

Yururi-Moyururi Male 208.3 278.5

Yokohama Male 153.8 223.2

Kaiho-2 (non-wintered) Male 193.3 281.3

Kaiho-2 (wintered) Male 137.7 220.3

*Pregnant females are excluded. Body weights were calculated from regression lines. See Figure 4.*

*Moyururi, Yokohama, and Kaiho-2 (non-wintered and wintered groups) [7].*

**Wintered\* Non-wintered\* Pregnant females**

**Body weight (g)**

Female 79.5 102.9

Female 156.3 205.1

Female 123.3 174.5

Female 168.9 259.9

Female 114.2 181.8

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 islands in the archipelago.

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 different between the Hahajima rats and the others.

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 their food habit.

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

**Figure 4.**

*Comparison of body weight in grams (*Y*) and log value of age in months (*X*) and resulting regression lines for male and female Norway rats from the Hahajima Islands, Yururi-Moyururi, and a business district in Yokohama excluding pregnant females, showing infection with rat lungworms (*Angiostrongylus cantonensis*) [7]. Circles and triangles show rats that were negative and those that were positive for the infection, respectively.*

low and flat Mukojima Islands [27]. Therefore, Norway rats probably thrive better in the Hahajima Islands than in the others.

Renal structures show that the ability to concentrate urinary water in Norway rats is like that in roof rats [59]. However, protein-rich diets demand a larger turnover of water than diets rich in carbohydrates or fat [48], and Norway rats feed on protein-rich diets, whereas roof rats prefer plant matter to animal matter [6]. Therefore, Norway rats require more water intake than roof rats. This difference in water requirements is probably one of the factors that separate the two species in the geographical distribution especially in tropical and subtropical climate zones [15]. Mild temperature is a secondary factor in determining the Norway rat distribution, after water balance and an appropriate diet. Even in the tropical climate zone, Norway rats are prosperous in large cities such as Bangkok (13° 44′ N, 100° 29′ E) and Chanthaburi (12° 36′ N, 102° 06′ E) in Thailand, which are surrounded by networks of watercourses and damp environments [15, 60]. Generally, in these habitats, there are protein-rich diets including garbage and invertebrates such as earthworms and insects [6]. Therefore, protein-rich diets and the means for avoiding dehydration such as creeks and sewage provide Norway rats with thriving habitats in large cities. These facts suggest that diets rich in animal matter or protein are associated with water balance, which are essential factors in the geographical distribution of Norway rats.

### **5. Conclusion**

Mild temperature is a secondary factor in the reproductive activities of Norway rats as was proved by the results in Yururi-Moyururi in the subarctic zone and

**31**

**Author details**

Rat Control Consulting, Yamato, Japan

provided the original work is properly cited.

\*Address all correspondence to: rccty@js8.so-net.ne.jp

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Tatsuo Yabe

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

Water balance and a protein-rich diet are essential factors in the reproductive activities and distribution of Norway rats as was shown by the results in Kaiho-2 and the Hahajima Islands. The rats on Kaiho-2 in the temperate zone stopped recruiting of new generations and lost body mass by consuming their body fat in the winter because of dehydration. In the Hahajima Islands in the subtropics, the rats fed mainly on plant matter to maintain water balance because of chronic dehydration, and as a result, they lost body mass. In this case, the rats probably avoided consuming animal matter or a protein-rich diet to maintain water balance, but they consumed protein from within their bodies instead. Norway rats usually feed on a protein-rich diet or animal matter, which differs from the food habits of roof rats, which prefer plant matter to animal matter (6). Thus, a protein-rich or

in an urban area in Yokohama in the temperate zone. In Yururi-Moyururi, the rats recruited new generations in their population under snow cover probably by preying on remnants of their own species, which were left by birds of prey such as common buzzards. In Yokohama, the rats showed peaks of recruitment even in the summer and winter, though the season of the peaks changed every year. Even in the tropics, the rats are prosperous in large cities such as Bangkok and Chanthaburi in Thailand, which are surrounded by networks of watercourses and damp environments [15, 60]. It is likely that watercourses supply the rats with an appropriate diet

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

discarded from houses as well as with moist conditions.

animal matter diet is an appropriate diet for Norway rats.

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

in an urban area in Yokohama in the temperate zone. In Yururi-Moyururi, the rats recruited new generations in their population under snow cover probably by preying on remnants of their own species, which were left by birds of prey such as common buzzards. In Yokohama, the rats showed peaks of recruitment even in the summer and winter, though the season of the peaks changed every year. Even in the tropics, the rats are prosperous in large cities such as Bangkok and Chanthaburi in Thailand, which are surrounded by networks of watercourses and damp environments [15, 60]. It is likely that watercourses supply the rats with an appropriate diet discarded from houses as well as with moist conditions.

Water balance and a protein-rich diet are essential factors in the reproductive activities and distribution of Norway rats as was shown by the results in Kaiho-2 and the Hahajima Islands. The rats on Kaiho-2 in the temperate zone stopped recruiting of new generations and lost body mass by consuming their body fat in the winter because of dehydration. In the Hahajima Islands in the subtropics, the rats fed mainly on plant matter to maintain water balance because of chronic dehydration, and as a result, they lost body mass. In this case, the rats probably avoided consuming animal matter or a protein-rich diet to maintain water balance, but they consumed protein from within their bodies instead. Norway rats usually feed on a protein-rich diet or animal matter, which differs from the food habits of roof rats, which prefer plant matter to animal matter (6). Thus, a protein-rich or animal matter diet is an appropriate diet for Norway rats.
