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.
Highlights
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
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]
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 in an urban area in Yokohama in the temperate zone
Summary
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. 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 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]. It seems that the appropriate diet changes depending on the habitat, and protein-rich diets do not always help Norway rats to thrive. 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 limit their geographical distributions
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