Nesting Characteristics of Harvest Mice (<i>Micromys minutus</i>) in Three Types of Japanese Grasslands with Different Inundation Frequencies

The harvest mouse (Micromys minutus) is a small rodent that inhabits grasslands. These mice weave herbaceous leaves and build spherical nests on plant stems from spring to autumn (Shiraishi 1969; Trout 1977). Nests are used for breeding and resting. Although the original habitats of this species are humid areas with longlasting grasslands located near rivers, lakes, and ponds (Dieterlen 1990), they are found not only in these natural wet grasslands but also in dry areas affected by artificial influences, such as cultivated lands, plantations, and even road verges (Harris 1979; Dickman 1986; Ozkan et al. 2003). In Japan, nests are most frequently found in flood plains (Kayanezumi Network Japan 2006). They are also found in other environments, which mostly include semi-natural grasslands such as fallow fields and embankments (Hata 2005). The difference in vegetation and inundation frequency between habitats might be responsible for the differences in nest heights and the plant species used for nesting materials; this is because harvest mice weave nests with living leaves that have not been separated from the plants. Therefore, in this study, three types of habitats with different inundation frequencies were included to determine the influence of vegetation type and degree of inundation on the nesting of harvest mice, by studying their nesting tendencies during the reproductive seasons from 1999 to 2005.