Different Nest Site Selection of Two Sympatric Arboreal Rodent Species, Siberian Flying Squirrel and Small Japanese Field Mouse, in Hokkaido, Japan

Abstract

Numerous vertebrates use tree cavities as nest resources. Mammals such as Carnivora (Zalewski 1997; Wilson and Nielsen 2007), rodents (Taulman 1999; Shibata et al. 2004; Holloway and Malcolm 2007), bats (Sedgeley and O’Donnell 1999; Boonman 2000; Willis and Brigham 2007), and marsupials (Lindenmayer et al. 1991; Smith et al. 2007; Crane et al. 2010), as well as birds (Aitken et al. 2002; Martin et al. 2004; Adamik and Kral 2008), use tree cavities for daily rest, reproduction, and/or overwintering. Sympatric cavity-users often partition their nest cavities to avoid interspecific competition (van Balen et al. 1982; Martin et al. 2004; Shafique et al. 2009). In Hokkaido, northern Japan, there are two cavitynesting rodents, the Siberian flying squirrel (Pteromys volans) (Nakama and Yanagawa 2009; Suzuki et al. 2011) and the small Japanese field mouse (Apodemus argenteus) (Nakata et al. 2009). There may be competition between these two rodents for tree cavities, but their favored nest sites have not yet been compared. Do they select tree cavities of different types, and, if so, what characteristics are their selections based on? These rodents have different physical and ecological characteristics. Pteromys volans (weight, 110 to 142 g; head and body length, 130 to 167 mm; Hanski et al. 2000; Asari et al. 2007; Oshida 2009) is 5 to 14 times heavier than A. argenteus (weight, 10 to 20 g; head and body length, 65 to 100 mm; Nakata et al. 2009). In addition, P. volans, which is an aerial and arboreal user, spends most of its time in the canopy and almost never walks on the ground, even when crossing wide fields (Selonen and Hanski 2003, 2004). Apodemus argenteus also uses arboreal space (Imaizumi 1978; Abe et al. 1989; Sekijima 2004), and it is able to climb to a height of 18 m (Ida et al. 2004). However, it more frequently uses foods on and under the ground than above the ground (Abe 1986). These differences in size and habit between the two species may result in different choices of tree cavities. Larger species of cavity-nesting mammals and birds tend to nest in larger cavities with larger entrances (Martin et al. 2004). Larger flying squirrels use larger trees with larger entrances than do smaller flying squirrel species (Shafique et al. 2009). Larger bird species also nest in larger entrance cavities than do smaller species (van Balen et al. 1982). Thus, we hypothesized that P. volans, with a larger body than that of A. argenteus, would nest in trees with greater diameters at breast height and in larger cavities with larger entrances. In addition, we hypothesized that P. volans, which is usually active in the upper layer of trees, would nest in higher cavities than A. argenteus, which commonly lives on and under the ground. We examined these hypotheses by surveys of tree cavities and nest boxes.