Nest Site Use by the Small Japanese Field Mouse: Possibility of Nest Height Change in the Presence of the Siberian Flying Squirrel

27 Mar Nest Site Use by the Small Japanese Field Mouse: Possibility of Nest Height Change in the Presence of the Siberian Flying Squirrel

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Abstract

同所的に生息し，類似した生態的特徴を持つ種同士は，他種との資源競争を避けるために，その資源に対する選択性を変化させることがある．本研究では，資源競争回避のメカニズムを解明するための基礎的情報を収集するために，樹洞を繁殖場所とするヒメネズミApodemus argenteusを対象として，タイリクモモンガPteromys volansとの営巣資源をめぐる競争回避のための選択性の変化を調べた．本研究では，タイリクモモンガの影響を受けない条件下と受ける条件下においてヒメネズミの営巣高を比較した．ヒメネズミは，タイリクモモンガの入れない入口径が小さい巣箱では低い位置から高い位置まで様々な高さに営巣した．それに対し，タイリクモモンガが入ることができる入口径が大きい巣箱では，一貫して低い位置に設置された巣箱に営巣した．一般的に低い樹洞は，クロテンMartes zibelli やアカギツネVulpes vulpesなど地上からくる捕食者に捕食される危険性が高い．加えて，入口径が大きい樹洞はより多くの種類の捕食者の侵入が可能であるため，捕食圧が高くなると考えられ，入口径が大きい巣箱で，ヒメネズミが好んで低い位置に営巣しているとは考え難い．本研究の結果は，ヒメネズミがタイリクモモンガとの営巣資源競争を避けるために，営巣場所の高さを変化させる可能性があることを示すものである．

English Abstract

Sympatric species with similar ecological characteristics can change their selection of resources to avoid competition. To gain knowledge to clarify the mechanisms of this avoidance, we surveyed nest height change of the small Japanese field mouse Apodemus argenteus to avoid competition with the Siberian flying squirrel Pteromys volans, which, like the mice, nests in tree cavities. We used two types of nest boxes: one with a small-sized entrance that only the mouse can use, and another with a large-sized entrance usable by both rodent species. The heights of the two types of nest boxes used by mice were compared. Mice nested in small-entrance boxes at various heights. Among the large entrance boxes, however, they only nested in those placed at relatively low heights. Generally, predation risk to cavity nesters increases with decreasing nest height, and larger entrances allow raids by potential predators. Thus, it is not likely that the mice prefer to nest in large entrance boxes installed at lower heights where predation risks are expected to be higher. These results suggest that small Japanese field mice change their nest height to avoid nesting resource competition with Siberian flying squirrels.