Winter aggregations, Dehnel Effect, and habitat relations in the Suisun shrew Sorex ornatus sinuosus

A live-trapping study on Suisun shrews Sorex ornatus sinuosus Grinnell, 1913, an endangered subspecies, was performed during the non-breeding season along the edge of a tidal marsh in California. During the winter, these shrews lived in distinct social aggregations composed of a single adult male, several adult females, and subadults. These groups remained stable even when the adult male died. As the breeding season approached, these groups were invaded by numerous adult males resulting in a nearly complete change in group membership. At the end of the breeding season, adult body mass declined by 30 to 40% (Dehnel Effect). Shrews reached high densities along the marsh/grassland ecotone, but the precise locations of social groups seemed unrelated to the presence of particular plant species or to the amphipod food supply. Subadult males wintered mostly outside of social groups in the marsh below high tide level, Conservation efforts need to focus on preserving the tidal marsh ecotone without promoting contact with the upland subspecies.