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ABSTRACT.-The hispid cotton rat (Sigmodon hispidus) has occurred in Nebraska for about 50 years and entered the state from the south via northward movements through Kansas. However, little is known about its status or distribution in the state in recent decades. We conducted surveys for S. hispidus in southern Nebraska at historical sites and sites without previous records to determine its current status. From 2004 to 2008, we documented cotton rats at 9 of 12 historical sites in south-central and extreme southeastern parts of the state, and we captured this species at 13 new sites in southwestern and south-central Nebraska. In the past 3 decades, the distribution of S. hispidus has expanded westward but not northward in the state. An explanation for such a shift is unclear. At this northern edge of its geographical range, pregnant females were captured from early April to early October, and nonreproductive females were captured from November to January. Males with enlarged testes occurred from late March to September. Aspects of its natural history in Nebraska are similar to those reported in Kansas.
Key words: Sigmodon hispidus, hispid cotton rat, Nebraska, distribution, expansion, reproduction, winter activity, seasonal activity.
The hispid cotton rat (Sigmodon hispidus) occurs from northern South America to southern Nebraska and from Virginia to California (Hall 1981, Cameron 1999). This species has occurred in the central Great Plains for only about a century. It was first recorded in Kansas in 1892 after moving northward from Oklahoma (Bailey 1902). The cotton rat continued to expand northward through eastern Kansas at 7 miles per year and approached the southern border of Nebraska by 1947 (Cockrum 1948). In 1958, S. hispidus was first reported in southeastern Nebraska (Richardson County; Jones 1960), with subsequent captures in south-central Nebraska in 1965 and 1966 (Adams County; Choate and Genoways 1967, Genoways and Schlitter 1967). From 1948 to 1965, cotton rats expanded at an estimated 5.5 miles per year in northern Kansas and southern Nebraska (Genoways and Schlitter 1967).
By 1975, hispid cotton rats were known from 6 counties in Nebraska-1 in the extreme southeastern corner and 5 in south-central parts of the state (Farney 1975). The northernmost locality was from Kearney County in 1966 and the westernmost was from Harlan County in 1973...