THE RICE RAT ORYZOMYS-PALUSTRIS IN A DELAWARE SALT-MARSH - ANNUAL REPRODUCTIVE-CYCLE

A population of the rice rat Oryzomys palustris inhabiting the Canary Creek salt marsh in Lewes, Delaware, was sampled on a monthly basis from September 1988 to December 1990. Males were assigned to the following age-classes on the basis of body mass: juveniles, 0-30 g; subadults, 31-50 g; adults, 51 g or greater. Testicular mass exhibited a seasonal pattern with lowest adult values in late autumn and early winter and peak values in late spring and early summer. Females showed similar patterns in uterine mass and vaginal patency. Pregnant and (or) lactating females were found from March to late September. The data suggest that the breeding season of the rice rat in southern Delaware begins in late winter and extends into late summer. Favorable environmental conditions may extend the breeding season for some individuals into autumn.