Demographic Studies of Sympatric Sigmodon hispidus and Reithrodontomys fulvescens (Rodentia)

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Abstract
S. hispidus (hispid cotton rat) and R. fulvescens (fulvous harvest mouse) are codominant rodent species of the southeastern Texas [USA] coastal prairie. Six isolated 1.6 ha plots were used to study the demographic properties of these sympatric rodents. Each plot was live-trapped 3-4 days/mo. for 19 mo. For 2 control plots all captured individuals were recorded as to species, sex, body weight, time and site of capture, and were marked and released. For 2 other plots all individuals were recorded, but Reithrodontomys was removed. In 2 others Sigmodon was removed. Sigmodon and Reithrodontomys reflected asynchronous breeding patterns in the control replicates. Sigmodon populations of higher densities and biomasses were maintained in the presence of Reithrodontomys. In the replicates where Reithrodontomys was removed, Sigmodon extended its breeding period. Reithrodontomys data did not yield strong directional differences with changes in species composition, but like Sigmodon, did show some evidence of higher densities and biomass when its codominant sympatric member was present. Possible explanations for this coexistence include differences in body size between species, differences in microhabitat utilization, and differences in temporal relfections of life-history patterns.