Variation in resource abundance affects population dynamics by alterin
g demographic processes and interactions among individuals in the popu
lation. For small mammals, food is likely to be a critical resource. P
opulation densities should vary directly with food abundance, but the
underlying demographic changes are more difficult to predict. We exper
imentally increased food available to populations of hispid cotton rat
s, Sigmodon hispidus, to examine how rates of recruitment and disappea
rance varied with food abundance. We expected supplemental food to inc
rease winter survival and to increase recruitment, perhaps advancing t
he date of first reproduction in the spring. We also thought larger be
haviorally dominant animals might dominate the point sources of supple
mental food, altering age or size ratios and producing an excess of tr
ansient animals. Supplemental food increased population densities but
did not dampen pronounced seasonal fluctuations. Supplemented populati
ons contained proportionately more juveniles and small adults than did
controls; social behavior may have limited increases in numbers of la
rge adults. Survival rates did not change; density increases were due
to increased reproduction and immigration. In supplemented populations
, reproductive effort by females increased, but the proportion of repr
oductive females decreased. Reproductive females and nonreproductive a
nimals of both sexes were less likely to be transients than were repro
ductive males. Transients constituted higher proportions of control po
pulations because of successful settlement, primarily by juveniles and
small adults, into supplemented areas. The strongly seasonal climatic
conditions under which northern S. hispidus populations exist produce
contrasting selective pressures which might favor season-specific for
aging strategies. Our data support a scenario of territorial females a
nd wandering males in the reproductive season, with females maximizing
resource acquisition for production of offspring. During winter, both
sexes may restrict foraging time (and exposure to predators and weath
er) to the minimum required for survival.