We predicted that female prairie voles (Microtus ochrogaster) would no
t increase in locomotor activity during ''induced'' proestrus. We deve
loped and tested two alternative a priori hypotheses to explain this p
redicted lack of activity. The Non-Response Hypothesis in which voles
cannot, physiologically, increase activity in response to estradiol an
d, the Threshold Effect Hypothesis in which a minimal concentration of
estradiol is necessary to achieve estrus, while higher concentrations
influence other behavioral parameters. The non-response hypothesis pr
edicts that hormone concentration will not affect locomotor activity,
while the threshold effects hypothesis predicts that voles achieve est
rus at low concentrations, while higher concentrations should trigger
increasing locomotor activity. Initial results using running wheels in
dicated that females decreased activity during induced proestrus. Radi
oimmunoassay revealed that induced proestrus was achieved at relativel
y low concentrations of estradiol. Results from a dose response experi
ment allowed for rejection of the nonresponse hypothesis and supported
the threshold effects hypothesis as females decreased activity at low
concentrations of estradiol, showed no relative change at moderate co
ncentrations, and increased activity at higher concentrations.