The relationship between levels of exogenous hormones and the display of lordosis by the female rat

In three experiments, the duration, intensity and probability of display of lordosis by ovariectomized female rats were measured in response to administration of progesterone and various levels of estradiol benzoate (EB). Behaviors of females in natural heat were compared with those receiving exogenous hormones. Lordosis duration was shown to be directly proportional to the amount of EB administered, although mean duration of hormone-treated females did not reach that shown by females in natural heat. Both Lordosis Quotient and intensity of lordosis reached asymptote with relatively low doses of EB when animals were treated with progesterone. Ovariectomy reduced the duration and intensity of lordosis even when amounts of EB and progesterone capable of restoring Lordosis Quotients were administered. Finally, the well-known synergistic relationship between estrogen and progesterone in bringing about lordosis in ovariectomized females was found to influence duration and intensity of lordosis as well as the probability of its display. Auxiliary heat behaviors (earwiggling, hopping, and display of "spontaneous" lordosis) were found to be related to level of replacement-EB as well as to the presence of progesterone. These results suggest that hormonally induced heat can be differentiated from natural heat on the basis of duration of lordosis.