Sex behavior was quantitated in 13 groups of 2-5 ovariectomized bitches receiving various estrogen and progesterone treatments. In addition, one of the scoring systems used to quantitate sex behavior was applied to the changes observed during proestrus and estrus in 7 intact cycling bitches for which daily serum progesterone and LH levels were assayed. In 6 of the 7 bitches, the onset of full sexual receptivity characteristic of estrus occurred 0-2 (1.2 ± 0.5) days after the LH peak and only after a detectable increase in serum progesterone levels.
Ovariectomized bitches administered estradiol-17β s.c. in 4.7 mm (o.d.) Silastic capsules, at a dose of 0.6 cm/kg for 30 days, became increasingly attractive to males and in some instances showed the passive behavior characteristic of late proestrus. However, none showed the positive reflexes associated with the full sexual receptivity characteristic of estrus. When bitches were administered progesterone implants (1.8 cm/kg) after 8 days of estradiol treatment they displayed within the next 1-5 days a full estrous behavior which lasted for about 1 week. When the estradiol dose was reduced from 0.6 to 0.1 cm/kg at the time of progesterone treatment, each of the bitches showed full sexual receptivity within the next 18 h and this behavior was maintained for 5 days.
Other groups of ovariectomized bitches were injected initially with an estradiol benzoate (EB) dose of 0.4 µg/kg and every 8 h thereafter with steadily increasing doses in increments of 0.2µ g/kg for 3, 6 or 9 days. After 3 days of EB treatment, proestrous but not estrous behavior was observed regardless of whether progesterone was administered after the EB treatment. EB treatment for 6 days caused some bitches to show proestrous behavior prior to termination of treatment and to display estrous behavior following the last EB injection. When 6 days of EB were followed in 8-32 h by progesterone implants (1.6 cm/kg), estrous behavior was observed within 8 h and lasted for 8 days. Estrus occurred earlier and lasted longer in these bitches than in those receiving no additional treatment or small estradiol implants instead of progesterone. Of 9 bitches receiving EB injections for 9 days, 3 showed estrous behavior 1-2 days prior to the last injection while the other 6 displayed estrus only after the last EB injection was given. In each bitch, including those displaying estrous behavior prior to the termination of EB treatment, serum estradiol levels immediately prior to the onset of estrus were lower than those present 12 h earlier. In bitches administered progesterone implants (3.2 cm/kg) 6 h after 9 days of EB treatment, the mean duration of estrous behavior was 9 days vs the 5 days in bitches not subsequently administered progesterone. When progesterone was administered 3 days prior to the end of EB treatment, 1 of 3 bitches showed estrus 8 h following progesterone while the others did not display estrus until after the last EB injection.
These results are considered consistent with the hypothesis that behavioral estrus in the bitch is potentiated by exposure to elevated levels of estrogen and subsequently initiated by a decrease in the estrogen:progesterone ratio effected by either a fall in estrogen and/or a rise in progesterone.
