Photoperiod modulates the effects of steroids on sociosexual behaviors of hamsters.

Female hamsters (Mesocricetus auratus) exposed to a short photoperiod for 8 weeks were ovariectomized, as were hamsters that were maintained in a long photoperiod. We then tested the effect of photoperiod on the ability of progesterone (P) to reduce aggression, as well as on the effects of estrogen (E), alone and with P, on the display of lordosis. Before hormone treatments, photoperiod did not affect the display of aggressive behaviors. When treated with steroids, short-day females were more aggressive than long-day housed animals, especially when tested using male intruders. Also, exposure to short days reduced the activational effects of E and E+P upon lordosis behavior. These findings confirm that photoperiod can modulate the behavioral effects of E and E+P in female hamsters. Furthermore, the results indicate that behavioral effects of P which do not depend upon E priming are also influenced by photoperiod.