Object Investigation by the oestrous rat and guinea-pig: the oestrous cycle and the effects of oestrogen and progesterone.

The time spent investigation small objects in an arena was found to vary systematically with the oestrous cycle of rats and guinea-pigs. Both the total time spent and the mean time per object were greater for oestrous animals, which show more long bouts of investigation per test than do dioestrous animals. The changes in investigation time were replicated by administering oestradiol benzoate followed after 40 h by progesterone, to ovariectomized rats, suggesting an endocrine basis. Most bouts of investigation were terminated by a move straight to another object, suggesting that it had been chosen before the move was made.