Cycle of lordosis behavior in female hamsters whose circadian activity rhythm has split into two components

Within 75 days of exposure to constant light (LL), the circadian activity rhythm of 21 of 32 hamsters had dissociated or 'split' into two distinct components. These components were labeled either evening (E) or morning (M) depending on whether they appeared to be derived from the E or M portion of the activity phase prior to the occurrence of 'splitting'. Both regular (i.e., 4-day) and irregular (i.e., non-4-day) cycles in lordosis behavior were observed in all of the animals with an intact activity rhythm and in 18 of the 21 animals with a split rhythm of activity. The onset of behavioral estrus always occurred near the time of the locomotor activity in animals with an intact activity rhythm. Among the females with a split activity rhythm, seven showed lordosis onsets only near the onset of the E component whereas five others began lordosis behavior only near the onset of the M component. Importantly, in five animals the onset of lordosis was associated on different days with either the E or M component. These results indicate that cycles in lordosis behavior persist even after the circadian rhythm of activity has dissociated into two components, and that at least one component of the estrous cycle, onset of lordosis behavior, can be coupled to either of two oscillators that underlie the circadian rhythm of locomotor activity in hamsters.