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

Abstract
Within 75 days of exposure to constant light (LL), the circadian activity rhythm of 21 of 32 hamsters had dissociated or split into 2 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 onset of locomotor activity in animals with an intact activity rhythm. Among the females with a split activity rhythm, 7 showed lordosis onsets only near the onset of the E component whereas 5 others began lordosis behavior only near the onset of the M component. In 5 animals the onset of lordosis was associated on different days with either the E or M component. Apparently, cycles in lordosis behavior persist even after the circadian rhythm of activity has dissociated into 2 components, and at least one component of the estrous cycle, onset of lordosis behavior, can be coupled to either of 2 oscillators that underlie the circadian rhythm of locomotor activity in hamsters.

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