The association between basal body temperature, sexual swelling and urinary gonadal hormone levels in the menstrual cycle of the chimpanzee.

Basal body temperature (BBT) was measured continuously by radiotelemetry throughout 14 chimpanzee menstrual cycles and correlated with daily observations of the sexual skin swelling. A biphasic BBT shift from a pre-nadir mean of 36-12 degrees C to a post-nadir mean of 36-67 degrees C was observed in 12 cycles. The temperature nadir showed a close temporal relationship with detumescence of the sexual skin swelling (an early luteal event), but the rate of temperature rise after the nadir was variable. In 3 normal cycles studied, the temperature nadir occurred the day after a urinary oestrone peak, but there was no consistent temporal association between BBT rise and pregnanediol increment. Progesterone secretion is therefore probably not the sole determinant of the BBT shift; the changing oestrogen/progestin ratio may be the more important factor regulating body temperature during the luteal phase.