Ultrasonography of the estrous cycle in female African elephants (Loxodonta africana)

Abstract

The endocrinology of the elephant estrous cycle has been well characterized, but little emphasis has been placed on evaluating corresponding changes in the reproductive tract. Ultrasound was used to document changes in reproductive tract morphology throughout the estrous cycle in four cycling female African elephants. During a 7-month period, frequent ultrasound examinations (n = 190) during the luteal and non-luteal phase were compared with serum progesterone and luteinizing hormone (LH) concentrations during a 7-month period. Ultrasonographic images documented vaginal and cervical edema and changes in mucus consistency during the non-luteal phase. The cross-sectional diameter of the endometrium showed a dramatic increase during the non-luteal phase and followed cyclic changes. A different pattern of follicular development on the ovary was associated with the two LH surges. Follicle growth associated with the first, anovulatory LH surge was characterized by the formation of multiple small follicles, in contrast to the maturation of a single large follicle at the second, ovulatory LH (ovLH) surge. Ovulation and the subsequent formation of a corpus luteum (CL) were observed only after the ovLH surge. Ultrasound data in combination with endocrine assessments suggest that the African elephant is non-ovulatory, although multiple non-ovulatory luteal structures developed during the late nonluteal phase of each cycle. Both ovulatory CL and non-ovulatory luteal structures were present only through one cycle and regressed at the end of the luteal phase in conjunction with the drop in serum progesterone. We conclude that