Concentrations of progesterone, testosterone and estradiol‐17β in the serum during the estrous cycle of Asian elephants (Elephas maximus)

The levels of progesterone, testosterone and estradiol‐17β in serum samples from two female Asian elephants were measured for the period of 32 months from February 1987 to September 1989. Serum samples were collected weekly from unanesthetized elephants. Each elephant showed eight ovarian cycles in 32 months. Ovarian cycles, characterized by changes in concentrations of serum progesterone, averaged 16.8 ± 0.6 (mean ± SEM. n = 14) weeks in length. The changes in concentrations of testosterone in the serum showed a similar pattern to those of progesterone with a striking increase noted during the luteal phase. The highest levels of serum estradiol‐17β were noted when progesterone levels showed low basal values. These results suggest that estradiol‐17β may be an index of follicular maturation during the estrous cycle in Asian elephants, and that the ovaries of Asian elephants may produce testosterone in the luteal phase.