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DISCOVERIES
Estrous Cycle of Female Asian Elephants
In 1974, as the population of Asian elephants rapidly declined, Oregon Zoo staff embarked on a comprehensive study of the animals’ reproductive biology to help in developing a sustainable population in North America. The first step to improving breeding at zoos was to understand the estrous cycle of female elephants. Though the duration of the estrous cycle for female Asian elephants had already been discovered, the frequency of cycles was still unknown. Zoo staff knew this information would prove vital in establishing a successful conservation program.
Initially, researchers decided they would use the male elephant’s proven ability to detect estrous in females to determine the frequency of cycles. A male was given daily opportunities to examine each female. If a female was in estrous, the male became very excited and interested in her, whereas he showed little or no interest when she was not. More than 12,000 bull-cow interactions were observed in this study. Though the information obtained gave researchers an idea of the length of estrous cycles, it did not provide a clear picture. Therefore, in 1977, the zoo began drawing weekly blood samples from females to study hormonal patterns. Within six months, a pattern emerged suggesting that the cycle had a between-estrous interval of about 16 weeks. Comparing this new data with the data from male-female interactions, researchers noted the same 16-week frequency.
From this research, Oregon Zoo elephant experts successfully defined periods of varying hormones during the estrous cycles of female Asian elephants. They found an 8- to 13-week period of high progesterone and a 3- to 14-week period of low progesterone. They also discovered that male elephants displayed much interest in the females during periods of low progesterone, and little interest during periods of high progesterone.
Following this discovery, Oregon Zoo staff took their research a step further and set out to determine precisely when ovulation occurred. Using hormonal and behavioral data, they predicted when ovulation would occur in two females. A male was allowed to breed with the females on each day of the predicted ovulation period until he became disinterested. In both cases, the female became pregnant, confirming that ovulation had occurred within a week of the predicted time. These discoveries provided zoos with the tools to better coordinate breeding.
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