To grow a calf weighing 200-plus pounds at birth, the average female elephant undergoes a pregnancy of some 640 days. While sheer size accounts in part for such length of time, this gestation period is still phenomenal. (Blue whales, for example, get the job done in a year.) Veterinarian Imke Lueders of the Leibniz Institute for Zoo and Wildlife Research in Berlin, with six colleagues, used ultrasounds of elephant ovaries and uteri to get a behind-the-scenes look at elephant pregnancy.
Of fifteen Asian and two African zoo elephants that were studied, five Asian elephants were artificially inseminated; all the rest conceived naturally. The researchers focused on a temporary endocrine gland that forms in mammalian ovaries following ovulation, the corpus luteum. After an egg’s release, a corpus luteum (CL) develops from the follicle and pumps out the hormone progesterone, preparing a thick uterine wall in which a fertilized egg can lodge. In most placental mammal species, the number of CLs matches the number of eggs released and, after implantation, those glands shut down as the placenta takes over the job of hormone production.
Elephants beg to differ. It was already known that during pregnancy their ovaries contain up to thirty CLs—not just one, as might be assumed in an animal that produces one offspring at a time. In each of the elephants studied, the researchers confirmed, there were two to twelve such glands. The team also showed that the CLs remain active during the entire pregnancy and that, therefore, elephants do not need progesterone produced by the placenta. As expected, the transient glands begin their activity following ovulation. Surprisingly, however, they then shrink and back off hormonal secretion until the mother’s body detects that embryo implantation has occurred—a full forty to forty-five days after ovulation, the researchers estimate.
Elephant mothers make a huge investment in their offspring, dedicating four to six years to nurturing their young after the long pregnancy. Seemingly, the mother
tests the developing embryo’s viability—signaled by implantation—before the hormone factories are reset and the marathon gestation is carried on. (Proceedings of the Royal Society B)