Nothing about elephants is small, and their pregnancies are no exception. Before giving birth to a 110-kilogram calf, mothers carry the fetus for 22 months, the longest gestation period of any mammal. And whereas most mammals have only one corpus luteum—a temporary gland that controls hormone levels during pregnancy—elephants have as many as 11. Now, by giving 17 elephants blood tests and ultrasound scans throughout their pregnancies, researchers have discovered a key to this remarkable form of motherhood.
"The study is exciting, surprising, and very pleasing," says veterinary researcher Twink Allen of the University of Cambridge in the United Kingdom, who was not involved in the work. "It's a very unusual strategy the elephant seems to have adopted, and it has puzzled people for 30 or 40 years."
In most mammals, one corpus luteum forms from a single egg follicle in the ovary during each menstrual cycle. The temporary gland produces progesterone, which in turn promotes thickening of the endometrium and, if an egg is fertilized, maintains the correct balance of hormones throughout a pregnancy to ensure that a female's body remains geared toward supporting her growing baby. If fertilization doesn't occur, the corpus luteum dies, only to reform during the next reproductive cycle. From dissected animals, scientists have known for more than 50 years that elephant ovaries contain multiple corpora lutea. But they didn't know how these structures formed or what roles they played in elephant pregnancies. And they'd never studied the corpus lutea in real-time during an elephant's life or pregnancy.
"There were all sorts of theories put forth," says Imke Lueders of the Leibniz Institute for Zoo and Wildlife Research in Berlin. "Some people thought that elephants accumulated the corpora lutea over many cycles, for example. Nobody could really prove any of the theories because elephants are so hard to study."
Hoping to change this, Lueders developed a method to follow elephant pregnancies using protocols originally designed for horses. She collaborated with researchers and zoos around the world to study 15 female Asian elephants and two female African elephants over a 5-year period. During normal menstrual cycles before pregnancies, as well as throughout the entire course of each female's pregnancy, the scientists took blood samples and performed ultrasounds. They used rectally inserted ultrasound probes on the elephants, which had to be specially trained not to kick and rear up during the uncomfortable exams.
The researchers found that the animals formed, on average, five corpora lutea during each menstrual cycle. And surprisingly, whereas one corpus luteum was derived from an egg-generating follicle, as happens in mammals such as humans, the rest of the structures formed from separate follicles at a different point in the reproductive cycle.
Over the course of each pregnancy, each gland slowly decreased its progesterone production. Having many glands likely helps keep the levels above a threshold for the entire 22 month gestation , the team reports online today in the Proceedings of the Royal Society B. The scientists hypothesize that the long gestation period allows for full brain development of elephants, which are born with complex cognitive skills and are immediately able to sense how to survive in their environment and interact with the herd.
"We still don't understand everything," Lueders says. "We'd like to look in more detail at the molecular aspects of this next. What genes are expressed in the ovaries that cause this?" She also wants to study whether the finding holds true in manatees, a close relative of elephants.
The observation explains a curiosity about elephant reproduction, says Allen. But it also could lead to methods for controlling elephant ovulation or timing artificial insemination. Such methods could come in handy when zoos are trying to breed animals—especially those that rarely mate and become pregnant on their own. But they also could help researchers develop elephant birth control for areas plagued by elephant overpopulation, he says.