Pheromone in Urine Spurs Mating in Elephants

Sharon Guynup
for National Geographic Today
October 4, 2002
How do female Asian elephants signal roaming males that it's mating season?

They leave "chemical love letters" in the form of a female pheromone, says L.E.L. "Bets" Rasmussen, a biochemist at the Oregon Health and Science University, in Portland, Oregon.

Rasmussen developed the research with Glenn D. Prestwich, a chemist at the University of Utah College of Pharmacy, in Salt Lake City. In 1996, Rasmussen identified the pheromone in female elephant urine. Now, Prestwich and Rasmussen have identified a related "odorant binding protein" in the male that helps stimulate him and then alerts him that the amorous encounter is finished.

The sex-chemical on-off switches "will help facilitate elephant breeding both in captivity and in the wild," Prestwich says. The study appeared in the October 1 issue of the journal Biochemistry.

For elephants, the ovulation window is "probably about a day," according to Rasmussen, so mating must be tightly choreographed for breeding success.

The chemicals cue the ritual. Four to six weeks before ovulation, a minuscule amount of the pheromone appears in the female elephant's urine. As she goes into heat, the pheromone reaches high concentrations—though not high enough for the male to sense at a distance.

Chemical Cues

"Males don't wander around sniffing the air with their trunk," Prestwich says.

Rather, a male tracks the female's daily cycle by "sampling" their urine, says Heidi Riddle, an elephant manager at Riddle's Elephant and Wildlife Sanctuary in Greenbrier, Arkansas, where the scientists conducted part of their research.

"A male touches the tip of his trunk to the female's privates, and she responds by urinating a little, and he sniffs (the urine) with his trunk," Riddle explains.

Then the male exhibits so-called "flehmen" behavior—touching the tip of his trunk to the roof of his mouth near the dual openings of a foot-long "vomeronasal organ," (VNO) the primary pheromone detector. This helps absorb other odors so that more of the female pheromone reaches the VNO to excite the male.

Over the millennia, humans and most primates have lost the VNO, but it's an important part of the olfactory system in many animals, from reptiles to horses, and almost always connected to sexual arousal, Prestwich says.

The mucus inside the male elephant's trunk helps deliver a concentrated whiff of the seductive scent. He double-checks the urine test, and if he senses a green light, mating begins.

Elephants and Moths

The researchers also discovered that the "odorant binding protein" in the mucus of the trunk puts the brakes on love. When mating is completed, the protein mops up leftover pheromones so the male is no longer stimulated

Heretofore scientists thought that the protein only helped deliver the message that the female was ready, "but it's not FedEx, it's the janitor," Prestwich says.

"Although the elephant has the largest nose and most incredible olfactory system of any animal, its chemical senses had never been studied before," Rasmussen says.

Rasmussen and Prestwich teamed up six years ago after meeting at a conference in Chile and, remarkably enough, realizing they were studying the same pheromone in two altogether different creatures: the elephant and the moth.

"The substance responsible for mating behavior and attraction was essentially the same (in both creatures) ... which strains the credibility of evolutionary biology," Prestwich says.

Collecting the Evidence

Prestwich worked only in the lab, but Rasmussen went into the field. For a month at a time she observed elephants from a jeep in Nagarhole National Park in southern India, running out to collect urine samples, and testing them on male elephants in Burmese logging camps.

She also reached inside the gaping mouths of captive elements to collect mucus—and help read the secrets there.

Knowledge of the elephant's love letters aids conservation. "Increasing birth rates in captive elephants while we still have them as ambassadors to help their wild counterparts is important," says Dennis Schmitt, a reproductive specialist at Dickerson Park Zoo, in Springfield, Missouri.

Mike Keele, assistant director of the Oregon Zoo, in Portland, Oregon, who wrote the master plan for the elephant "Species Survival Program" for all zoos, points out another application of the research. Wild elephant populations are declining worldwide, from about 1.4 million in 1971 to approximately 350,000 today—partly from human-elephant conflict.

Since bulls are drawn to females in heat, the pheromone could possibly be used to steer rogue elephants away from crops and villages—and destruction.

"The more we know about elephant reproduction, the better chance we have to save them," Riddle says.

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