for National Geographic Today
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.
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