Mucus is not particularly romantic for most humans. But for elephants, mucus is key to sexual arousal. Males depend on mucus in their trunks to detect a pheromone released by ready females. Now a study shows that a mucus protein may also help turn off males' sex drive by mopping up this pheromone.
When a female elephant is in heat, her urine contains a pheromone called (Z)-7-dodecen-1-yl acetate. A male in search of a willing partner checks for this pheromone by dipping his trunk in puddles of urine or by probing a female's genital area. As the acidic mucus drips into the urine, the urine pH falls, freeing the pheromone from an attached protein. The male then touches his urine-laden trunk to sensory areas in his mouth. If the male detects pheromone, he attempts to mate.
But what cools off the ardor? A report in the 1 October issue of Biochemistry points to a specific mucus protein. A collaborative effort between the labs of medicinal chemist Glenn Prestwich at the University of Utah College of Pharmacy in Salt Lake City and elephant researcher and chemist L. E. L. “Bets” Rasmussen at the Oregon Health and Science University in Portland identified this protein as one of a family of odorant-binding proteins (OBPs).
The team added a radioactive version of the pheromone to puddles of urine from females that were not in heat and gave the male elephants a whiff. Then Rasmussen collected mucus from elephants trained to exhale forcefully on command. (“There is nothing harder to get out of your hair than elephant mucus,” she says.) Back in the lab, Prestwich's group isolated the OBP from the mucus and found that it binds tightly to the pheromone and releases it very slowly. This suggested to them that the OBP is a scavenger that hangs on to the pheromone until the mucus is flushed from the trunk by normal dripping.
“The role of OBPs as scavengers has been hypothesized previously, but it has never before been proven so elegantly,” says chemist Erika Plettner of Simon Fraser University in Burnaby, British Columbia. “It is an important step forward,” agrees protein chemist Robert Beynon at the University of Liverpool, United Kingdom.