Mixed Message: Pheromone blend sends signal

The meaning of a chemical message released by male Asian elephants depends on the chemical’s total concentration as well as the balance of the chemical’s two forms, say the authors of a new report.

The pheromone frontalin, a signal well studied in bark beetles, has two versions that are molecular mirror images of each other. Previously, L. Elizabeth L. Rasmussen of the Oregon Health and Science University in Beaverton and David R. Greenwood of Hort Research in Auckland, New Zealand, reported that Asian elephants also use frontalin as a pheromone. Mature males release this chemical during musth, a period when they become aggressive and seek mates.

In their new study, Rasmussen, Greenwood, and their colleagues set out to determine which of the two forms of frontalin elicits a behavioral response. They analyzed more than 100 secretion samples from the temporal glands of six male Asian elephants. These glands are located on the sides of an animal’s face, near its eyes.

The researchers found that frontalin first appears in the gland’s secretions when males are in their teens. Its concentration rises 15-fold over the next 25 years, such that sexually mature, socially dominant males between the ages of 31 and 43 years secrete the largest amounts.

Furthermore, the ratio of the two mirror-image forms of frontalin varies with a male’s age. Young males tend to secrete more of one form, but as they enter their 20s, they produce an increasingly balanced ratio of the two forms.

The length of time that a male remains in musth increases with age, from less than a week in young males to as long as 3 months in mature males. Secretion of the two pheromone forms was most evenly balanced during the middle period of musth in mature males.

The researchers also observed the responses of various elephants to collected secretions. Sniffing the sample, for example, indicates attraction, while circling it suggests repulsion, the scientists say.

The team reports in the Dec. 22 Nature that samples from young males—containing a low concentration of frontalin, mostly in the young-elephant form—aroused mild interest from males, while females largely ignored the pheromone. Mature-male secretions—with high concentrations of frontalin in even ratio—repulsed other males and non-ovulating and pregnant females but attracted ovulating females.

“Up until now, we thought that there was only one signaling compound involved, but here we have a blend,” says Rasmussen.

Glenn Prestwich of the University of Utah in Salt Lake City comments, however, that the experiment didn’t tease apart the effects of concentration and of ratio. He’d like to see a behavioral test that compares samples with the same concentrations of frontalin but different ratios of the two forms.

Aimee Cunningham

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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