Funny smell may have split bee species

A group of bees may have evolved into a new species because the guys couldn’t pick the right aftershave.

Flashy metallic green bees currently called Euglossa viridissima and found in Central America should be reclassified as two species, reports Thomas Eltz of the University of Düsseldorf in Germany. DNA analysis shows little, if any, interbreeding between the two forms, he and colleagues say online December 4 in Current Biology.

Tests of what the male bees can smell, and what those males smell like, suggest an unusual scenario for how the bees split, Eltz says.

Males of the two forms look similar, although one has two little toothlike jags on its lower jaw and the other has three. In smell, the difference is more dramatic.

These are orchid bees, one of some 200 species named for the males’ habit of scraping fragrant compounds off orchids or other materials to blend a perfume inside oversized leg pockets. Blends differ among species. Despite the bee group’s pretty name, “all sorts of smelly objects can be sources of scents — rotting wood, tree wounds, feces,” Eltz says.

Males carry around their mix of scents in their hind leg pockets. During the two months or so of trying to attract a mate, males periodically transfer the mixture to little tufts of hair at the base of their wings and fan the perfume into the air.  

When Eltz and his colleagues compared the perfumes of the two forms of E. viridissima males, the main ingredient of the three-toothed bees’ scented mix came from a relatively large, benzaldehyde-based compound referred to as HNDB. Where the bees collect the compound isn’t clear, but Eltz says fungus-riddled wood is one possibility. Yet this chemical didn’t show up in the otherwise similar scents of the two-toothed males.

Those two-toothers probably didn’t collect HNDB because they aren’t able to smell it very well, Eltz says. He and his colleagues used a lab setup to waft both natural and synthetic HNDB across antennae removed from male bees. Nerves in antennae from two-toothed bees didn’t register much of a response but three-toothed bee antennae responded strongly.

Human noses can smell the perfume blend. “You don’t get a kick out of it, but it’s not unpleasant,” he says. And he says in some cases he can sniff the difference between a version that doesn’t have HNDB and one that does.

If female bees care about the perfumes at the mating sites, a changed odor might not interest females who have traditional tastes, Eltz says. Also, a mutation that changed male sensitivity to an odor might affect some of the females too. Eltz isn’t ready to say whether the HNDB-rich scent is the original or the new version. But either way, a new perfume might have led to a new species.

Linking the difference in perfumes to the difference in perception “is the impressive part of the paper,” says Mark Elgar of the University of Melbourne in Australia. “Thus far, they have a potential mechanism for species divergence.”

What the researchers need to test the idea is information on whether the females of the proposed new species actually do differentiate between males based on the perfumes, Elgar says. “The technical difficulties of conducting such an experiment must be very frustrating for this team.”

They are, Eltz agrees. A female mates only once during her two-month adult life, and otherwise doesn’t seem much interested in males, scented or un. Field tests have proved so difficult that Eltz is now trying to set up a laboratory colony of the bees. “They aren’t really laboratory animals,” he says, but he notes he’s making progress.