The art of blending perfumes has been practised by humans for thousands of years, distilling essential oils and aromatic compounds to produce desirable fragrances. However, it appears that we are not the only species that indulges our olfactory senses: male orchid bees also gather scents from the environment and each species blends their own unique fragrance. ‘The males expose them at the places where mating occurs, so the perfumes may be chemical signals to females’, says Thomas Eltz, from the Ruhr-University Bochum, Germany, adding that the males could use the quality of their perfume blends to convey their superiority. Eltz also explains that many species of orchid bee live in close proximity, forcing species whose territories overlap to develop different blends that can be distinguished from those of their neighbours to prevent interbreeding. But it wasn't clear whether each of these species had also developed a sense of smell that is finely tuned to their own particular perfume – with close relatives barely responding to components from each other's scents – or whether differences in sensitivity had built up gradually over time as the species became more distantly related.
Working with Santiago Ramírez, from the University of California, Davis, USA, Eltz set out to determine how sensitive male orchid bees are to the constituent odours of their own unique perfume blend, but first he had to collect some bees. Eltz and his student, Lukasz Mitko, travelled to Panama, where they collected males from 15 species of Euglossa orchid bees by enticing them to trees with strips of filter paper dipped in attractive scents ranging from vanillin to methyl cinnamate. ‘It is quite a spectacular sight to have dozens of green, blue or red metallic bees appear out of nowhere around a bait’, says Eltz, who explains that Lukasz had to cycle to the forest collection site to collect the bees each day before rushing back to Bill Wcislo's Smithsonian Tropical Research Institute lab in Panama City. Having identified each species, Mitko then measured the response of their antennae to a puff of air containing one of 18 scents that the bees were known to gather for perfume production. ‘Several of the test substances were not commercially available, so we had to isolate tiny quantities from bee extracts using preparative gas chromatography’, says Eltz, adding that Erik Hedenström also synthesized one of the compounds, 6-(4-methylpent-3-enyl)-naphtalene-1,4-dione.
Fortunately, the males responded strongly to a selection of the odours that they incorporated into their own scents, ‘confirming the idea of olfactory tuning to species-specific compounds,’ says Eltz. However, when Ramírez and Marjorie Weber constructed a family tree of the relationships between the 15 species, instead of large differences in the sensitivities of closely related species – as the team had expected – the family tree showed that the most closely related species had the most similar odour sensitivities, while the most distantly related species showed the greatest differences. Instead of evolving dramatically different senses of smell to accompany their individual scents, the bees’ senses of smell had diverged more gradually over time.
However, Eltz points out that despite the divergence in the bees’ tastes over time, some distantly related species – the Panamanian Euglossa mixta and the Mexican Euglossa dilemma – are both sensitive to 2-hydroxy-6-nonadienylbenzaldehyde. He explains, ‘This is a case of convergent sensory adaptation to a shared major perfume ingredient,’ and is keen to track how these odorant receptors have evolved over the course of time through the orchid bee family tree.
