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Fragrance biology of orchid bees

DFG Projects EL 249/2 and EL 249/3

Neotropical orchid bees or euglossine bees (Apidae: Euglossini) are characterized by their very long tongues and by a unique behavior of males. Male orchid bees collect scents (fragrances) from all sorts of fragrant objects, including flowers, tree wounds, decaying wood, and feces, and store them in hind leg pockets. That behavior is exploited by a great variety of neotropical orchids that attract euglossines as their exclusive pollinators. The males can also be baited with artificial fragrance compounds (cineole, methyl salycilate, vanillin etc.), and this technique has been used by a number of studies in population ecology. It is however unresolved why the males collect fragrances. Together with Mark Whitten (Florida Museum of Natural History) and David Roubik (Smithsonian Tropical Research Institute) I found that individual males continuously forage for scents over much of their long lives and finally accumulate large quantities of complex blends (Eltz et al. 1999). As fragrances are hard to come by we hypothesized that they serve as indicators of male quality (viability, survival) and are judged by female bees prior to mating. Matings take place in small territories that are established by males around the stems of small trees in the forest. Here, the males show a typical display behavior that involves frequent landings on the perch and short inspection flights to the near neighborhood. Female visits to these territories are very rare. If females prefer to mate with males that have rich and sexy bouquets, than the behavior of fragrance collection could have evolved through sexual selection. We tested this hypothesis (female preference) with cage experiments in Panama and obtained the first video shots of orchid bee matings (Eltz, Roubik & Whitten, 2003). Using high-speed video and tracer experiments we have demonstrated that fragrances are exposed by males during display. We observed an intricate and highly repetitive leg movement during display of caged males of Euglossa cognata. Single-frame analysis demonstrated that the behavior involves several morphological structures of hitherto unknown function and suggests transfer of substances from the hind leg pocket to a tuft of hairs on the contralateral mid leg. Body-side-specific fluorescent dye application and consecutive detection of signals on males after display confirmed this transfer (Eltz, Sager & Lunau, 2005). Deposited on the mid tibial tufts, the fragrances are ideally placed in order to become ventilated by jugal combs on the wing bases, as suggested by Bembé (2004). Being clearly distinct from motor patterns involved in fragrance collection, the described movement is continuously performed by displaying males, suggesting an equally continuous exposure of volatiles. Chemically the fragrances are species-specific even when individuals from distant and ecologically divergent localities are considered (Eltz, Roubik & Lunau, 2005). Hexane extracts of male hind legs of two sympatric species of Eulaema exposed at their respective display sites on Barro Colorado Island quickly and exclusively attracted males of the "correct" species, which demonstrates that tibial fragrances can potentially mediate specific attraction (Zimmermann, Roubik & Eltz, 2006). However, it is unclear whether conspecific males are the true signal adressees or whether they are simply eavesdropping on other males' mating "calls". More recently we have extended our geographical scope to work with Euglossa viridissima in the Mexican state of Yucatán, where we collaborate with Javier Quezada-Euan (Departamento de Apicultura, Mérida). We have shown in cage experiments with isotopically labelled materials that males use an intricate conveyor belt mechanism for fragrance collection and concentration (Eltz et al. 2007). Further research is currently done on male fragrance morphs and the potential for fragrance driven sympatric speciation. For this we combine chemical, experimental and molecular approaches.

References: Eltz, T., Whitten, W. M., Roubik, D. W. & Linsenmair, K. E. 1999. Fragrance collection, storage, and accumulation by individual male orchid bees. Journal of Chemical Ecology, 25, 157-176.

Eltz, T., Roubik, D. W. & Whitten, W. M. 2003. Fragrances, male display and mating behaviour of Euglossa hemichlora - a flight cage experiment. Physiological Entomology, 28, 251-260.

Bembé, B. 2004. Functional morphology in male euglossine bees and their ability to spray fragrances (Hymenoptera, Apidae, Euglossini). Apidologie, 35, 283-291.

Eltz, T., Sager, A. & Lunau, K., 2005. Juggling with volatiles: exposure of perfumes by displaying male orchid bees. Journal of Comparative Physiology A, 191, 575-581.

Eltz T., Roubik D.W. & Lunau K., 2005. Experience-dependent choices ensure species-specific fragrance accumulation in male orchid bees. Behavioral Ecology and Sociobiology, 59, 149-156.

Zimmermann Y., Roubik D.W. & Eltz T., 2006. Species-specific attraction to pheromonal analogues in orchid bees. Behavioral Ecology and Sociobiology, 60, 833-843.

Eltz, T., Zimmermann, Y., Haftmann, J., Twele, R., Francke, W., Quezada-Euan, J. J. G. & Lunau, K., 2007. Enfleurage, lipid recycling, and the origin of perfume collection in orchid bees. Proceedings of the Royal Society B-Biological Sciences 274: 2843-2848. PDF Nature Research Highlight

sleeping males of Euglossa hemichlora
male Euglossa cognata at fragrance bait
male E. cognata hovering in front of perch, legcrossing for fragrance exposure
male E. cognata hovering in front of perch, legcrossing for fragrance exposure; red arrow indicates transfer of substances