An embellishment that became a mutualism: Inquiries on male bee tibial bouquets and fragrance-producing orchids in Panama and oceanic islands (Apidae: Apinae, Euglossini; Orchidaceae: Epidendroideae)

- Odors of pollinating male orchid bees and fragrance host orchids reflect isolation.
- Mating “bouquets” of male Euglossa contain fewer chemicals on islands.
- Reduced interspecific mating interference may produce reduced species-specific odor bouquets.
- Most components of bee odors and orchid fragrances are rare among individuals.
- Benzenoids absent in orchids are major components of Euglossa mating bouquets.

We used comparative studies to investigate how and why floral and bee fragrances evolve, including courtship odors collected by male Euglossa mixta to form their “tibial bouquet”, on Coiba Island and other Panama forests. Fragrances of four orchid genera, two used extensively by E. mixta - Coryanthes and Mormodes - and two never used, Clowesia and Catasetum - were also analyzed. From among 636 chemicals in 93 male tibiae, 66 were also found in 30 floral head-space samples of orchids, in which 315 total volatile compounds were detected. Geographic variation was noteworthy in E. mixta, but no significant difference was found between mainland and island populations. The aromatic benzenoids methyl salicylate, 2-hydroxy-6-nona-1,3-dienylbenzaldehyde (HNDB), and the monoterpene 1,8 cineole, nearly always occurred. Coryanthes or other orchids produce two of the chemicals, but no source of HNDB is known. No statistical evidence was found of bee preference for orchids with bouquets like those formed in bee hindlegs, yet Coryanthes and Mormodes produced the most monoterpenes and more resembled the bees, when compared to Catasetum and Clowesia. Coiba bee tibial bouquets averaged 56% as diverse as on mainland and Coiba has <50% the euglossine species of nearby mainland, but lacks those most similar to E. mixta, both in phylogeny and tibial bouquet. Coiba's diverse rain forest should contain many volatiles the bees seek. Because odor collection and production are costly, our findings strengthen hypotheses that odors are used to avoid interspecific reproductive interference. Despite finding large differences in the same orchid species, we do not know whether isolation of between 107 and 104 years produced differentiation. Fragrances seem analogous among orchids and bees, thus may lessen interspecific interference or competition, and promote outcrossing or favor embellishments, via female choice. Such adaptive reasons for fragrance variation within bee or orchid populations remain largely untested.

Male Eulaema meriana visiting and collecting fragrances from the flower of Gloxinia, in Colón Province, Panama. Photo: D.W. Roubik. This is an example of a non-orchid fragrance S. Vogel documented for euglossine male bees elsewhere.161