A Commissaris's long-tongued bat (Glossophaga commissarisi) with its tongue in a sea bean triggering slot, a split second prior to landing and having pollen fired onto its rump. The sea bean (Mucuna holtonii), is a legume that grows along streams, rivers and other rain forest edges from southern Mexico and Belize throughout Central America. It is a woody vine whose inflorescences open flowers only at night. Its flowers are highly specialized for exclusive bat pollination. Small bats, such as Glossophaga commissarisi (Brown long-tongued bat) and Hylonycteris underwoodi (Underwood’s long-tongued bat). It is apparently ignored by larger glossophagine species, such as Lonchophylla robusta (orange nectar bat). Its anthers and stamens are sized and shaped to effectively fire pollen onto the rumps of bats. Inflorescences hang below the surrounding foliage on 3-6-foot-long stems known as peduncles that facilitate approaches by flying bats. Each inflorescence is about 5-7 inches in diameter and contains many buds with up to 8 flowers opening each night for approximately 6 weeks. A flower that is ready to be pollinated signals its readiness by raising its dish-shaped banner (the vexillum), adapted to strongly reflect echoes from many different directions. Bats normally ignore this flower until its banner has risen to a nearly vertical position. The reflected echos guide an approaching bat to put its nose in an approximately 2-mm-wide space between 2 petals known as wings. When a nose pushes into this small opening, the flower is triggered to fire pollen onto the bat’s rump. Another pair of petals forms a “keel” which encloses the stamens and anthers. When the flower is triggered by a bat nose, the keel suddenly opens, releasing these male reproductive organs in a manner that fires pollen onto the bat’s butt. This process is described in detail by Helversen and Helversen (Nature 398, 759-760; 29 April 1999). The banner serves as “a small concave 'mirror' that works like an optical cat's eye, but in the acoustic domain, reflecting most of the energy of the bats' echolocation calls back into the direction of incidence.
As flower firing is triggered, the bat is rewarded by receiving an average of 100 microliters of nectar (a relatively large quantity). Bats that subsequently visit already fired flowers get approximately 10-20 microliters of nectar. Each flower can fire only one time. Banners do not open all at once. Openings are staggered between sunset and about midnight, the first ones not reaching full readiness till an hour or more after sundown. This discourages nectar thievery by diurnal visitors, such as hummingbirds, who seldom achieve pollination, but attempt to feed on the nectar. Also, by staggering flower readiness, bats are encouraged to make multiple visits, ensuring improved pollination. At a bat’s first visit, a flower’s stigma (female reproductive organ) is not yet receptive, preventing self-pollination. It becomes receptive as subsequent pollen-bearing bats approach. The large, bean-like seeds of this plant are called sea beans, because they are extremely buoyant and adapted for sea dispersal. They are commonly found on beaches as far away as the southeastern United States and are a beautiful jet-black. When polished by natives, they are made into lovely necklaces, bracelets and other handcraft items. This plant is also a good nitrogen fixer and is fed upon by butterfly caterpillars, such as those of the blue morpho. Pollination