It has long been known that plants use brightly colored and distinctively scented flowers to attract pollinators such as bees, birds, and moths. Less has been understood, however, about whether they can also tailor their floral and vegetative structures to attract another important group of pollinators, which primarily navigate and forage using echolocation—bats. Being colorful or smelling sweet loses its value when the pollinator is using its ears to search for food over long distances and through cluttered habitats. Hence, plants that rely on bats to spread their pollen must develop customized ways to attract their acoustically-oriented helpers.
A new study, published in the July 29 2011 edition of the journal Science, has just described the first evidence for a plant, the tropical vine Marcgravia evenia, that uses a type of “echo beacon” to reflect a bat’s sonar signals directly back, slicing through the waves of echoes from other objects in the environment.
The unique plant, found in Cuba, bears a cluster of nectar-producing flowers that hang below a concave reflector leaf. The shape of this leaf allows a constant, direct echo to be picked up within a 100-degree range as a bat approached its distinctive, dish-shaped leaves. Experiments conducted by researchers from the University of Ulm and the University of Erlangen-Nuremburg in Germany and the University of Bristol in the U.K. showed that sonar signals reflected from M. evenia allowed Pallas’s long-tongued bats (Glossophagia soricina) to find the flowers twice as fast as they could without the help of the beacon leaves. It’s as if the plant were a lighthouse in the forest, providing the bat an uncluttered signal towards it despite the surrounding sound waves.
Another plant, Mucuna holtonii, has yet another structural adaptation to bats. A 1999 study, conducted by one of the same researchers (Otto von Helversen) that worked on the recent M. evenia experiments, showed that M. holtonii has small reflector structures inside the flowers, as opposed to a parabolic reflector-type leaf positioned above the flowers themselves. These small structures don’t have the power of M. evenia’s reflector leaves in signaling over long distances and through ambient noise, but M. holtonii‘s petal arrangement depends on whether the flower has been fed from or not; bats preferentially seek the unexploited, nectar-rich inflorescences as they travel amongst flowers. After the nectar has been depleted from a flower, the petals take on a different arrangement, altering the acoustic signal a bat “sees” and letting it know that that flower’s nectar supply has already been consumed.
Both of these plants display remarkable adaptations for attracting bats, each with their own significance. The ability of M. evenia to reflect back a powerful beacon through the forest is unmatched by any plant yet discovered, and the neat trick developed by M. holtonii to signal which flowers still need to be pollinated—and thus hold a tasty meal for the bat—is an entirely different adaptation to that allows the plant to cooperate with another species using acoustic signals.
Bats are important pollinators for a host of plants in both the tropic and temperate zones. The ability to fly allows them to cover a much larger home range than many small mammals, meaning that they can spread pollen and seeds over relatively broad distances. This makes them extremely valuable symbiotic partners to plants that have patchy distributions and low population densities, such as M. evenia.
The same features—distance and isolation—that make these plants dependent upon pollinators may also make it difficult for animals to locate them, which can be a crisis for animals that function on a very tight energy budget. In fact, the nectar-eating bat that appears to have co-evolved with M. evenia, the Pallas’s long-tongued bat, has the fastest metabolism ever recorded in a mammalian species—its internal motors run as fast as those of some hummingbirds. Over 80% of the sugars they consume is burned immediately, without ever being stored as fat—and they burn half of their fat stores each day as well. They are analogous to small cars with a tedious combination of tiny gas tanks and low gas mileage, which means they get into trouble if they have a hard time finding a station at which to fill up. Hence, finding food in a timely and efficient manner is much more than a matter of satisfying the bat’s sweet tooth—it is critical for the survival of these small animals.
An adaptation that enhances the signals from a bat’s echolocation is beneficial for both the plant, which needs a pollen courier, and the bat, which needs the energy provided by the flowers’ nectar. This strongly suggests that what we’re seeing is and example of co-evolution between plant and animal, which each adapting to the other’s needs while filling their own, improving the match and honing the adaptation from generation to generation. Intricate ecological relationships such as these are a boon for both organisms involved, but may also put them at risk if populations of one or the other are diminished due to environmental pressures. Such complicated webs make conservation efforts challenging, but serve as reminders about the importance of the ecological integrity of an area in addition to pure numbers of species.