Shape Changing Flowers Attract Bats

For nectar feeding bats, finding food in a dense tropical rainforest is a complex task. For many years it was thought that nectar feeding bats relied solely on scent to find their food. Unlike insects that move around, flowers are stationary. That all changed once we developed microphones sensitive enough to pick up bat calls.

It turns out, nectar feeding bats utilize different frequencies for echolocation that are primed for resolving finer details than those of their insect feeding cousins. Obviously this works quite well for the bats as they are very important pollinators. Whereas many bat pollinated plants position their flowers in easy to reach places for bats, others take it a step further.

Some plant species couple their floral displays with specialized structures that are prime reflectors for bat sonar. Recently, a species of vine known scientifically as Marcgravia evenia (http://bit.ly/2h358BF) was made famous the world over for the satellite dish shaped leaves it produces just above its inflorescence. However, this species is not alone. Other plants have managed to tap into bat sonar in some very interesting ways. Take, for instance, the bizarrely beautiful blooms of the sea bean.

A member of the legume family, Mucuna holtonii is neotropical in its distribution. It is a vining species that snakes its way up into the canopy. When in flower, the plant produces a stunning pendulum-shaped inflorescence, the end of which is ringed in flowers. Although highly derived for this group, the flowers are nonetheless representative of the family. Their secret to success lies in the single large petal known as the banner or vexillum coupled with some explosive power.

This banner petal acts as a nectar guide, though not in a strictly visual sense. These flowers open at night when nectar feeding bats are out and about. They emit a scent, which likely lets the bats know roughly when and where a meal is available. Unlike bees, however, these bats have no interest in pollen. Instead, they are after the energy-rich nectar reserves that only virgin flowers produce. And they produce quite a bit (relative to their size), upwards of 100 microliters.

Bats that have honed in on the scent are further attracted to the flowers by the shape of the vexillum. Supremely adapted to the specific frequency of these nectar feeding bats, the vexillum of each virgin flower reflects sound waves over a greater range of directions than the clutter of the surrounding forest, thus helping the bats zero in on exactly where they need to be.

Unlike many other bat pollinated flowers, those of M. holtonii cannot be accessed simply by hovering. A bat must land on them in order to access the nectar within. The weight of the bat is what the flowers require to complete the process. When a visiting bat lands on the flower, it triggers an explosive mechanism that snaps the anthers outward, causing pollen to explode from the flower, spattering it all over the bat's back.

Once the bat drinks its fill, nectar production ceases. However, the flowers don't senesce at this point. They still provide a calling card for any other flowers yet to be visited. Despite the fact that they stick around, it has been found that bats are significantly less likely to visit spent flowers. How do they know?

The answer again lies in that banner petal. Once the flowers have been triggered, its shape changes. This results in a change in the way in which bats perceive the flowers via echolocation. Bats soon learn that flowers with this altered shape no longer offer a nectar reward. The plant benefits from this because it reduces the chance that a bat will end up depositing its pollen right back on the flower it came from. It's win win when you think about it. Bats maximize their meals and the plant maximizes its chances of cross pollination.

Photo Credit: Merlin Tuttle

Further Reading: [1] [2]