Fruit bats might not have the greatest eyesight, but they know a good smell when they sniff one.
Even in the dark of night, the olfactory receptors in their brains can bind to a whole array of specific odors wafting into the air, allowing these tropical pollinators to track down nearby fruit for a midnight snack.
In fact, we might have bats to thank for the spicy scent of peppers. Recent fieldwork in Costa Rica has now found evidence that pepper plants and short-tailed fruit bats have coevolved over millions of years.
Researchers say the volatile organic compounds (VOCs), responsible for the smell of ripe peppercorn berries, appear to be uniquely tuned to the olfactory receptors in local bats.
The study is based on hundreds of hours of work spent searching for and collecting pepper plants in Costa Rica, before extracting the chemicals that form their scents.
In the end, the team was able to identify 249 VOCs across 22 species of plant belonging to the Piper genus.
Some of the VOCs were common across all the plants, like the chemicals that form the spicy scents of cinnamon or cloves; other VOCs were only found in a few species.
Each plant had its own unique bouquet of scents, but the authors still needed to figure out if this perfume was tailored to a pollinator.
When researchers examined the feces of three local fruit bat species – Carollia castanea, C. sowellii, and C. perspicillata – they found all of the bats consumed quite a lot of fruit from pepper plants, as suggested by previous research.
According to bats' droppings, the fruits that gave off a rare VOC known as 2-heptanol were the ones bats seemed to eat the most, as well as two common VOCs found in almost every pepper plant species: α-caryophyllene or α-phellandrene.
To further test these results, researchers gave wild bats a choice between a normal unripe fruit from a pepper plant and an unripe fruit enriched with one of the three VOCs listed above.
The video recordings of the experiments add weight to the idea that wild bats are more likely to investigate and consume unripe fruit if it contains certain key scents that indicate ripeness.
"These findings suggest bats use specific chemicals in the fruit scent bouquet not only to select ripe fruits but to find the specific Piper species that make up the bulk of their diet," explains biologist Sharlene Santana from the University of Washington.
"By helping them communicate with the bats, these chemical signals are likely a component of a dispersal syndrome in these plants."
Bats are important pollinators in tropical and desert environments. When these winged creatures eat fruit, they poop out the seeds elsewhere, dispersing the plant's genes and adding to the species' diversity.
Similar to how certain flower colors are thought to have evolved to attract hummingbirds, some scientists have postulated certain fruit scents might have arisen to attract bats. But this is some of the first research to actually test whether fruit and bat really have evolved together.
Careful statistical analysis of the data from Costa Rica strongly suggests the scent chemistry of pepper fruits and the diet of fruit bats are indeed linked – a relationship the authors say has contributed "to the extreme diversity of tropical fruiting plants worldwide."
"Olfaction is the bridge between the plant signal and bat fruit consumption, and finding the specific VOCs bats respond to opens the door to matching olfactory receptor genes to important VOCs, which has been impossible until now," says ecologist and evolutionary scientist Liliana Dávalos from Stony Brook University.
The authors are calling for further research to better understand how bats perceive VOCs and to what extent their affinity for scents is learned as opposed to innate.
The study was published in the Proceedings of the Royal Society B.