It’s night, and plant
biologist Arthur Domingos de Melo is looking up at the open, ivory flowers of a
tropical, hardwood tree. Though it’s the dry season in the arid, thorny
Caatinga region of northeast Brazil, a slow drizzle begins to fall. But not
from the sky. Domingos de Melo is under the tree’s canopy, and the “rain” is sweet.

Behold Hymenaea
, a species whose flowers make so much nectar that it overflows
and falls in unusually copious and fragrant showers, even though the price of
water in this part of the world is steep.

Domingos de Melo and colleagues at the Universidade Federal de Pernambuco in Recife, Brazil, had been studying bat pollination of local plants for two decades in the region when, in 2015, one type of bat-pollinated tree struck them as odd. Its nectar, rather than the just the flower petals, was imbued with its own perfume — a phenomenon poorly understood in bat-pollinated plants — and the plant made loads of it.

From 2015 to 2018, the team
studied a population of H. cangaceira in
Brazil’s Catimbau National Park. Each day after sunset during the trees’
reproductive season, between December and March, hundreds of flowers bloom on
each tree and drip with nectar before wilting with the dawn. 

An individual flower
produced up to 1.5 milliliters of nectar per night, the team found. That meant
that one full-sized tree making some 624,000 flowers in a season could produce a
920 liters or so of nectar
in that
time — more
than enough to fill 15 beer kegs — the team estimates in a study published online
October 15 in Ecology.

The only other plant known to
make a similar “sweet rain” is the immense kapok tree, which grows to heights of
around 70 meters in the resource-rich Amazon rainforest. But in the parched,
dusty Caatinga, where H. cangaceira can grow up to about 10 meters tall, producing liters of
sticky, pungent nectar probably takes a lot of a tree’s water and energy.

“I might have expected that order
of magnitude of nectar investment from a larger tree, such as a kapok or a
balsa,” says Robert Raguso, a biologist
at Cornell University not involved with this study. “Those little trees are
investing an awful lot in their floral rewards.”

Such sugary “rewards” are
usually for pollinators: an enticement
to get them close enough to collect a dusting of pollen (SN: 5/9/06). But producing so much energy-rich nectar suggests it’s
particularly crucial for the tree.

The researchers speculate
that the trees may have developed the ability to produce so much sweet nectar
under evolutionary pressure to attract bat pollinators
(SN: 10/16/15). While the researchers
saw other animals visit the trees’ flowers, bats were the only ones that got
close enough to pick up pollen. About one in eight plants in the Caatinga is pollinated by bats, and there are at least 96 bat species
in the region.

Raguso notes, however, that
the nectar may be beneficial to the tree in other ways, for example, by soaking
into the soil beneath the canopy and providing nutrients that enhance
root-microbe relationships.

Chemical analysis of the
nectar revealed 38 different scent compounds, dominated by trans-cinnamaldehyde
and gamma-decalactone — the odors of cinnamon and fermenting fruit, respectively.
Together, those two compounds made up almost 68 percent of the odor mix. This chemical
identification of nectar scent compounds is among the first achieved for a bat-pollinated
plant. The researchers note that bats generally are enticed by the smell of rotten
or fermenting fruit, but Domingos de Melo wants next to investigate whether the
nectar’s fragrant compounds actually do attract bats.  

While the study details H.
’s “wildly cool”
pollination scheme, evolutionary ecologist Amy Parachnowitsch of the University
of New Brunswick in Fredericton, Canada, suggests the team’s isolation of
individual, potentially bat-attracting compounds in nectar is the tip of the

“There are so few studies
that have tested nectar for scent that once we start looking there is likely to
be many more examples,” says Parachnowitsch. “Scents in nectar are probably
common, but we are a very long way from understanding their functional roles
and if there is any differences with various pollinators.”