A flying lesser long-nosed bat (Leptonycteris yerbabuenae) approaching a flower. Photo credit: Marco Tschapka.

A flying lesser long-nosed bat (Leptonycteris yerbabuenae) approaching a flower. Photo credit: Marco Tschapka.

Flight is the most metabolically demanding life process, with some creatures running their powerful engines on a high energy liquid fuel: nectar. Although some nectars are packed with up to 50 g of sugar for every 100 g of fluid (50% sugar), the nectars produced by bat pollinated flowers are more dilute; the bats never encounter nectars containing more than 33% sugar. ‘Foragers should prefer foods that yield more energy’, explains Michael Walter from the University of Tübingen, Germany, so it would make sense that bats could taste the difference between concentrated and dilute nectars, to ensure that they select the most calorific blooms when refuelling. But no one had checked. Knowing that the ability to discriminate between the sweetness of different flowers is even more essential for nectar feeders when the nectar is watery, Hans-Ulrich Schnitzler and colleagues from the University of Tübingen and the University of Ulm, Germany, tested the lesser long-nosed bat's (Leptonycteris yerbabuenae) sweet tooth.

Offering eight bats a choice between two syrup solutions – some differing by as little as 0.5% (4.75% sugar versus 5.25%), while other differences were larger (21% versus 29%) – Walter, Aaron Verdong, Vanessa Olmos and Christina Weiss filmed the bats each night as they foraged from the artificial feeders in the lab to determine whether the mammals could differentiate between the different syrup concentrations. ‘It took us over 7 months to run the whole experiment, as we offered each bat the choice between at least 13 different sugar solution pairs’, says Walter. After filming almost 31,000 feeder visits, the team counted how many times each bat sipped from the most concentrated syrup, reasoning that if the animal could distinguish between the two, it would prefer to frequent the feeder that provided it with the most sugar and energy.

Incredibly, when the results were in, seven of the animals were capable of discerning a difference of just 0.5% when offered the choice between the 4.75% and 5.25% syrups. ‘No one had tried to offer them such small concentration differences before and they could easily distinguish between the two’, says Walter, adding, ‘I'm always amazed by the capabilities of these small animals’. However, as the sugar solutions became stronger (around 20% sugar), only one bat was able to differentiate between syrups differing by 1.5% (19.25% versus 20.75%), while the remaining animals were only capable of tasting sweetness differences of 4% at an average concentration of 20% sugar (18% versus 22%). And when the team calculated how much syrup each bat consumed while foraging, it was clear that the bats dining from flowers stocked with stronger nectars (greater than 10% sugar) could easily sip sufficient nectar to meet their energy needs over the course of a night. However, the bats that were offered syrups averaging 5% sugar only consumed 2/3 of the energy imbibed by the better fed bats. There simply wasn't enough energy in the dilute diet for the bats to satisfy their energy demands; ‘There is only so much water that a small bat belly can absorb’, chuckles Walter.

Lesser long-nosed bats seem to have a finely tuned palate, which allows them to discriminate between dilute nectars to identify the most bountiful blooms that will satisfy their appetite. And when the team compared the bats’ abilities with those of other creatures that thrive on nectar, it was apparent that lesser long-nosed bats have an even more discerning sense of taste than hummingbirds, bumblebees and even other nectar sipping bats, which might help them to get to the front of the line when filling up at the best stocked flowers.

Walter
,
M. H.
,
Verdong
,
A.
,
Olmos
,
V.
,
Weiss
,
C. C.
,
Vial
,
L.-R.
,
Putra
,
A.
,
Müller
,
J.
,
Tschapka
,
M.
and
Schnitzler
,
H.-U.
(
2020
).
Discrimination of small sugar concentration differences helps the nectar-feeding bat Leptonycteris yerbabuenae cover energetic demands
.
J. Exp. Biol.
223
,
jeb215053
.