Whiskers help nectar-eating 'acro bats' hover like hummingbirds

From dragonflies to hummingbirds, hovering flight is among the most complex and captivating forms of animal movement -- a physiological feat of size, musculature and wing development.

For nectar-feeding bats that hover as they feed from flowers, this aerial maneuver also depends on extra-long whiskers unlike those of most other bat species, according to a Dartmouth College-led study in the journal Proceedings of the Royal Society B. The researchers used high-speed cameras to capture how the stiff hairs jutting forward from the face of nectar-eating bats provide enhanced spatial information that guides the animals as they swoop in to quickly feed -- within a second or less -- on succulent flowers without landing.

"The whiskers of nectar-feeding bats are critical sensory organs that provide high-quality input the brain works with to optimize hovering. It's a cool junction between sensory biology and bio-kinematics, between form and function," said lead author Eran Amichai, a postdoctoral researcher in biological sciences at Dartmouth who studies echolocation in bats. Co-authors are postdoctoral fellow David Boerma from the American Museum of Natural history, animal behavioralist Rachel Page at the Smithsonian Tropical Research Institute in Panama, Sharon Swartz, a professor of biology and engineering at Brown University, and Hannah ter Hofstede, a past assistant professor of biological sciences at Dartmouth now at the University of Windsor in Canada.

The researchers worked at the Smithsonian Tropical Research Institute recording Pallas's long-tongued bats -- a South and Central American bat that has the fastest metabolism of any mammal -- as they drank from hand-blown glass flowers designed for the study to replicate the plants the animals feed from. High-speed infrared cameras captured photos and video of the bats as they descended upon the glass flowers and navigated their muzzles and tongues into the "bloom" to eat the nectar. Feedings typically lasted between a half- to one second.

The researchers found that bats with clipped whiskers were less agile and accurate during feeding and flight than animals with untouched whiskers. The animals with clipped whiskers were held for a few days until the hairs regrew, then released back into the jungle. "Clipping the whiskers doesn't reduce the bats' ability to feed, they just do it a little less gracefully," Amichai said. "If it were gymnastics, they'd get an 8.5 instead of a 9.8."

The role of long whiskers in nectar-feeding bats' flight control provides new insight into the coevolution of the bats with the flowers they feed on, Amichai said. The majority of bats possess short whiskers not arranged in any particular pattern or direction. But the researchers found that whisker length in nectar-eating bats evolved at least twice to -- along with long tongues and faces -- potentially help them better navigate the deep chambers of the flowers they prefer. In turn, the long reach these flowers require results in more pollen sticking to their pollinators and thus the broader proliferation of their kind.

The researchers plan to continue their work using higher-resolution images, flowers that move, interactions with predators and other expansions on the experimental model, Amichai said.

In the meantime, the latest study offers a fascinating glimpse into how nectar-feeding bats combine various forms of sensory information to navigate the world around them, Amichai said. Their world is a combination of scent, echolocation, spatial memory, knowledge of the seasons and the physical sensation and equilibrium provided by their whiskers.

"I find thinking in these terms of switching back and forth between completely different ways to perceive the world -- and seamlessly integrating their input -- to be a mind-blowing concept," Amichai said. Understanding how animals perceive and interact with their surroundings helps scientists develop better conservation strategies, he said.

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Hummingbirds can smell their way out of danger

In less time than it takes to read this sentence, hummingbirds can catch a whiff of potential trouble. That's the result of new UC Riverside research showing, contrary to popular belief, the tiny birds do have an active sense of smell.

Researchers have known for some time that vultures have a highly sensitive sense of smell, with some species being compared to "airborne bloodhounds." This is due in part to their large olfactory bulbs -- tissue in the brain that controls smell.

However, hummingbirds' olfactory bulbs are, like the rest of their bodies, extremely small. Earlier studies were unable to demonstrate that hummingbirds showed a preference for the smell of flowers containing nectar. In addition, flowers pollinated by birds generally don't have strong odors, unlike those pollinated by insects. For these reasons, scientists did not previously believe the birds possessed the ability to smell things.

UCR scientists have now shown for the first time that not only can hummingbirds smell insects, but also that scent may help them stay out of danger while looking for nectar to eat. A paper describing their experiments has now been published in the journal Behavioral Ecology and Sociobiology.

"This is pretty exciting, as it is the first clear demonstration of hummingbirds using their sense of smell alone to make foraging decisions and avoid contact with potentially dangerous insects at a flower or feeder," said Erin Wilson Rankin, associate entomology professor and study co-author.

For their experiments, the researchers allowed more than 100 hummingbirds to choose between two feeders, either sugar water alone, or sugar water plus one of several chemicals whose scent signaled the presence of an insect. There were no visual differences between the two feeders offered in each of the experiments.

Tests included the scent deposited on flowers by European honeybees, an attraction chemical secreted by Argentine ants, and formic acid, a defensive compound produced by some Formica ants which is known to harm birds as well as mammals.

"If a bird has any exposed skin on their legs, formic acid can hurt, and if they get it in their eyes, it isn't pleasant," Rankin said. "It's also extremely volatile."

The hummingbirds avoided both of the ant-derived chemicals, especially the formic acid. However, they had no reaction at all to the honeybee scent, which is known to deter other bees from visiting flowers.

To ensure it was the chemical itself the birds were reacting to, and not simply a fear of new smells, the researchers did an additional test with ethyl butyrate, a common additive in human food.

"It smells like Juicy Fruit gum, which is not a smell known in nature," Rankin said. "I did not enjoy it. The birds did not care about it though and didn't go out of their way to avoid it."

Rankin said the study raises new questions about the underrated importance that scent plays in birds' foraging decisions and specifically, hummingbird foraging.

Ashley Kim, first author on the paper and current ecology doctoral student at UC San Diego, was based in the Rankin Lab at UCR while participating in this project.

"This research made me understand the importance of studying the basic biology and natural history of animals that are commonly overlooked," she said.

Kim's participation was supported by the National Science Foundation, through its Research Experiences for Undergraduates program, which helps undergraduates get hands-on experience conducting research.

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