For all the wonders of flight, most animals that have mastered it stay quite close to the ground. Exceptions are mostly birds of prey that ride thermal drafts high into the sky without using up precious energy flapping. So detections of a few bat species at heights of 1.6 kilometers (1 mile) up have made zoologists wonder how they got there. The answer apparently lies in updrafts created when wind hits hillsides.
Even for strong fliers, it takes a lot of energy to gain altitude without help. On clear days hot air rising from the sun-warmed ground can allow birds of prey to reach great heights with only the occasional flap, but nocturnal species don't have this sort of assistance, making the presence of certain bats at high altitude a puzzle.
Dr Teague O'Mara of Southeastern Louisiana University put GPS loggers on European free-tailed bats in northern Portugal and published observations of their movements in three dimensions in Current Biology.
"We show that wind and topography can predict areas of the landscape able to support high-altitude ascents, and that bats use these locations to reach high altitudes while reducing airspeeds," Dr O'Mara said in a statement. "Bats then integrate wind conditions...deftly exploiting vertical wind energy in the nocturnal landscape." The rising air occurs when a horizontal wind encounters a slope and sweeps up it.
The bats don't fly high often, O'Mara found, seldom going above 300 meters (1,000 feet). When they do reach for the (upper) skies, they take less than 20 minutes to reach their peak, and seldom stay there long, instead going on rollercoaster rides or returning to 100 meters (330 feet) above ground and staying there. The moths that make-up much of their food often congregate at heights of about 600 meters, but like raptors using height to spot smaller birds, the bats may go high to find their prey.
"These free-tailed bats seem to find ways to minimize how much energy they have to spend to find food each night," O'Mara said. "It's a pretty incredible challenge for an animal that can only really perceive the 30 to 50 meters [100-160 feet] ahead of it in detail.” The bats' short-sightedness is relevant because it is thought birds that ride daytime updrafts use their excellent eyesight to spot terrain likely to provide wind in the right wind conditions.
O'Mara also found these bats can reach an impressive 135 kilometers per hour (84 mph) in short bursts of level flying.
The study leaves many questions unanswered, such as why animals that feed on relatively slow-moving moths would waste energy on such bursts of speed. The paper raises the possibility fast flying occurs during mating displays. Even less clear is how such small animals, whose wings are formed from a thin and vulnerable membrane, are able to reach such airspeeds. O'Mara calls this an “unsolved problem” adding, "but it's now clear that bats can fly incredibly fast when they choose. It's up to us to figure out how they do that.”
O'Mara hopes answering this question will inspire ways for humans and our machines to fly faster with less energy.