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Bats can get lift from their landscape. The flying mammals surf on air currents, which sweep upwards as they hit hills or slopes, to reach altitudes of up to 1600 metres. Birds do this too, but it wasn’t clear whether nocturnal bats could take advantage of the technique given that winds are a lot less active in the night sky.

Riding thermals is relatively easy for a bird flying during the day, says Teague O’Mara at Southeastern Louisiana University.

“Because the sun heats up the landscape, there’s rising warm air. Winds come in and lift and push birds through,” says O’Mara. Birds also benefit from long-distance visibility, which may allow them to “read” and exploit the landscape.

“But at night the energy in the atmosphere drops, and the wind drops, and there’s just not a lot going on,” he says. Even so, prior studies have shown that bats can soar to great heights on night flights, so O’Mara and his colleagues set out to discover how they do it.

The researchers studied a of European free-tailed bats (Tadarida teniotis) in northeastern Portugal. Using lightweight GPS collars or backpacks, they gathered high-resolution GPS data from eight lactating female bats on night flights.

The researchers also built a digital model containing information on the local topography and weather patterns. They found that the bats sought out hillsides and cliffs with south or west-facing slopes, where they could benefit from prevailing night-time winds that sweep up these slopes upon meeting the topography. This strong upward push allowed the bats to gain altitude while using very little energy, says O’Mara.

The bats would then dip back down towards the ground and find a new slope that would allow them to rise again, repeating the process many times – making a flight path reminiscent of a roller-coaster ride, the team writes.

The flying style suggests the bats have a detailed mental map of the region’s topography, despite flying in low-visibility night-time conditions. Even echolocation isn’t particularly helpful: it allows a bat to perceive no more than 50 metres ahead.

“They seem to know their surroundings and remember them,” says O’Mara, adding that they might also use each other’s echolocation calls to “find out what’s going on”.

Exactly how the bats orientate their bodies and “huge flappy wings” when soaring upwards isn’t yet clear – but O’Mara is keen to find out, hopefully in future studies, he says.