Why do bats tuck in their wings?

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By flexing their wings inward to their bodies on the upstroke, bats use only 65 per cent of the inertial energy they would expend if they kept their wings fully outstretched, according to a new research. The study suggests that they use their flexibility to compensate for that mass. Unlike insects, bats have heavy, muscular wings with hand-like bendable joints. Whether people are building a flying machine or nature is evolving one, there is pressure to optimise efficiency.

A new analysis at Brown University reveals the degree to which pressure has literally shaped the flapping wings of bats. "Wing mass is important and it’s normally not considered in flight," said Attila Bergou, who along with Daniel Riskin is co-lead author of the study. The findings not only help explain why bats and some birds tuck in their wings on the upstroke, but can also help inform human designers of small flapping vehicles. "If you have a vehicle that has heavy wings, it would become energetically beneficial to fold the wings on the upstroke," said Sharon Swartz, professor of ecology and evolutionary biology at Brown. She and Kenneth Breuer, professor of engineering, originally set out to study something different: how wing motions vary among bats along a wide continuum of sizes. They published those results in 2010 but as they analysed the data further, they started to consider the intriguing pattern of the inward flex on the upstroke.

They tracked markers on the bats, who hailed from six species, and measured how frequently the wings flapped, how far up and down they flapped, and the distribution of mass within them as they moved. They measured the mass by cutting the wing of a bat that had died into 32 pieces and weighing them. The team fed the data in to a calculus-rich model that allowed them to determine what the inertial energy costs of flapping were and what they would have been if the wings were kept outstretched. Bergou, a physicisist, said he was surprised that the energy savings was so great, especially because the calculations also showed that the bats have to spend a lot of energy — 44 per cent of the total inertial cost of flapping — to fold their wings inward and then back outward ahead of the downstroke. — ANI

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