Which adaptation in bats is primarily for flight?

Thinner and lighter bones in bats are a crucial adaptation for flight. This skeletal structure reduces overall body weight, making it easier for bats to achieve the lift necessary for flying. Lighter bones minimize the energy expenditure required during flight, enabling bats to be more agile and maneuverable in the air.

While strong hind limbs, extra elongated phalanges, and increased radius and humerus length all contribute to a bat's ability to fly, the primary adaptation that directly facilitates this ability is the reduction in bone density and weight. This characteristic allows bats to maintain a lightweight framework essential for sustained flight and aerial navigation.