About a million bats fly out of the Mulu Cave in Sarawak, Malaysia, every evening to look for food. They fly out in batches and it sometimes takes two hours before they are all in the air. When they leave the cave they form a circle (see photo), before forming sinusoidal waves that stretch great distances. Why do they do this?
The sinusoidal wave still seems to be a matter of conjecture 鈥� Ed
鈥� Mexican free-tailed bats emerging from caves in the southern US fly in the same circular pattern, and around the British Isles puffins approaching their burrows also fly in a 鈥渨heel flock鈥�. Fast-flying bats or birds adopt this as a protection against predators such as hawks, gulls or skuas.
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I am unsure about the sinusoidal movement. Perhaps it arises because bats are safer from predators near the ground but have to seek their food higher in the sky. This could mean that sections of the flock briefly climb high in food-gathering sorties 鈥� while it is still light enough for the hawks to see them 鈥� before ducking back to low altitude for safety. Mexican free-tailed bats, for example, feed very high in the air after darkness has descended.
Sean Neill, University of Warwick, Coventry, UK
鈥� Bat sonar is oriented directly ahead of the bat and would be blocked if they flew in a straight line. By flying in a spiral each can maintain a forward view and use its sonar to detect prey or predator. Seen from behind (as in the photo) the spiral appears as a circle and from the side as a sinusoidal wave. It is remarkable how perfect the spiral is and it would be interesting to know if it is always clockwise or anticlockwise for a given species.
Jerry Whitman, Barnham, West Sussex, UK