Figure 1

Rationale for the experiments. (a) Three examples of mine entrance closures using a corrugated metal pipe. Bats roosting in tunnels with corrugated culvert pipes for access may abandon the roosts, which is detrimental to bat populations in the US Southwest. (b) Illustration of hoop tunnel. (c) Design of experiments with alternating trials through the hoop tunnel (92 cm diameter) and chain corridor (70 cm corridor width). The bat’s flight path is recorded with a thermal-imaging video camera. Echolocation pulses during flight are recorded with two ultrasonic microphones placed on the end wall of the tunnel or corridor. (d) Explanation of acoustic rationale for special case of tunnel through hoops compared to rows of vertical chains. When the bat is positioned exactly in the center of the chain corridor (left) or hoop tunnel (right), its echolocation sounds (red arrows) impinge on the chains to left and right exactly at the same instant and return to the left and right ears simultaneously with the same spectra, leading to perception of a phantom vertically-extended obstacle (gray ellipses) located immediately to the bat’s front. Even a slight deviation of the bat’s location off-center in the chain corridor makes the echoes at the left and right ears different in arrival-times and spectra, disambiguating the phantom object so the left and right vertical chains are perceived as separate images in the chains’ true locations (gray ellipses), with no perceived obstacle to the front. In the hoops, when the bat is centered, the aggregate reflection stimulates both ears equally, so each hoop’s image is located in front of the bat (gray circle). When the bat moves off the center of the circle, the continuous reflection creates a phantom obstacle that remains in front and follows the bat’s movement.