Vision impairs the abilities of bats to avoid colliding with stationary obstacles

Abstract

Background: Free-flying insectivorous bats occasionally collide with stationary objects they should easily detect by echolocation and avoid. Collisions often occur with lighted objects, suggesting ambient light may deleteriously affect obstacle avoidance capabilities. We tested the hypothesis that free-flying bats may orient by vision when they collide with some obstacles. We additionally tested whether acoustic distractions, such as "distress calls" of other bats, contributed to probabilities of collision.

Methodology/principal findings: To investigate the role of visual cues in the collisions of free-flying little brown bats (Myotis lucifugus) with stationary objects, we set up obstacles in an area of high bat traffic during swarming. We used combinations of light intensities and visually dissimilar obstacles to verify that bats orient by vision. In early August, bats collided more often in the light than the dark, and probabilities of collision varied with the visibility of obstacles. However, the probabilities of collisions altered in mid to late August, coincident with the start of behavioural, hormonal, and physiological changes occurring during swarming and mating. Distress calls did not distract bats and increase the incidence of collisions.

Conclusions/significance: Our findings indicate that visual cues are more important for free-flying bats than previously recognized, suggesting integration of multi-sensory modalities during orientation. Furthermore, our study highlights differences between responses of captive and wild bats, indicating a need for more field experiments.

Figure 1. Total approaches to the obstacle.

The combined passes (dark: grey, dim: white, bright: dotted) and collisions (black) of bats with the obstacle for the three fabric types (transparent: n = 74 trials, opaque: n = 73 trials, reflective: n = 79 trials) for the three light levels (dark: n = 99 trials, dim: n = 61 trials, bright: n = 66 trials).

Figure 2. Probabilities of colliding as a function of fabric type and date.

The probabilities of bats colliding with obstacles for each fabric type, with light levels fixed for (a) dark (n = 99 trials), (b) dim (n = 61 trials), and (c) bright (n = 66 trials) conditions. The arrow marks the cross-over point in the month where there is a change in the probability of colliding with obstacles.

Figure 3. Probabilities of colliding as a function of light level and date.

The probabilities of bats colliding with obstacles for each light level, with fabric types fixed for (a) transparent (n = 74 trials), (b) opaque (n = 73 trials), and (c) reflective (n = 79 trials) conditions. The arrow marks the cross-over point in the month where there is a change in the probability of colliding with obstacles.