Visual obstacle avoidance by echolocating bats

Abstract

Evidence is presented that two species of echolocating bats, Carollia perspicillata and Phyllostomus hastatus, while in flight in subdued daylight can visually detect and avoid 30-cm wide strips of white cloth suspended across their flight space. Bats deafened with earplugs avoided the cloth strips significantly better than did individuals that were both deafened and blindfolded. Blindfolding alone did not impair the bat's ability to negotiate these obstacles. Control experiments indicate that loss of vision—rather than any possible secondary effects of the various treatments—was responsible for the differences in behaviour and obstacle avoidance scores between deaf and deaf-blind bats. Deafened bats showed striking differences from non-deafened individuals in the pattern and amplitude of their orientation cries emitted as they approached the strips of cloth. Whereas normal or blind bats increased their pulse repetition rate when approaching the barrier, deaf or deaf-blind individuals maintained high repetition rates and pulse intensities which did not change as the barrier was approached, indicating that it was not detected. Certain limitations of echolocation and its possible supplementation by vision are discussed. It is suggested that vision may provide a passive surveillance system for resting bats and thus reduce the energy expenditure associated with sound production and the scanning of the environment for echoes. The range limitations of acoustic orientation may make visual detection of familiar landmarks especially useful to these frugivorous bats when finding their way to and from feeding sites.