a doctoral thesis - Johan Eklöf Zoology dept. Göteborg university 2003
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(svensk sammanfattning)
Abstract
The use of ultrasonic echolocation (sonar) in air is seriously constrained by the attenuation of high frequency sounds and unwanted echoes from the background (called clutter). Therefore, in many situations, echolocating bats have to rely on other sensory cues. The aim of this thesis is to investigate the use of vision by echolocating bats.
Bat eyes are generally small, especially among aerial hawking insectivores, with the exception of members of the family Emballonuridae. In gleaning, and in frugivorous species, however, the eyes tend to be larger and more prominent. The eyes of all bats are well adapted to low illumination, having mainly rod-based retinas, large corneal surfaces and lenses, and generally large receptor fields. Bats can easily detect small differences in brightness on clear nights, and the visual acuity remains relatively good in dim illuminations. The visual resolving power (as obtained from counts of retinal ganglion cells or by optomotor response tests) varies considerably among the different species of bats, from less than 0.06° of arc in Macrotus californicus (Phyllostomidae) to almost 5° in aerial hawking Myotis species (Vespertilionidae). Generally, the visual acuity is similar to that of rats and mice, suggesting that cm-sized object can be discriminated at ranges less than a few metres. Studies on pattern discrimination have yielded highly variable results. Fruit and nectar eating species respond to patterns to a larger extent than aerial insectivores.
One of the most fundamental roles of the eyes is to register the amount of ambient light, in order to establish photoperiodic cycles. Some tropical bats avoid too bright conditions, i.e. moonlit nights probably due to increased predation risk, a behaviour not found in high latitude species.
As sonar only works well at short ranges, vision is primarily used for detection of landmarks and to avoid objects when moving over long distances, for example during seasonal migration and when commuting between feeding sites. In these situations, there seems to be precedence of vision over sonar. At short range, within that of echolocation, bats may defer to visual cues in addition to sonar and spatial memory to solve different tasks of orientation, especially when there is conflicting information. Light conditions and time of the day may determine the behaviour of the bats and thus which sensory cues will be used.
Although echolocation is the key innovation that have made it possible for bats to fly at night, vision is retained as an important complement; and indeed bats use an array of different sensory inputs to solve the different tasks of life.
Although echolocation is the key innovation that have made it possible for bats to fly at night, vision is retained as an important complement; and indeed bats use an array of different sensory inputs to solve the different tasks of life.
There is an increasing amount of data suggesting that vision might be of importance in some situations and some aspects of foraging, especially for frugivorous and nectarivorous bats, which can make use of differences in brightness and spectral composition, to find different food items. But even in species traditionally considered to rely heavily on echolocation, such as most insectivorous bats, vision seems to play a more important role than has been recognised previously. The gleaning brown long-eared bat (Plecotus auritus, Vespertilionidae), known to forage mostly by using passive listening, detects prey more readily by using vision than by using sonar, and the aerial hawking northern bat (Eptesicus nilssonii, Vespertilionidae), use visual information in addition to sonar to find large stationary prey in clutter.
More on vision in echolocating bats - here
