How to Fool a Bat, Pt. II

November 15, 2010

Bats, for all their acoustical superpowers, sometimes fly into stuff. Oddly, the things they often hit are either sufficiently big (like television towers) that a bat ought to be able to echolocate around them, or bright enough — windows, lighthouses — to be visibly seen even with a bat’s relatively poor eyesight. And that’s exactly the problem, it turns out. A recent study in PLoS reveals that bats in the wild collide with objects not because they don’t see them, but because they do. In effect, they’re looking but not listening.

Bats may be blind as bats, but they’re not blind. Some bats have surprisingly sharp vision; the California leaf-nosed bat preys on crickets and other insects that it visually detects on the ground. Most see fuzzily, less attuned to details than to degrees of light and dark, especially around dusk and dawn. When a bat tries to navigate with its eyes and ears simultaneously, however, its brain can become confused by conflicting visual and acoustic information. A captive bat flying around in a room in the daytime will repeatedly fly into the window, like a desperate bird; relying on sight, it sees the window as open space. Blindfold the bat, though, and it acoustically senses an impenetrable window and avoids it.

The Canadian team wondered whether bats in the wild have the same trouble. One August, a time of year when bats begin to congregate and swarm, they set up a simple experiment near the entrance of a busy bat cave. Well after dark, in alternating trials, they set up one of three lengths of fabric for the bats to navigate around: an opaque tablecloth, a transparent tablecloth, or a reflective emergency blanket. In addition, they erected each of the obstacles in one of three lighting conditions: in ambient light (basically, darkness), or in dim or bright light, courtesy of a carefully calibrated spotlight aimed at the fabric. They set up the obstacles night after night, then counted how many bats — mostly little brown bats (Myotis lucifugus) — collided with them, and under what circumstances.

The results shed some light. Over the course of the month, and some 2,200 approaches, bats collided with the obstacles 26 percent of the time. Most of the collisions occurred in the dark, or ambient, condition, mainly because most of the approaches occurred in the dark; the bats seemed to be avoiding the experimental setup when the spotlights were turned on. Still, per approach, collisions were more likely to occur in the dim or bright conditions than in the dark. The bats were echolocating (as a set of microphones made clear), but the greater collision rate suggests that the animals indeed were orienting by vision when they hit some obstacles, and that the visual input was somehow disorienting. (It also reveals that wild bats regularly fly into stuff even in the pitch dark.) Whether the fabric was transparent or opaque made no difference; the bats hit both with equal regularity under all lighting situations. Bats were less likely to collide with the reflective cloth, at least initially, further suggesting that the bats were attempting to navigate by sight.

The researchers did notice something peculiar, however: as the month progressed, the collision statistics shifted. Although the bats initially were less likely to hit obstacles in the dark, after August 22nd they were more likely to hit them then. Likewise, the reflective blankets at first were less likely to be hit than the other fabrics, but after August 24 all the fabrics were equally likely to be hit. As it happens, the researchers note, bats undergo major hormonal and behavioral changes in mid to late August as they prepare for mating season and hibernation. These changes, it seems, make the bats more visually oriented — and more likely to become disoriented. In a separate study earlier this year, the same researchers found that certain tropical bats, which regularly eat fermented fruits and nectar, can fly and echolocate perfectly well even with high blood-alcohol levels. Apparently the effects of lust are harder to shake off.

Orbach, D., & Fenton, B. (2010). Vision Impairs the Abilities of Bats to Avoid Colliding with Stationary Obstacles PLoS ONE, 5 (11) DOI: 10.1371/journal.pone.0013912

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