BEHAVIORAL BIOLOGY:

SUMMARY: If an extended surface is determined to be smooth via acoustic signaling, echolocating bats innately view it as water, often overriding other sensory cues.

The fact that many bats utilize echolocation to find prey is well-known. This ability is well-developed; some bats even dynamically adjust the width of the sound pulse during the pursuit of prey, thereby broadening their field of view without sacrificing attention, useful for capturing erratically-flying prey.

In contrast, much less is known regarding how echolocating bats utilize sensory cues to recognize their habitat, an ability which is clearly important to their survival. Furthermore, to what extent is such recognition innate or learned?

Stefan Greif and Björn Siemers (Max Planck Insitute for Ornithology, Germany) and coworkers have investigated how echolocating bats differentiate water, often a prominent feature of their habitat, from other surfaces. Their research demonstrates that echolocation is given prominence even in the face of contradictory sensory cues, and further suggests that this recognition ability is innate (not learned).

Bat water perception.

The scientists started with the observation that significant bodies of water are generally acoustically smooth. They hypthesized that such a surface would be perceived as water via echolocation, whether or not it is in fact water.

They tested six bats total (gender balanced) from four species, captured in Rusenski Lom Nature Park (further species were tested on a limited basis). The bats all tried to drink from smooth plates (i.e. touched the plate head down, as they do with water), in preference to textured plates, over the course of dozens of attempts for each species.

The plates were of differing composition (wood, plastic, or metal), and did not smell, look, taste, or feel like water; i.e., there is clearly contradictory sensory information, if the bats were attuned to it. The bats never flew down to the textured surfaces in an attempt to obtain a drink; they only flew down to the acoustically-smooth surfaces.

Experiments performed in dqrkness yielded roughly 50% more drinking attempts from the smooth surfaces. This suggests that while echolocating bats do use additional sensory information to identify water, the acoustic properties of smooth surfaces are often sufficient to trick them.

The scientists further tested pre-flight juvenile bats captured with their mothers in a cave, and never previously exposed to a significant body of water. Once they learned to fly, they attempted to drink from the smooth, but not rough, surfaces, i.e. the same behavior as the adult bats.

Future directions.

The neural and genetic basis of this water recognition behavior in echolocating bats are unknown, and have been identified by the scientists as possible future directions of inquiry. It's difficult for me to envision such experiments in other mammals; echolocating bats are ideal candidates due to their unambiguous, readily measurable system of navigation.

NOTE: The scientists' research was funded by the Human Frontier Science Program and the Max Planck Society.

Greif, S., & Siemers, B. M. (2010). Innate recognition of water bodies in echolocating bats Nature Communications, 1 (8) DOI: 10.1038/ncomms1110