The Web of Life (8-2-2) Sound continued


Sound is a tricky medium. Low frequencies travel farther, though lack the precision of shorter wavelengths.

Male green tree frogs use a dual-frequency call. A female picks up the lower frequency at a distance. As she approaches the male, the higher frequency component kicks in.

Elevation helps. Cricket chirps from shrubs or trees travel 14 times farther than those sung from the ground, resulting in a better female draw. Territorial bird songs are typically delivered from an elevated post, enhancing effective range. Territorial grassland birds, such as meadowlarks and pipits, deliver their declaratory pips on the fly.

There is less attenuation of sound in water than air, partly because sound in water travels faster, and water is a more stable momentary medium. Hence many aquatic animals extensively communicate by sound.

The benthos off Los Angeles is an acoustic cacophony: shrimp, lobsters, crabs, and fish snapping, rasping, humming, squeaking, and grunting. California mantis shrimp reside there on the ocean floor; burrow-dwellers in muddy water. Each shrimp has its own recognizable voice. By muscle vibration, males sing in a 3-part low frequency harmonic rhythmic rumble to attract females to their lovely burrow, or to defend territory.

Bird warning calls illustrate how precision can matter. The source of an alarm call should be difficult for the predator to locate. To achieve this, several facets apply.

The call should gradually fade in and out. It should be a thin, pure tone, optimally pitched at a frequency determined by the distance between the predator’s ears.

For a mid-sized hawk or owl, 7 kilohertz makes it perfectly vexing for these birds of prey to locate the sound source. This matches the actual alarm call used by several small bird species to warn of an approaching aerial predator. Thus, prey alarms blend with the predator’s own vocalizations, and so are not as distinctive as they would be if at a different frequency.

Whale songs can be heard for hundreds of miles, but distance is not everything. Whales are good listeners and adjust their singing to local conditions to optimize the quality of reception, not just range.