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Tuesday, March 22, 2011

Echolocation – The Compass of Swiftlets

Echolocation, also called biosonar, is the biological sonar used by several animals, most notably microchiropteran bats and odontocetes (toothed whales and dolphins), but has also been demonstrated in simpler form in other groups such as shrews, one genus of megachiropteran bats (Rousettus) and two cave dwelling bird groups, the so called cave swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated Oilbird Steatornis caripensis. The term echolocation was coined by Donald Griffin, whose work with Robert Galambos was the first to conclusively demonstrate its existence in bats in 1938. However, long before that, the Italian 18th century scientist Lazzaro Spallanzani had, by means of a series of elaborate experiments concluded that bats navigated by hearing and not vision. Echolocation in odontocetes was not properly described before two decades later, by Schevill and McBride. Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects in the environment. They use these echoes to locate, range, and identify the objects. Echolocation is used for navigation and for foraging (or hunting) in various environments.

The genus, Aerodramus was thought to be the only echolocating swiftlets. These birds use echolocation to locate their roost in dark caves. Unlike a bat’s echolocation, Aerodramus swiftlets make clicking noises that are well within the human range of hearing. The clicks consist of two broad band pulses (3-10 kHz) separated by a slight pause (1-3 milliseconds). The interpulse periods (IPPs) are varied depending on the level of light; in darker situations the bird emits shorter IPPs, as obstacles become harder to see, and longer IPPs are observed when the bird nears the exit of the cave. This behavior is similar to bats as they approach targets. The birds also emit a series of low clicks followed by a call when approaching the nests; presumably to warn nearby birds out of their way. It is thought that the double clicks are used to discriminate between individual birds. Aerodramus sawtelli, the Atiu Swiftlet, and Aerodramus maximus, the Black-nest Swiftlet are the only known species which emit single clicks. The single click is thought be used to avoid voice overlap during echolocation. The use of a single click might be associated with a evolutionary shift in eastern Pacific swiftlets; determining how many clicks the Marquesan Swiftlet emits, could shed light on this. It was also discovered that both the Atiu Swiftlet (Fullard, 1993) and the Papuan Swiftlet (Price et al., 2005) emit clicks while foraging outside at dusk; the latter possibly only in these circumstances considering it might not nest in caves at all. Such behavior is not known to occur in other species (Fullard, 1993) but quite possibly goes given that the Papuan and Atiu Swiftlets are not closely related. However, it has recently been determined that the echolocation vocalizations do not agree with evolutionary relationship between swiftlet species as suggested by DNA sequence comparison (Thomassen & Povel, 2006). This suggests that as in bats, echolocation sounds, once present, adapt rapidly and independently to the particular species' acoustic environment.

Three hypotheses are considered to describe how echolocation evolved in the genus Aerodramus and, as determined more recently, other taxa in the Apodidae. One hypothesis states that echolocation evolved from an ancestral species of swiftlets and was lost in the genera which lack echolocation. A second hypothesis is that echolocation evolved independently several times. The third scenario involves a combination of the first two, i.e. a gain-loss-regain scenario.

Several functional subunits (like vocal muscles and brain areals) are needed to produce the echolocating system. Past studies have thought that the loss of one of these subunits was more likely to occur than acquiring all the traits needed to echolocate. But a recent study (Thomassen et al., 2005) suggests that the echolocation subunits were mainly located in the central nervous system, while the subunits in the vocal apparatus were already present and capable of use before echolocation even evolved. This study supports the second hypothesis of independent evolution of echolocation in Aerodramus and Collocalia, with the subsequent evolution of complex behavior needed to complement the physical echolocation system, or even the third approach, as the vocal apparatus-parts of the echolocation system might even be inherited from some prehistoric nocturnal ancestor.


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