ScienceWeek

SENSORY PHYSIOLOGY: ON ECHOLOCATION

The following points are made by Gareth Jones (Current Biology 2005 15:R484):

1) Echolocation, or biosonar, is an active process used by the species that have it for sensing the environment when vision is ineffective, for example at night or in turbid water. It involves the production of sound, and the reception of echoes that return from objects. By comparing the outgoing pulse with the returning echoes -- which are modified versions of the outgoing pulse --the brain can produce images of the surroundings. The location of a target in three dimensions can be determined from its range and direction. Echolocating animals can determine how for away objects are -- their range -- by measuring the time delays between call production and reception. Sound travels at 340 meters per second in air, and travels to the object and back again, so a delay of 2 milliseconds corresponds to a range of 34 centimeters.

2) Object direction can be determined in the vertical and horizontal planes. Many bats determine the vertical angle (elevation) of targets by interpreting interference patterns caused by sounds reflecting from the tragus, a flap of skin in the external ear. Horseshoe bats move their ears up and down independently, and may calculate elevation from intensity differences received at each ear. Bats determine the horizontal angle, or azimuth, of targets from differences in the intensity of sound received at each ear. Echo strength can give cues about target size, and surface texture may be determined from peaks and troughs in the frequency spectrum of the echo. Overall then, echolocation can provide rich detail about the environment.

3) Echolocation has evolved to its greatest sophistication in bats and toothed whales (dolphins and their relatives), though simple forms of echolocation are also used by cave swiflets and oilbirds, and by small nocturnal mammals such as shrews and rats. The main function of echolocation is orientation -- calculating one's own position relative to the surroundings -- although many bats and dolphins also use echolocation for detecting, localizing and even classifying prey. Although echolocation can give bats and dolphins sophisticated information about their surroundings, in certain situations it becomes of little use. For example, mouse-eared bats use echolocation to detect airborne prey, but almost "switch off" echolocation when detecting prey under leaf litter. Echoes from leaves mask echoes from prey, and in these situations the bats must rely on rustling sounds made by the insects as they move through the leaf litter for successful prey detection.

4) Echolocation is such a remarkable process that its discovery involved several incidences of disbelief. In 1793, the Italian scientist Lazzaro Spallanzani (1729-1799) discovered that blinded bats were still able to negotiate obstacle courses. The Swiss naturalist Charles Jurine reported in 1794 that bats use hearing in orientation, as they collided with wires when their ear canals were closed with wax. An influential doubter of these results was the palaeobiologist Georges Cuvier (1769-1832), who dismissed Spallanzani's and Jurine's experiments as flawed, and concluded that bats use the organs of touch for orientation. Cuvier's influence did much to hinder further research on bat orientation for over a century. Most bat echolocation calls are ultrasonic (>20 kHz), and hence inaudible to human ears. High frequencies have short wavelengths, and therefore reflect strongly from very small targets such as insects. Because people cannot hear bat sounds, how bats use their hearing to detect obstacles remained difficult to fathom.[1-5]

References:

1. Au, W.W.L. (1993). The Sonar of Dolphins. Springer-Verlag, New York

2. Griffin, D.R. (1986). Listening in the Dark. Cornell University Press, Ithaca

3. Houston, R., Parsons, S., Jones, G., and Bennett, A. (2001). Biosonar: Seeing with Sound. www.biosonar.bris.ac.uk (2001).

4. Pollak, G.D. and Casseday, J.H. (1989). The Neural Basis of Echolocation in Bats. Springer-Verlag, Berlin

5. Thomas, J.A., Moss, C.F., and Vater, M. (2004). In: Echolocation in Bats and Dolphins.. (2004). The University of Chicago Press, Chicago

Current Biology http://www.current-biology.com

--------------------------------

Related Material:

ZOOLOGY: ON BATS

The following points are made by Nancy B. Simmons (Science 2005 307:527):

1) The only mammals capable of powered flight, bats constitute more than 20% of living mammal species [1]. Unlike birds and other terrestrial vertebrates, most bats use echolocation -- a biological form of sonar -- to locate and track their prey [2]. Bats are found on every continent except Antarctica, and they exploit a wide variety of food sources including insects, small vertebrates, fruit, nectar, pollen, and even blood [1-3]. More than 110 bat species may coexist in some ecological communities, a number that far exceeds that of any other mammalian group [1, 3]. Despite their prominent position among mammals, the evolutionary history of bats is largely unknown because of a limited fossil record and incomplete phylogenies.[4]

2) Living bats are classified into 18 families on the basis of shared anatomical specializations and echolocation habits, and another six families are known from fossils [1,5]. Although biologists have long agreed that these groups represent distinct evolutionary lineages, there has been no consensus concerning relationships among them. The lack of a well-resolved phylogeny (evolutionary tree) for bats has hindered attempts to understand the origins of major specializations in these mammals, and has complicated efforts to untangle the temporal and biogeographic history of the group.

3) One extant family (Pteropodidae, or Old World fruit bats) lacks the sophisticated echolocation abilities of other bats. Because bat echolocation is a complex system involving specialization of the respiratory system, ear, and brain [2], it has generally been assumed that echolocation evolved only once in bats. This hypothesis has been supported by phylogenetic analyses of morphological data by a number of groups [5]. These analyses revealed a basal split among bats between a single lineage leading to all echolocating bats (Microchiroptera) and another lineage leading to non-echolocating pteropodids (Megachiroptera). However, recent analyses of DNA sequence data have challenged this hypothesis, instead suggesting that some echolocating bats (rhinolophoids) are more closely related to pteropodids than to other echolocating bats. These relationships imply that echolocation either evolved twice in bats or evolved once but was later lost in pteropodids. Either scenario would require a complete rethinking of our understanding of the evolutionary history of bats, including new evolutionary explanations for more than 20 different anatomical specializations shared by living echolocating bats, except for pteropodids [5].

4) Weaknesses in prior molecular studies have left some doubt about their interpretation. For example, analyses of different genes yielded incompatible phylogenetic trees, and sampling of living bat families was incomplete, leaving open the possibility that sampling biases were responsible for the surprising molecular results. A new study by Teeling et al [4] overcomes these difficulties by simultaneously analyzing portions of 17 nuclear genes sampled in all extant families. The resulting phylogenetic tree, which is strongly supported by the data, confirms the results of earlier molecular studies. Non-echolocating pteropodids nest among lineages of echolocating bats, implying a dual origin for echolocation or its loss in pteropodids.

References (abridged):

1. N. B. Simmons, in Mammal Species of the World: A Taxonomic and Geographic Reference, D. E. Wilson, D. M. Reeder, Eds. (Johns Hopkins Univ. Press, Baltimore, in press)

2. H. T. Arita, M. B. Fenton, Trends Ecol. Evol. 12, 53 (1997)

3. N. B. Simmons, T. M. Conway, in Bat Ecology, T. H. Kunz, M. B. Fenton, Eds. (Univ. of Chicago Press, Chicago, 2003), pp. 493-535

4. E. C. Teeling et al., Science 307, 580 (2005)

5. N. B. Simmons, J. H. Geisler, Bull. Am. Mus. Nat. Hist. 235, 1 (1998)

Science http://www.sciencemag.org

--------------------------------

Related Material:

ANIMAL BEHAVIOR: ON BAT ECHOLOCATION

The following points are made by B. Fenton and J. Ratcliffe (Nature 2004 429:612):

1) In 1794, Lazzaro Spallanzani (1729-1799) reported experimental results supporting his earlier proposal that bats could "see" with their ears. The famed Georges Cuvier (1769-1832) found the suggestion preposterous(1), however, and it took almost another 150 years for Spallanzani to be vindicated. After repeating many of Spallanzani's experiments, Donald Griffin(2) published the same conclusions in 1940, coining the term "echolocation" to describe how bats use echoes of the sounds they produce to locate objects in their path. A microphone sensitive to sound frequencies above the range of human hearing, a bat detector, allowed Griffin to eavesdrop on what bats said as they flew through an obstacle course in the dark.

2) Today, we know that there is variation between bat species in the design of echolocation calls, which often coincides with differences in their behavior and ecology(3). Kingston and Rossiter(4) and Siemers and Schnitzler(5) have advanced this line of investigation further. Kingston and Rossiter(4) examined the situation in a single species, the large-eared horseshoe bat (Rhinolophus philippinensis), which occurs from southeast Asia to Australia. They showed how echolocation signals can diverge within a species and how this divergence might promote sympatric speciation -- the division of one species into two or more without a geographical barrier. This is a hot and contentious topic in evolutionary biology. In three study areas, Kingston and Rossiter found three distinct variants of large-eared horseshoe bats differing in size, echolocation calls and relatedness. The largest was almost twice as heavy as the smallest, and the sounds dominating their echolocation calls ranged from 27.20.2 kHz in the largest to 53.60.6 kHz in the smallest.

3) The level of detail available to an echolocating bat is a function of the wavelength of the sounds in its echolocation calls, and so differences in the frequencies that dominate its calls influence a bat's auditory scene. Bats using high frequencies (shorter wavelengths) can detect smaller prey than can bats using lower-frequency calls (longer wavelengths). Kingston and Rossiter suggest that the range of echolocation calls in one species would generate "disruptive selection" because larger bats do not have the same access to small prey as do smaller ones. Theirs is the first demonstration of how adaptive evolution in bats, and so speciation, might have been driven through divergences in echolocation signals.

4) Siemers and Schnitzler(5) examined the behavioral consequences of differences in echolocation signals used by similar species of bats to detect prey. In a portable flight-room, they challenged flying individuals of five European species of mouse-eared bats (Myotis species) to detect and attack prey sitting on or close to vegetation. This is presumed to be difficult for the bats because echoes from prey could be masked by echoes -- "clutter" -- from the background. Siemers and Schnitzler standardized the degree of clutter in which the bats operated, and documented their behavior and foraging performance. The five species they used have similar hunting behavior and are placed in the same "foraging guild" of bats (the "edge space aerial/trawling foragers"). The five species might have been expected to perform at the same level, but they did not. Their study is the first to provide empirical evidence that seemingly minor differences in call design can have real behavioral consequences.

References (abridged):

1. Griffin, D. R. Listening in the Dark (Yale Univ. Press, New Haven, 1959)

2. Galambos, R. & Griffin, D. R. Anat. Rec. 78, 95 (1940)

3. Thomas, J., Moss, C. & Vater, M. (eds) Echolocation in Bats and Dolphins (Univ. Chicago Press, 2004)

4. Kingston, T. & Rossiter, S. J. Nature 429, 654-657 (2004)

5. Siemers, B. M. & Schnitzler, H. -U. Nature 429, 657-661 (2004)

Nature http://www.nature.com/nature

ScienceWeek http://scienceweek.com