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ScienceWeek
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
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NEUROBIOLOGY: ULTRAVIOLET VISION IN A BAT
The following points are made by Y. Winter et al (Nature 2003 424:612):
1) Most mammals, with the exception of primates, have dichromatic vision and correspondingly limited color perception(1). Ultraviolet vision was discovered in mammals only a decade ago(2), and in the few rodents and marsupials where it has been found, ultraviolet light is detected by an independent photoreceptor(2,3). Bats orient primarily by echolocation, but they also use vision.
2) All bat species have functional eyes that are used in various contexts(4,5), but their spectral sensitivities and their capacity for spectral discrimination are not known(1). Some bat-pollinated, neotropical plant species have violet blossoms and can even reflect ultraviolet light to a remarkable degree. This raised the question of whether flower bats have dichromatic color vision and whether they perceive the ultraviolet light reflected by some flowers.
3) The authors examined these questions in three behavioral discrimination experiments with neotropical flower bats (four individuals: three female, one male) of the species Glossophaga soricina (Phyllostomidae). First, the detection thresholds for light stimuli at different wavelengths, including the ultraviolet range, were determined. The result was a bimodal spectral-sensitivity function. This led the authors to investigate the ability of G. soricina to discriminate between colors, which as a property of the whole system can be tested only by psychophysical experiments. In a final experiment, the underlying photoreceptor mechanism was examined by using spectral chromatic adaptation. The authors suggest the result proves that these bats are capable of ultraviolet perception through a single receptor mechanism not previously demonstrated in mammals with intact eyes.
4) In summary: The authors demonstrate that a phyllostomid flower bat, Glossophaga soricina, is color-blind but sensitive to ultraviolet light down to a wavelength of 310 nm. behavioral experiments revealed a spectral-sensitivity function with maxima at 510 nm (green) and above 365 nm (ultraviolet). A test for color vision was negative. Chromatic adaptation had the same threshold-elevating effects on ultraviolet and visible test lights, indicating that the same photoreceptor is responsible for both response peaks (ultraviolet and green). Thus, excitation of the beta-band of the visual pigment is the most likely cause of ultraviolet sensitivity. This is a mechanism for ultraviolet vision that has not previously been demonstrated in intact mammalian visual systems.
References (abridged):
1. Jacobs, G. H. The distribution and nature of color vision among the mammals. Biol. Rev. 68, 413-471 (1993)
2. Jacobs, G. H., Neitz, J. & Deegan, J. F. II Retinal receptors in rodents maximally sensitive to ultraviolet light. Nature 353, 655-656 (1991)
3. Jacobs, G. H. & Deegan, J. F. II Sensitivity to ultraviolet light in the gerbil (Meriones unguiculatus): Characteristics and mechanisms. Vision Res. 34, 1433-1441 (1994)
4. Suthers, R., Chase, J. & Braford, B. Visual form discrimination by echolocating bats. Biol. Bull. 137, 535-546 (1969)
5. Chase, J. Differential responses to visual and acoustic cues during escape in the bat Anoura geoffroyi: cue preferences and behavior. Anim. Behav. 31, 526-531 (1983)
Nature http://www.nature.com/nature
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REORGANIZATION OF THE FREQUENCY MAP OF THE AUDITORY CORTEX EVOKED BY CORTICAL ELECTRICAL STIMULATION IN THE BIG BROWN BAT
The following points are made by S.A. Chowdhury and N. Suga (J. Neurophysiol. 2000 83:1856):
1) In a search phase of echolocation, big brown bats, Eptesicus fuscus, emit biosonar pulses at a rate of 10/s and listen to echoes. The authors report that when a short acoustic stimulus was repetitively delivered at this rate, the reorganization of the frequency map of the primary auditory cortex occurred at and around the neurons tuned to the frequency of the acoustic stimulus. Such reorganization became larger when the acoustic stimulus was paired with electrical stimulation of the cortical neurons tuned to the frequency of the acoustic stimulus.
2) This reorganization was mainly due to the decrease in the best frequencies of the neurons that had best frequencies slightly higher than those of the electrically stimulated cortical neurons or the frequency of the acoustic stimulus. Neurons with best frequencies slightly lower than those of the acoustically and/or electrically stimulated neurons slightly increased their best frequencies. These changes resulted in the over-representation of repetitively delivered acoustic stimulus.
3) Because the over-representation resulted in under-representation of other frequencies, the changes increased the contrast of the neural representation of the acoustic stimulus. Best frequency shifts for over-representation were associated with sharpening of frequency-tuning curves of 25% of the neurons studied. Because of the increases in both the contrast of neural representation and the sharpness of tuning, the over-representation of the acoustic stimulus is accompanied with an improvement of analysis of the acoustic stimulus.
Journal of Neurophysiology http://jn.physiology.org
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