Maximum Detection Distance Research Articles

Echolocation calls in Central American emballonurid bats: signal design and call frequency alternation

AbstractIn southern Central America, 10 species of emballonurid bats occur, which are all aerial insectivores: some hunt flying insects preferably away from vegetation in open space, others hunt in edge space near vegetation and one species forages mainly over water. We present a search call design of each species and link signal structure to foraging habitat. All emballonurid bats use a similar type of echolocation call that consists of a central, narrowband component and one or two short, frequency‐modulated sweeps. All calls are multi‐harmonic, generally with most energy concentrated in the second harmonic. The design of search calls is closely related to habitat type, in particular to distance of clutter. Emballonurid bats foraging in edge space near vegetation and over water used higher frequencies, shorter call durations and shorter pulse intervals compared with species mostly hunting in open, uncluttered habitats. Peak frequency correlated negatively with body size. Regular frequency alternation between subsequent calls was typical in the search sequences of four out of 10 species. We discuss several hypotheses regarding the possible role of this frequency alternation, including species identification and partitioning of acoustic channels. Furthermore, we propose a model of how frequency alternation could increase the maximum detection distance of obstacles by marking search calls with different frequencies.

Hull-mounted hydrophones for passive acoustic detection and tracking of sperm whales ( Physeter macrocephalus)

Acoustic monitoring has proven increasingly important for detection and tracking of marine mammals. Most systems rely on towed hydrophones that can be cumbersome to deploy and pose a risk in terms of fouling propellers and failing in rough seas. To alleviate these problems, an acoustic detection system designed for monitoring and tracking vocally active marine mammals, sperm whales in particular, has been developed. The system uses two hydrophones, designed for hull-mounting on each side of the keel of small as well as larger vessels. The system is relatively simple to use and does not require deployment of any equipment. Hydrophone outputs are amplified and filtered by a custom built conditioning circuitry. Bearing is estimated by manoeuvring the vessel to minimize the difference in level between the two hydrophones. Forward-aft ambiguity is resolved by turning the vessel. The system permits acoustic bearing estimations to be carried out while cruising at speeds of 5 knots. The theoretical maximum detection distances of sperm whales are several kilometres, depending on the relative orientation of the animal.

Echolocation call intensity in the aerial hawking batEptesicus bottae(Vespertilionidae) studied using stereo videogrammetry

Aerial hawking bats use intense echolocation calls to search for insect prey. Their calls have evolved into the most intense airborne animal vocalisations. Yet our knowledge about call intensities in the field is restricted to a small number of species. We describe a novel stereo videogrammetry method used to study flight and echolocation behaviour, and to measure call source levels of the aerial hawking bat Eptesicus bottae (Vespertilionidae). Bats flew close to their predicted minimum power speed. Source level increased with call duration; the loudest call of E. bottae was at 133 dB peSPL. The calculated maximum detection distance for large flying objects (e.g. large prey, conspecifics) was up to 21 m. The corresponding maximum echo delay is almost exactly the duration of one wing beat in E. bottae and this also is its preferred pulse interval. These results, obtained by using videogrammetry to track bats in the field, corroborate earlier findings from other species from acoustic tracking methods.

Measurement of an individual silver perch Bairdiella chrysoura sound pressure level in a field recording

Simultaneous audio and video were recorded of a silver perch Bairdiella chrysoura producing its characteristic drumming sound in the field. The background noise contribution to the total sound pressure level is estimated using sounds that occurred between the pulses of the silver perch sound. This background contribution is subtracted from the total sound to give an estimate of the sound pressure level of the individual fish. A silver perch source level in the range 128-135 dB (re: 1 microPa) is obtained using an estimate of the distance between the fish and the hydrophone. The maximum distance at which an individual silver perch could be detected depends on the background sound level as well as the propagation losses. Under the conditions recorded in this study, the maximum detection distance would be 1-7 m from the hydrophone.

Development of high-frequency micro echo sounder

: The authors have developed a micro echo sounder operating at a frequency as high as 1 MHz, which can be attached to aquatic animals in combination with a depth and temperature sensor to obtain information on the animal's prey as well as its behavior. The performance of the sounder was evaluated in an experimental tank and its detection range was estimated to confirm its effectiveness. The beam width is 5.9° and the maximum detection distance is estimated as 34 m when the target strength of an object animal is −50 dB.

LOCALIZATION OF NORTH ATLANTIC RIGHT WHALE SOUNDS IN THE BAY OF FUNDY USING A SONOBUOY ARRAY

AbstractA free‐drifting 14‐sonobuoy array was used to localize North Atlantic right whales (Eubalaena glacialis) in the Grand Manan Basin area of the Bay of Fundy. This area is a primary summer/autumn right whale habitat and overlaps an international shipping lane. The three‐hour deployment on a single day provided two‐dimensional localization of 94 right whale sounds based on arrival time differences determined from spectrogram cross‐correlation analysis. The sounds were of two distinct types: tonal and gunshot. Maximum detection distances were about 30 km for both types of sound. The mean RMS location error was 1.8 km for tonal‐type sounds and 2.5 km for gunshot‐type sounds. The average RMS error was 20% of the average distance from the receiving hydrophones, the primary source of error being uncertainty in the sonobuoy positions.

Assessing visual detection ability for mobility in individuals with low vision

We investigated the mobility performance of legally blind patients with age-related maculopathy (ARM) as assessed by their ability to visually detect hazards in their travel path. The three types of hazards were: drop-offs (a curb), obstacle on the travel surface (a small wastebasket), and a head-height obstacle (a piece of foam insulation projecting across the travel path at head height). We also recorded the patient’s logMAR visual acuity and Pelli-Robson contrast sensitivity. Three distinct daylight assessments were made in a naturalistic outdoor environment: an initial assessment of distance to visually detect each of the three types of hazards; an assessment designed to assess the subject’s maximum potential visual detection distance; and a final assessment following training and fitting with filters to reduce glare to determine the increase, if any, in visual detection distance for the three types of hazards. The inter-rater and test/retest reliabilities of the assessment procedure were studied. The results indicated that many ARM patients are unable to visually detect some hazards at a safe distance, that their ability to improve can be predicted from the assessment, and that most subjects can improve their ability to visually detect obstacles if provided appropriate training and filters. The limitations of this study and their implications are discussed.

Field Study of Driver’s Curve-Detection Performance in Daytime and Nighttime

Effects of road scenes and traffic-control devices on driver’s curve-detection performance were investigated. Field experiments were conducted on a 19-km section of highway running through a hilly area in Hokkaido, Japan. The driver obtained directional information from the road scene ahead and traffic-control devices at the beginning of the target curve. Road scene characteristics were determined subjectively. Configurations of traffic-control devices at 32 curves were obtained from the road maintenance database and were measured on site for each curve. Each of the 17 participating subjects drove an instrument-equipped vehicle and pressed a button as soon as he or she recognized the direction of the target curve. Detection distance of each curve was measured. The experimenter determined the maximum detection distance (MDD) of each target curve. A curve-detection index, defined as the detection distance divided by the MDD, was used to compare detection performance for each curve. Characteristics of detection performance for the curves were determined by a cluster analysis and regression analysis. Curves were classified into five groups according to the results of the cluster analysis. Results of the within-subject regression analysis revealed that subjects driving in the daytime obtained directional information about the curve from the road scene, whereas in the nighttime the lighting midway through the curve had a greater effect on detection performance. Results indicate that visual cues should be considered when traffic-control devices are installed at a curve, and appropriate traffic-control devices should be selected to increase the detection performance of a curve.

Size, peripheral auditory tuning and target strength in noctuid moths

Summary We investigated relationships among body size, the frequency of peak auditory sensitivity (best frequency) and acoustic conspicuousness (measured as target strength) to simulated bat echolocation calls in a range of tympanate moths (Lepidoptera: Noctuidae). Audiograms of Amphipyra pyramidea Linnaeus, Agrotis exclamationis Linnaeus, Omphaloscelis lunosa Haworth and Xestia xanthographa Denis and Schiffermuller are described for the first time. Best frequency was inversely related to forewing length, an index of body size. Models predict that target strength falls off rapidly once wavelength (1/frequency) exceeds some defined feature of target size (e.g. circumference for spheres). We investigated how target strength varies in relation to target size and emitted frequency for simple targets (paper discs) and for moths. Target strength fell rapidly when target radius/wavelength < 2 for paper discs of similar size to many noctuid moths. Target strength fell rapidly below wing-length/wavelength ratios of 2 in relatively small (O. lunosa, wing-length = 15.2 ± 0.4 mm, best frequency = 45 kHz) and large (N. pronuba, wing-length = 24.6 ± 0.8 mm, best frequency = 15 kHz) noctuid species, and decreased rapidly at frequencies below 25 kHz in both species. These target strengths were used to predict the detection distance of the moths by bat sonar between 10 and 55 kHz. Predicted detection distances of both species were maximal for fictive call frequencies of 20 kHz, and were reduced at lower frequencies due to decreased target strength and at higher frequencies by excess atmospheric attenuation. Both relatively large and small noctuid moths are therefore strong acoustic targets to bats that echolocate at relatively low frequencies. Bats may emit allotonic calls at low frequency because the costs of reduced detection range are smaller than the benefits of reduced audibility to moths. Because best frequency scales with body size and maximum detection distance is not very sensitive to body size, noctuid moths in the size range examined do not necessarily have best frequencies that would match the call frequencies of bats that may detect the moths at greatest distance precisely. Hence, best frequency may be constrained in part by body size.

Size, peripheral auditory tuning and target strength in noctuid moths

SummaryWe investigated relationships among body size, the frequency of peak auditory sensitivity (best frequency) and acoustic conspicuousness (measured as target strength) to simulated bat echolocation calls in a range of tympanate moths (Lepidoptera: Noctuidae). Audiograms of Amphipyra pyramidea Linnaeus, Agrotis exclamationis Linnaeus, Omphaloscelis lunosa Haworth and Xestia xanthographa Denis and Schiffermüller are described for the first time. Best frequency was inversely related to forewing length, an index of body size. Models predict that target strength falls off rapidly once wavelength (1/frequency) exceeds some defined feature of target size (e.g. circumference for spheres). We investigated how target strength varies in relation to target size and emitted frequency for simple targets (paper discs) and for moths. Target strength fell rapidly when target radius/wavelength &lt; 2 for paper discs of similar size to many noctuid moths. Target strength fell rapidly below wing‐length/wavelength ratios of 2 in relatively small (O. lunosa, wing‐length = 15.2 ± 0.4 mm, best frequency = 45 kHz) and large (N. pronuba, wing‐length = 24.6 ± 0.8 mm, best frequency = 15 kHz) noctuid species, and decreased rapidly at frequencies below 25 kHz in both species. These target strengths were used to predict the detection distance of the moths by bat sonar between 10 and 55 kHz. Predicted detection distances of both species were maximal for fictive call frequencies of 20 kHz, and were reduced at lower frequencies due to decreased target strength and at higher frequencies by excess atmospheric attenuation. Both relatively large and small noctuid moths are therefore strong acoustic targets to bats that echolocate at relatively low frequencies. Bats may emit allotonic calls at low frequency because the costs of reduced detection range are smaller than the benefits of reduced audibility to moths. Because best frequency scales with body size and maximum detection distance is not very sensitive to body size, noctuid moths in the size range examined do not necessarily have best frequencies that would match the call frequencies of bats that may detect the moths at greatest distance precisely. Hence, best frequency may be constrained in part by body size.

Cognitive Characteristic of Driver's Curve Detection in terms of Road Scene Factors

In this study, the effects of road scenes and traffic control devices on a driver's curve detection performance were investigated. The field experiments were conducted on an 11 -km section of highway running through Shikotsu-Toya National Park in Hokkaido. The driver obtains directional information from the road scene ahead and from traffic control devices on the corner of the target curve. The characteristics of the road scene were determined subjectively. The configurations of traffic control devices at 21 curves were obtained from the road maintenance database and measured at site for each curve. Twenty-three subjects participated in the experiment. Each of the subjects drove an inst rumented vehicle and pressed a button as soon as she/he recognized the direction of the target curve. The detection distance of each curve was measured. The maximum detection distance (MDD) of each target curve was determined by the experimenter. A curve detection index (CDI), defined as (MDD-detection distance) divided by MDD, was employed in order to compare detection performance for each of 21 curves. The characteristics of detection performance for all of the 21 curves were determined by the cluster analysis and multiple regression analysis. The 21 curves were classified into 4 groups according to the results of cluster analysis. The results of multiple regression analysis revealed that the subject's driving in the daytime obtained directional information about the curve from the road scene, while their driving at night obtained directional information from traffic control devices such as guardrails, utility poles and chevron signs. The results of this study indicate that viewing cues should be considered when traffic control devices set up at the side of curve, and appropriate traffic control devices should be selected to enable safe detection of a curve. It should be noted, however, that further investigations in terms of cues and CDI are required for various curves and conditions.

Target detection by an echolocating harbor porpoise (Phocoena phocoena).

Two echolocation experiments are described. They were conducted on the same harbor porpoise housed in a sea pen, one year apart at Neeltje Jans, The Netherlands. The aims were to determine the target detection ability of an echolocating harbor porpoise, with the ultimate goal to predict the distance at which harbor porpoises can detect fishing nets. In experiment 1, the maximum distance at which the 3-year-old porpoise could detect a 7.62-cm diameter water-filled stainless-steel sphere by echolocation was determined psychophysically. The 50%-current detection threshold was reached when the sphere was at a distance of 26 m from the porpoise's rostrum. In experiment 2, conducted a year later, the maximum detection distance for a 5.08-cm water-filled stainless-steel sphere was 15.9 m. The target strengths of both targets were measured using simulated harbor porpoise echolocation signals and the results, coupled with transmission-loss calculations, indicated that the echo levels received by the porpoise with the targets at the threshold ranges in the two experiments were only 1.3 dB apart. Together with information on the target strengths of various fishing nets, the results of the present study can be used to predict the distance at which the nets can be detected by harbor porpoises.

Does echolocation wavelength restrict bats’ choice of prey?

The size range of insects encountered by bats that use narrowband echolocation signals (FM-CF or quasi-CF) extends to sizes much smaller than the wavelengths used by these bat species. It was hypothesized that the expected Rayleigh scattering sets a lower limit on the size of prey which can be detected. To test the Rayleigh scattering theory empirically, insects were ensonified with pure-tone pulses ranging from 20 to 85 kHz and their target strengths were measured. The target strength of the smaller insects was frequency-dependent within the frequency range used by aerial-hawking bats. The target strength diminished sharply as the wing length to wavelength ratio decreased below unity. A model was developed to predict maximum detection distances, which lead to an estimate of the optimum frequency to be used for detecting each insect size. In a dietary analysis, it was found that the bat species using the shortest wavelengths took the smallest insects. The minimum prey size detectable by each bat species was predicted. The longest wavelength bats (Vespertilio murinus, Nyctalus noctula) took smaller prey than were estimated. The diets of the shorter wavelength bats agreed with predictions of the model. [Work supported by BBSRC.]

Density-dependent predation by skunks using olfactory search images.

The formation of search images can create density-dependent predation. Predators have been shown to form search images when searching for many small prey items in one feeding session. This paper reports experiments that test whether striped skunks can form olfactory search images in other situations: when prey are found over several days, when prey are large, and when prey are found in certain habitats. Striped skunks were raised in captivity, and their reaction distance to food was measured outside in a natural grassy area. In experiment 1 skunks were offered many small food items for several days in a row. From one day to the next, skunks initially detected food from further away, they increased detection distance faster and their maximum detection distance increased - i.e., they formed olfactory search images faster and stronger from one day to the next. In experiment 2 skunks formed search images over several days when finding only one large food item per day. In experiment 3 skunks lost olfactory search images when they entered habitats in which they had previously searched for another type of food. These long-term search images magnify the effects of short-term search images, extend the effects of short- term search image to longer time spans, and affect different species from short-term search images.

Comparative foraging adaptations of small raptors in a dense African savanna

Our aim was to investigate how ecologically similar species with different evolutionary adaptations may coexist in a moist Guinean savanna, which is the optimal habitat of the Lizard Buzzard Kaupifalco monogrammicus whereas the Grey Kestrel Falco ardosiaceus and the Grasshopper Buzzard Butastur rufipennis are of more Sudanian origin. Activity budgets, foraging behaviour and success were assessed by focal sampling and were related to habitat parameters and prey availability. Each species had its own daily foraging pattern which seemed to be related to the activity patterns of their main prey, grasshoppers or lizards. Interspecific habitat segregation reflected their morphological and behavioural adaptations and their respective tolerance to grass height, the most important factor in habitat selection. The resident Lizard Buzzard selected the densest woodlands, whereas the Grey Kestrel was restricted to the most open areas and the dry season migrant Grasshopper Buzzard to intermediate, recently burnt, savanna types. The main foraging variables (perch height, inter‐perch distance and frequency of moves) differed significantly between species. Maximum detection distances increased with perch height but decreased with increasing grass cover. Perch height was lower when the grass became too high. When attack rates increased, birds tended to move more often, but for shorter distances, and stayed longer within restricted patches. The highly insectivorous Grasshopper Buzzard had the highest and least variable attack rates, negatively correlated with grass height. It exhibited a behaviour typical of a migrant—superficial and opportunistic exploitation of superabundant, easily accessible, and seasonal food sources. Conversely, the residents were associated with denser, more stable habitats and larger, less abundant, but more predictable prey.