An acoustic valve within the nose of sperm whales Physeter macrocephalus

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Determinants of echolocation click frequency characteristics in small toothed whales: recent advances from anatomical information

1. The pulse-like clicking sounds made by odontocetes for echolocation (biosonar) can be roughly classified by their frequency characteristics into narrow-band high-frequency (NBHF) clicks with a sharp peak at around 130 kHz and wide-band (WB) clicks with a moderate peak at 30–100 kHz. Structural differences in the sound-producing organs between NBHF species and WB species have not been comprehensively discussed, nor has the formation of NBHF and WB clicks.
2. A review of the sound-producing organs, including the latest findings, could lead to a new hypothesis about the sound production mechanisms. In the current review, data on echolocation click characteristics and on the anatomical structure of the sound-producing organs were compared in 33 species (14 NBHF species and 19 WB species).
3. We review interspecific information on the characteristics of click frequencies and data from computed tomography scans and morphology of the sound-producing organs, accumulated in conventional studies. The morphology of several characteristic structures, such as the melon, the dense connective tissue over the melon (the ‘porpoise capsule’), and the vestibular sacs, was compared interspecifically.
4. Interspecific comparisons suggest that the presence or absence of the porpoise capsule is unlikely to affect echolocation frequency. Folded structures in the vestibular sacs, features that have been overlooked until now, are present in most species with NBHF sound production and not in WB species; the vestibular sacs are therefore likely to be important in determining echolocation click frequency characteristics. The acoustical properties of the shape of the melon and vestibular sacs are important topics for future investigations about the relationship between anatomical structure and sound-producing mechanisms for echolocation clicks.

     

Hearing sensation levels of emitted biosonar clicks in an echolocating Atlantic bottlenose dolphin

Emitted biosonar clicks and auditory evoked potential (AEP) responses triggered by the clicks were synchronously recorded during echolocation in an Atlantic bottlenose dolphin (Tursiops truncatus) trained to wear suction-cup EEG electrodes and to detect targets by echolocation. Three targets with target strengths of -34, -28, and -22 dB were used at distances of 2 to 6.5 m for each target. The AEP responses were sorted according to the corresponding emitted click source levels in 5-dB bins and averaged within each bin to extract biosonar click-related AEPs from noise. The AEP amplitudes were measured peak-to-peak and plotted as a function of click source levels for each target type, distance, and target-present or target-absent condition. Hearing sensation levels of the biosonar clicks were evaluated by comparing the functions of the biosonar click-related AEP amplitude-versus-click source level to a function of external (in free field) click-related AEP amplitude-versus-click sound pressure level. The results indicated that the dolphin's hearing sensation levels to her own biosonar clicks were equal to that of external clicks with sound pressure levels 16 to 36 dB lower than the biosonar click source levels, varying with target type, distance, and condition. These data may be assumed to indicate that the bottlenose dolphin possesses effective protection mechanisms to isolate the self-produced intense biosonar beam from the animal's ears during echolocation.     

The relationship between the acoustic behaviour and surface activity of killer whales (<Emphasis Type="Italic">Orcinus orca</Emphasis>) that feed on herring (<Emphasis Type="Italic">Clupea harengus</Emphasis>)

We describe the acoustic behaviour of piscivorous killer whales in Norwegian and Icelandic waters. Whales were assigned to one of three activities (feeding, travelling or other), and sound recordings were made in their proximity with a single hydrophone and a digital audiotape (DAT) recorder. A quantitative analysis of the production of pulsed calls, whistles and echolocation clicks in the three activities revealed that there was a significant effect of activity on the production of these sound types. Both killer whales in Icelandic and Norwegian waters produced high rates of clicks and calls during feeding and low rates of click, calls and whistles during travelling. The differences can be used as acoustical markers and provides new possibilities for acoustic monitoring of killer whales in these areas. Based on the similarity between their prey choice, hunting strategies, phenotype and acoustic behaviour, we suggest that the killer whales in Icelandic and Norwegian waters belong to the same ecotype: Scandinavian herring-eating killer whales.     

Underwater acoustic communication in the macrophagic carnivorous larvae of Ceratophrys ornata (Anura: Ceratophryidae)

Natale, G.S., Alcalde, L., Herrera, R., Cajade, R., Schaefer, E.F., Marangoni, F. and Trudeau, V.L. 2011. Underwater acoustic communication in the macrophagic carnivorous larvae of Ceratophrys ornata (Anura: Ceratophryidae). —Acta Zoologica (Stockholm) 92 : 46–53. We provide the first evidence for sound production by anuran larvae. In this study, we describe the sounds, their context‐specific emission and the structures related to sound production of the carnivorous larvae of Ceratophrys ornata (Amphibia, Anura, Ceratophryidae). Tadpoles emit a brief, clear and very audible metallic‐like sound that consists of a short train of notes that occur at all stages of larval development. Tadpoles make sound only when a conspecific tadpole is preying upon it or when touched by an object. Ceratophrys ornata larvae possess the basic required anatomical structures for sound production via expulsion of atmospheric air from the lungs through the open soft‐tissue glottis. The glottis is opened and closed via the larval laryngeal muscles (constrictor laryngis and dilatator laryngis). The arytenoid cartilages appear at stage 40 and the cricoid cartilage does at stage 43. Adult laryngeal muscles differentiate from the larval ones at stage 46 together with the vocal sac formation from the adult interhyoideus muscle. We demonstrate (n = 2160 conspecific predator–prey interactions) that larval sounds occur always under predatory attack, probably serving to diminish the chances of cannibalism. These data raise the possibility that other macrophagic carnivorous anuran larvae may produce sound.     

KILLER WHALE (ORCINUS ORCA) AUDITORY EVOKED POTENTIALS TO RHYTHMIC CLICKS

Temporal auditory mechanisms were measured in killer whales ( Orcinus orca ) by recording auditory evoked potentials (AEPs) to clicks. Clicks were presented at rates from 10/sec to 1,600/sec. At low rates, clicks evoked an AEP similar to the auditory brainstem response (ABR) of other odontocetes; however, peak latencies of the main waves were 3–3.7 msec longer than in bottlenose dolphins. Fourier analysis of the ABR showed a prominent peak at 300–400 Hz and a smaller one at 800–1,200 Hz. High-rate click presentation (more than 100/sec) evoked a rate-following response (RFR). The RFR amplitude depended little on rate up to 400/sec, decreased at higher rates and became undetectable at 1,120/sec. Fourier analysis showed that RFR fundamental amplitude dependence on frequency closely resembled the ABR spectrum. The fundamental could follow clicks to around 1,000/sec, although higher harmonics of lower rates could arise at frequencies as high as 1,200 Hz. Both RFR fundamental phase dependence on frequency and the response lag after a click train indicated an RFR group delay of around 7.5 msec. This corresponds to the latency of ABR waves PIII-NIV, which indicates the RFR originates as a rhythmic, overlapping ABR sequence. The data suggest the killer whale auditory system can follow high click rates, an ability that may have been selected for as a function of high-frequency hearing and the use of rapid clicks in echolocation.     

Male courtship ultrasound produced by mesothoracic tymbal organs in the yellow peach moth <Emphasis Type="Italic">Conogethes punctiferalis</Emphasis> (Lepidoptera: Crambidae)

Insect sound-producing apparatuses are mostly classified into two types: file–scraper and tymbal. Structures and locations of these organs are conserved in some phylogenetic groups, e.g., crickets, grasshoppers, and cicadas. However, moths have evolved diversified sound-producing organs, such as wing castanets and proboscis, in addition to the file–scraper and tymbal, in each species. Here we demonstrate that the yellow peach moth Conogethes punctiferalis (Guenée) (Lepidoptera: Crambidae) has developed mesothoracic tymbal organs never reported so far in insects. Tymbals are male specific and used for generating ultrasonic clicks in mating. We found eight to nine striae on the smooth surface of the tymbal membrane, suggesting the production of several clicks by a single buckle of the membrane in association with contraction/relaxation of the mesothoracic muscles. Acoustic data from click sequences support the idea that the series is generated by side-to-side asynchrony with an active/passive half cycle by an inward/outward buckle, and thus in click group (pulse) production, males emit 28 clicks with the right and left tymbals. The click-producing mechanism is similar, but not homologous, to those of other clicking species in five moth families. Thus, moths have acquired tymbal organs through independent and convergent evolution.     

Pulsed signal properties of free‐ranging bottlenose dolphins (Tursiops truncatus) in the central Mediterranean Sea

This study describes pulsed signals from bottlenose dolphins of the central Mediterranean Sea. Data were collected during 2011 and 2012 in 27 surveys in the Sicilian Channel, during which 163 animals were sighted. Based mainly on the pulse repetition rate, the signals were classified as Low‐frequency click (LF; single clicks without a regular pulse rate), Train click (TC; with a interclick interval of 80 ± 2 ms), Burst (with a interclick interval of 3.4 ± 0.2 ms), or Packed click (with a lower number of clicks per train and median interclick interval of 3.2 ± 0.0 ms). The measured parameters were peak sound pressure level (SPL_pk); signal duration; the 1st, 2nd, and 3rd peak of frequency; number of peaks frequency; bandwidth; centroid frequency; and the 10th, 25th, 75th, and 90th percentiles of the power spectrum distribution. Most of the parameters were significantly different among the groups, reflecting the different functions of these signals. LF clicks showed a lower peak frequency and percentiles and a longer duration and could be used to explore a wide area without a specific target focalization and with less resolution. The TC showed a higher SPL_pk, higher peak frequency, lower duration, and lower number of secondary peaks frequency, showing a better resolution to investigate a specific target.     

'Megapclicks': acoustic click trains and buzzes produced during night-time foraging of humpback whales (Megaptera novaeangliae)

Humpback whales (Megaptera novaeangliae) exhibit a variety of foraging behaviours, but neither they nor any baleen whale are known to produce broadband clicks in association with feeding, as do many odontocetes. We recorded underwater behaviour of humpback whales in a northwest Atlantic feeding area using suction-cup attached, multi-sensor, acoustic tags (DTAGs). Here we describe the first recordings of click production associated with underwater lunges from baleen whales. Recordings of over 34000 'megapclicks' from two whales indicated relatively low received levels at the tag (between 143 and 154dB re 1 microPa pp), most energy below 2kHz, and interclick intervals often decreasing towards the end of click trains to form a buzz. All clicks were recorded during night-time hours. Sharp body rolls also occurred at the end of click bouts containing buzzes, suggesting feeding events. This acoustic behaviour seems to form part of a night-time feeding tactic for humpbacks and also expands the known acoustic repertoire of baleen whales in general.     

THE CLICK-SOUNDS OF NARWHALS (MONODON MONOCEROS) IN INGLEFIELD BAY, NORTHWEST GREENLAND

We studied the sounds of narwhals ( Monodon monoceros ) foraging in the open waters in Northwest Greenland. We used a linear, vertical array of three hydrophones (depth 10 m, 30 m, 100 m) with a fourth hydrophone (depth 30 m) about 20 m from the vertical array. A smaller fifth hydrophone (depth 2 m) allowed for registering frequencies up to 125 kHz (± 2 dB) when signals were recorded at 762 mm/set on an instrumentation tape recorder. Clicks were the prevalent signals, but we heard whistles occasionally. We separated the clicks into two classes: click trains that had rates of 3-10 clicks/sec and click bursts having rates of 110-150 clicks/sec. The spectra of train clicks had maximum amplitudes at 48 ± 10 kHz and a duration of 29 ± 6 psec. The spectra of burst clicks had maximum amplitudes at 19 ± 1 kHz and a duration of 40 ± 3 psec. By analogy with other dolphin species, narwhals presumably use the clicks for echolocation during orientation and for locating prey. The narwhal click patterns resemble those of insectivorous bats. Click trains might correspond to bat searching signals and click bursts to the bat's terminal "buzz", emitted just before prey capture.     

Transmission and perception of acoustic signals in the desert clicker,Ligurotettix coquilletti (Orthoptera: Acrididae)

Males of the desert clicker, Ligurotettix coquilletti (Acrididae: Orthoptera) defend a femalerequired resource, the creosote bush Larrea tridentata, in desert habitats of the southwestern United States. Males signal acoustically to each other as well as to searching females. The call is produced by tegminal/femoral stridulation where one or both legs are used for sound production. Sound pressure levels, measured laterally, are influenced by the intervening tegmen between the stridulating leg and the microphone. Differences in measured sound pressure levels between sides can vary up to 7 dB. When clicks are produced multiply,these multiple clicks may be 4 dB louder than single clicks. We examine the structure of the call and the effective broadcast area of single males by monitoring acoustic ascending neurons of the ventral nerve cord in the neck. By taking the neurophysiological preparation into the field, we were able to map the broadcast area of isolated males and also of males calling within aggregations. The distance over which the signal of isolated males could be detected was 8–14 m, whereas neural representation of the calls of males within aggregation were detectable within 4–6 m. The sound spectrum of the song, although having a major lower-frequency component around 10 kHz, has extensive power in the ultrasonic range. The tuning characteristics of the ascending auditory neuron matched the overall structure of the male call. The importance of the acoustic cue, as compared to visual cues, is discussed in relation to female attraction.     

Short-latency auditory evoked potentials during change in the physical parameter of a sound stimulus

The amplitude-temporal and spectral characteristics of the short-latency auditory evoked potentials (SLAEP) recorded under conditions of monoaural stimulation with sound clicks with initial phase of rarefaction followed by compression and alteration, with the intensity of 60 dB and frequency of 11.1 Hz, were studied in ipsi- and contralateral derivations. Substantial changes in SLAEP morphology in response to polarity inversion of the acoustic stimulus were found. Waves II, IV, VI, and VII changed to the greatest extent. The spectral analysis detected three main SLAEP components: low- (LF), medium- (MF), and high-frequency (HF) components as well as the respective frequency bands. Change in the click phase from rarefaction to compression resulted in bilateral redistribution of power between the MF and HF components. This was expressed as a decrease in the HF peak power and simultaneous rise of MF power. Selective effects of the polarity inversion of the sound stimulus on the MF and HF components support the finding that the activity of SLAEP-generating structures are mainly reflected in the mentioned components. It is suggested that two populations of phase-sensitive units are represented in the auditory analyzer. These populations determine the characteristic changes in SLAEP morphology and spectral characteristics.     

Experimental analysis of lens-forming capacity in Xenopus borealis larvae

Previously, the only anuran amphibians known to have the capacity to regenerate a lens after lentectomy were Xenopus laevis and Xenopus tropicalis. This regeneration process occurs during the larval life through transdifferentiation of the outer cornea promoted by inductive factors produced by the retina and accumulated inside the vitreous chamber. However, the capacity of X. tropicalis to regenerate a lens is much lower than that of X. laevis. This study demonstrates that Xenopus borealis, a species more closely related to X. laevis than to X. tropicalis, is not able to regenerate a lens after lentectomy. Nevertheless, some morphological modifications corresponding to the first stages of lens regeneration in X. laevis were observed in the outer cornea of X. borealis. This suggested that in X borealis the regeneration process was blocked at early stages. Results from histological analysis of X. borealis and X. laevis lentectomized eyes and from implantation of outer cornea fragments into the vitreous and anterior chambers demonstrated that: (i) in X. borealis eye, the lens-forming competence in the outer cornea and inductive factors in the vitreous chamber are both present, (ii) no inhibiting factors are present in the anterior chamber, the environment where lens regeneration begins, (iii) the inability of X. borealis to regenerate a lens after lentectomy is due to an inhibiting action exerted by the inner cornea on the spreading of the retinal factor from the vitreous chamber towards the outer cornea. This mechanical inhibition is assured by two distinctive features of X. borealis eye in comparison with X. laevis eye: (i) a weaker and slower response to the retinal inducer by the outer cornea; (ii) a stronger and faster healing of the inner cornea. Unlike X. tropicalis and similar to X. laevis, in X. borealis the competence to respond to the retinal factor is not restricted to the corneal epithelium but also extends to the pericorneal epidermis.     

Superfast vocal muscles control song production in songbirds

Birdsong is a widely used model for vocal learning and human speech, which exhibits high temporal and acoustic diversity. Rapid acoustic modulations are thought to arise from the vocal organ, the syrinx, by passive interactions between the two independent sound generators or intrinsic nonlinear dynamics of sound generating structures. Additionally, direct neuromuscular control could produce such rapid and precisely timed acoustic features if syringeal muscles exhibit rare superfast muscle contractile kinetics. However, no direct evidence exists that avian vocal muscles can produce modulations at such high rates. Here, we show that 1) syringeal muscles are active in phase with sound modulations during song over 200 Hz, 2) direct stimulation of the muscles in situ produces sound modulations at the frequency observed during singing, and that 3) syringeal muscles produce mechanical work at the required frequencies and up to 250 Hz in vitro. The twitch kinematics of these so-called superfast muscles are the fastest measured in any vertebrate muscle. Superfast vocal muscles enable birds to directly control the generation of many observed rapid acoustic changes and to actuate the millisecond precision of neural activity into precise temporal vocal control. Furthermore, birds now join the list of vertebrate classes in which superfast muscle kinetics evolved independently for acoustic communication.     

Anti-bat tiger moth sounds: Form and function

The night sky is the venue of an ancient acoustic battle between echolocating bats and their insect prey. Many tiger moths (Lepidoptera: Arctiidae) answer the attack calls of bats with a barrage of high frequency clicks. Some moth species use these clicks for acoustic aposematism and mimicry, and others for sonar jamming, however, most of the work on these defensive functions has been done on individual moth species. We here analyze the diversity of structure in tiger moth sounds from 26 spe-cies collected at three locations in North and South America. A principal components analysis of the anti-bat tiger moth sounds reveals that they vary markedly along three axes: (1) frequency, (2) duty cycle (sound production per unit time) and frequency modulation, and (3) modulation cycle (clicks produced during flexion and relaxation of the sound producing tymbal) structure. Tiger moth species appear to cluster into two distinct groups: one with low duty cycle and few clicks per modulation cycle that supports an acoustic aposematism function, and a second with high duty cycle and many clicks per modulation cycle that is con-sistent with a sonar jamming function. This is the first evidence from a community-level analysis to support multiple functions for tiger moth sounds. We also provide evidence supporting an evolutionary history for the development of these strategies. Further-more, cross-correlation and spectrogram correlation measurements failed to support a "phantom echo" mechanism underlying sonar jamming, and instead point towards echo interference.     

Tiger moths and the threat of bats: decision-making based on the activity of a single sensory neuron

Echolocating bats and eared moths are a model system of predator–prey interaction within an almost exclusively auditory world. Through selective pressures from aerial-hawking bats, noctuoid moths have evolved simple ears that contain one to two auditory neurons and function to detect bat echolocation calls and initiate defensive flight behaviours. Among these moths, some chemically defended and mimetic tiger moths also produce ultrasonic clicks in response to bat echolocation calls; these defensive signals are effective warning signals and may interfere with bats'' ability to process echoic information. Here, we demonstrate that the activity of a single auditory neuron (the A1 cell) provides sufficient information for the toxic dogbane tiger moth, Cycnia tenera, to decide when to initiate defensive sound production in the face of bats. Thus, despite previous suggestions to the contrary, these moths'' only other auditory neuron, the less sensitive A2 cell, is not necessary for initiating sound production. However, we found a positive linear relationship between combined A1 and A2 activity and the number of clicks the dogbane tiger moth produces.