Papers by Jakob Christensen-dalsgaard
Sensory losses or reductions are frequently attributed to relaxed selection. However, anuran spec... more Sensory losses or reductions are frequently attributed to relaxed selection. However, anuran species have lost tympanic middle ears many times, despite anurans' use of acoustic communication and the benefit of middle ears for hearing airborne sound. Here we determine whether pre-existing alternative sensory pathways enable anurans lacking tympanic middle ears (termed earless anurans) to hear airborne sound as well as eared species or to better sense vibrations in the environment. We used auditory brainstem recordings to compare hearing and vibrational sensitivity among 10 species (six eared, four earless) within the Neotropical true toad family (Bufonidae). We found that species lacking middle ears are less sensitive to high-frequency sounds, however, low-frequency hearing and vibrational sensitivity are equivalent between eared and earless species. Furthermore, extratympanic hearing sensitivity varies among earless species, highlighting potential species differences in extratympanic hearing mechanisms. We argue that ancestral bufonids may have sufficient extratympanic hearing and vibrational sensitivity such that earless lineages tolerated the loss of high frequency hearing sensitivity by adopting species-specific behavioural strategies to detect conspecifics, predators and prey.
bioRxiv (Cold Spring Harbor Laboratory), May 10, 2022
List of Symbols: c sound velocity f frequency λ wavelength k wave number a radius p sound pressur... more List of Symbols: c sound velocity f frequency λ wavelength k wave number a radius p sound pressure Z specific impedance vp particle velocity v vibration velocity acc acceleration ρ (rho) density .
Hearing Research, Jun 1, 2023
Hearing Research, Jul 1, 2022
In-air and underwater audiograms and directional hearing abilities were measured in humans. The l... more In-air and underwater audiograms and directional hearing abilities were measured in humans. The lowest underwater thresholds were 2.8 µW/m2 or 3.6 mPa at a frequency of 500 Hz. The underwater hearing thresholds were 4-26 dB and 40-62 dB higher than in-air hearing thresholds when measured in intensity and pressure units, respectively. This difference is considerably smaller than what has been reported earlier. At frequencies below 1 kHz, when measured in units of particle velocity, the underwater threshold was much lower than published bone conduction thresholds, suggesting that underwater hearing is not always mediated by bone conduction pathways to the inner ear, as previously thought. We suggest it is the resonance of air in the air-filled middle ear that produces the low underwater thresholds, at least at frequencies below 1 kHz. The ability to determine the direction of a 700 Hz underwater sound source while being blindfolded was extremely poor, with submerged test subjects showing only coarse directional hearing abilities at azimuths of less than 50˚. The physical cues to sound direction are different in air and water, and the poor directional hearing abilities indicate that, in spite of low hearing thresholds, humans have no special adaptations to process directional acoustic cues under water.
The Journal of Experimental Biology, Jun 15, 2022
ABSTRACTThe ability to sense and localize sound is so advantageous for survival that it is diffic... more ABSTRACTThe ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear – which we normally regard as a prerequisite for sensitive hearing on land – in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in ‘earless’ atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in ‘earless’ atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.
The last several decades of research have seen a burgeoning of data on the morphology, physiology... more The last several decades of research have seen a burgeoning of data on the morphology, physiology, and evolutionary history of vertebrate auditory organs. This chapter briefly describes the status of our understanding of ear structure and function and their origins in fish, which hear using their vestibular epithelia, and land vertebrates that early evolved dedicated hearing structures. The various major lineages of land vertebrates—amphibians, lepidosaurs, archosaurs, and mammals—each have unique hearing organs. From humble beginnings as a small epithelium in their common ancestor, each lineage evolved specialized hair-cell populations and divisions of labor that led to highly sensitive and frequency-selective hearing. This chapter covers the origins, morphology, and physiological characteristics of the ears of all major groups.
Elsevier eBooks, 2017
The central auditory system is organized similarly in all vertebrates and may reflect the organiz... more The central auditory system is organized similarly in all vertebrates and may reflect the organizational pattern common to the octaval system. In fishes, multiple endorgans project to all of the octaval nuclei, and there is no single nucleus dedicated to auditory processing. In contrast, distinct auditory pathways characterize all tetrapods. Middle ears developed independently in the different tetrapod lineages and resulted in both increased sensitivity to airborne sound and to sensitivity to a wider range of frequencies than their aquatic ancestors. The increase in frequency response was accompanied by differentiation and expansion of the central auditory system. Physiological studies of all vertebrate groups reveal similar mechanisms to encode sound. Phase locking, frequency tuning, the emergence of binaural comparisons, feature detection, and convergence of sensory modalities are found throughout. The central auditory systems in modern vertebrates are marked by many examples of similar solutions to the problem of how to process auditory stimuli.
Biology Letters, Sep 8, 2010
Lungfishes are the closest living relatives of the tetrapods, and the ear of recent lungfishes re... more Lungfishes are the closest living relatives of the tetrapods, and the ear of recent lungfishes resembles the tetrapod ear more than the ear of ray-finned fishes and is therefore of interest for understanding the evolution of hearing in the early tetrapods. The water-to-land transition resulted in major changes in the tetrapod ear associated with the detection of airborne sound pressure, as evidenced by the late and independent origins of tympanic ears in all of the major tetrapod groups. To investigate lungfish pressure and vibration detection, we measured the sensitivity and frequency responses of five West African lungfish (Protopterus annectens) using brainstem potentials evoked by calibrated sound and vibration stimuli in air and water. We find that the lungfish ear has good low-frequency vibration sensitivity, like recent amphibians, but poor sensitivity to airborne sound. The skull shows measurable vibrations above 100 Hz when stimulated by airborne sound, but the ear is apparently insensitive at these frequencies, suggesting that the lungfish ear is neither adapted nor pre-adapted for aerial hearing. Thus, if the lungfish ear is a model of the ear of early tetrapods, their auditory sensitivity was limited to very low frequencies on land, mostly mediated by substrate-borne vibrations.
Uploads
Papers by Jakob Christensen-dalsgaard