The complex evolutionary history of the tympanic middle ear in frogs and toads (Anura)

Re-examination of the oldest known frog from South America: New data prompt new evolutionary interpretations

Available paleontological evidence, although scarce, points to the early diversification of salientian lissamphibians in the Triassic and Early Jurassic. This study provides new key anatomical information on the earliest mostly articulated frog currently known, thereby improving our understanding of the early evolution of this lissamphibian lineage. Herein, available specimens of the Early Jurassic Vieraella herbstii from Patagonia, which consist of dorsal and ventral imprints of the incomplete, partially articulated skeleton of a single individual, are thoroughly redescribed. Although we comment on its known features, we focus on those that had been misinterpreted or overlooked previously. Among other features, we address the relative proportions of the skull regions and limbs, the morphology of the vomers, the peculiar articulation of the palatine flanges of premaxilla and maxilla that suggests the presence of well-developed cristae subnasales, and the presence of prepollex. The most surprising, significant findings are the presence of stapes, indicating the possibility of a complete tympanic middle ear, and of a short, tapering urostyle and postsacral vertebral elements. Based on available information, we present partial reconstructions of the pectoral girdle and the skeleton of Vieraella in the dorsal aspect. Discussion of the evolutionary significance of these features and perusal of the phylogenetic and functional analyses that included Vieraella in the taxon sampling highlight the necessity of thorough revision of the scorings and measurements, incorporating information presented herein. This reassessment will be relevant not only to clarify its relationships but also to provide sound insights into the early diversification of frogs.

Auditory sensitivity and tympanic middle ear in a vocal and a non-vocal frog

The tympanic middle ear is important for anuran hearing on land. However, many species have partly or entirely lost their tympanic apparatus. Previous studies have compared hearing sensitivities in species that possess and lack tympanic membranes capable of sound production and acoustic communication. However, little is known about how these hearing abilities are comparable to those of mutant species. Here, we compared the eardrum and middle ear anatomies of two sympatric sibling species from a noisy stream habitat, namely the "non-vocal" Hainan torrent frog (Amolops hainanensis) and the "vocal" little torrent frog (Amolops torrentis), the latter of which is capable of acoustic communication. Our results showed that the relative (to head size) eardrum diameter of A. hainanensis was smaller than that of A. torrentis, although the absolute size was not smaller. Unlike A. torrentis, the tympanic membrane area of A. hainanensis was not clearly differentiated from the surrounding skin. The middle ear, however, was well-developed in both species. We measured the auditory brainstem responses (ABRs) of A. hainanensis and compared the ABR thresholds and latencies to those previously obtained for A. torrentis. Our results suggested that these two species exhibited significant differences in hearing sensitivity. A. hainanensis (smaller relative eardrum, nonvocal) had higher ABR thresholds and longer initial response times than A. torrentis (larger relative eardrum, vocal) at lower frequencies. Neurophysiological responses from the brain were obtained for tone pips between 800 Hz and 7,000 Hz, with peak sensitivities found at 3,000 Hz (73 dB SPL) for A. hainanensis, and at 1,800 Hz (61 dB SPL) for A. torrentis. Our results suggest that the non-vocal A. hainanensis has lower hearing sensitivity than its vocal sister species (i.e., A. torrentis), which may be related to differences in tympanic or inner ear structure and morphology.

Harlequin frog tadpoles-comparative buccopharyngeal morphology in the gastromyzophorous tadpoles of the genus Atelopus (Amphibia, Anura, Bufonidae), with discussion on the phylogenetic and evolutionary implication of characters

The Neotropical genus Atelopus is the most diverse genus of bufonids comprising 99 species. Tadpoles of these frogs are readily distinguished based on the presence of a belly sucker, used by them to stay attached to rocks in fast-flowing streams. Despite their intriguing biology, information about their anatomy is scarce and many morphological systems are unknown. We describe the buccopharyngeal cavity of five Atelopus species. The Atelopus buccopharyngeal cavity is characterized by (1) presence of a pendulum-like papillae in the prenarial arena, (2) presence of a glandular zone in the prenarial arena, (3) narial vacuities, (4) conical median ridge, (5) absence of buccal roof arena papillae, (6) absence of buccal roof pustulations, (7) single pair of infralabial papillae, (8) absence of lingual papillae, and (9) absence of pustulations in the buccal floor. We propose that characters 1, 2, and 3 are new synapomorphies for the genus. We also propose that the presence of a single pair of infralabial papillae is a synapomorphy for bufonid. Finally, we discuss the convergent evolution of gastromyzophorous and suctorial tadpoles withing anurans.

Convergent Anuran Middle Ear Loss Lacks a Universal, Adaptive Explanation

INTRODUCTION

Shared selection pressures often explain convergent trait loss, yet anurans (frogs and toads) have lost their middle ears at least 38 times with no obvious shared selection pressures unifying "earless" taxa. Anuran tympanic middle ear loss is especially perplexing because acoustic communication is dominant within Anura and tympanic middle ears enhance airborne hearing in most tetrapods.

METHODS

Here, we use phylogenetic comparative methods to examine whether particular geographic ranges, microhabitats, activity patterns, or aspects of acoustic communication are associated with anuran tympanic middle ear loss.

RESULTS

Although we find some differences between the geographic ranges of eared and earless species on average, there is plenty of overlap between the geographic distributions of eared and earless species. Additionally, we find a higher prevalence of diurnality in earless species, but not all earless species are diurnal. We find no universal adaptive explanation for the many instances of anuran tympanic middle ear loss.

CONCLUSION

The puzzling lack of universally shared selection pressures among earless species motivates discussion of alternative hypotheses, including genetic or developmental constraints, and the possibility that tympanic middle ear loss is maladaptive.

The continued importance of comparative auditory research to modern scientific discovery

A rich history of comparative research in the auditory field has afforded a synthetic view of sound information processing by ears and brains. Some organisms have proven to be powerful models for human hearing due to fundamental similarities (e.g., well-matched hearing ranges), while others feature intriguing differences (e.g., atympanic ears) that invite further study. Work across diverse "non-traditional" organisms, from small mammals to avians to amphibians and beyond, continues to propel auditory science forward, netting a variety of biomedical and technological advances along the way. In this brief review, limited primarily to tetrapod vertebrates, we discuss the continued importance of comparative studies in hearing research from the periphery to central nervous system with a focus on outstanding questions such as mechanisms for sound capture, peripheral and central processing of directional/spatial information, and non-canonical auditory processing, including efferent and hormonal effects.

Evolutionary rates and shape variation along the anuran vertebral column with attention to phylogeny, body size, and ecology

The vertebral column is critical to a vertebrate species' flexibility and skeletal support, making vertebrae a clear target for selection. Anurans (frogs and toads) have a unique, truncated vertebral column that appears constrained to provide axial rigidity for efficient jumping. However, no study has examined how presacral vertebrae shape varies among anuran species at the macroevolutionary scale nor how intrinsic (developmental and phylogenetic) and extrinsic (ecological) factors may have influenced vertebrae shape evolution. We used microCT scans and phylogenetic comparative methods to examine the vertebrae of hundreds of anuran species that vary in body size as well as adult and larval ecology. We found variation in shape and evolutionary rates among anuran vertebrae, dispelling any notion that trunk vertebrae evolve uniformly. We discovered the highest evolutionary rates in the cervical vertebrae and in the more caudal trunk vertebrae. We found little evidence for selection pressures related to adult or larval ecology affecting vertebrae evolution, but we did find body size was highly associated with vertebrae shape and microhabitat (mainly burrowing) affected those allometric relationships. Our results provide an interesting comparison to vertebrae evolution in other clades and a jumping-off point for studies of anuran vertebrae evolution and development.

Plant phylogenetics: The never-ending cycle of evolutionary gains and losses

The Zygnematophyceae is the sister clade to the land plants, but their biology remains mysterious. In a new study, a resolved phylogeny and a scenario for the evolution of multicellularity in that clade are proposed.

Six new species of Pristimantis (Anura: Strabomantidae) from Llanganates National Park and Sangay National Park in Amazonian cloud forests of Ecuador

We describe six new species of rainfrogs of the genus Pristimantis (Strabomantidae) from Amazonian cloud forests in Ecuador. We also present a phylogeny showing the relationships of the new species. The phylogeny is based on mitochondrial genes 16S rRNA (16S), 12 rRNA (12S), NADH-ubiquinone oxidoreductase chain 1 (ND1) and the nuclear gene recombination-activating 1 (RAG1). We also describe the osteology of two of the new species using high-resolution x-ray computed tomography. The new species belong to two clades. The first clade is sister to the subgenus Huicundomantis and includes P. tamia sp. nov., P. miktos, and P. mallii. Pristimantis tamia sp. nov. is morphologically similar to P. miktos, P. mallii, P. martiae, and P. incomptus, but differs from them by lacking vocal slits and tympanic membrane and by having light greenish blue iris. Based in our results we expand the subgenus Huicundomantis to include the P. miktos species group. The second clade is remarkable by being highly divergent and consisting exclusively of new species: P. anaiae sp. nov., P. glendae sp. nov., P. kunam sp. nov., P. resistencia sp. nov., and P. venegasi sp. nov. The new species resemble P. roni, P. yanezi, P. llanganati, P. katoptroides, P. verecundus, and P. mutabilis but can be distinguished from them by lacking vocal slits and tympanic membrane and by having large dark round areas with thin clear borders in the sacral region. All six new species occur in the eastern slopes of the Ecuadorian Andes and are known from a single locality in Llanganates or Sangay National Park. We recommend assigning all of them to the Data Deficient (DD) Red List category. Based in our high-resolution x-ray tomographies, we report the presence of structures that appear to be intercalary elements. This would be the first report of such structures in Terrarana.

Hearing without a tympanic ear

The 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.

Unraveling the heritage of lost traits

We synthesize ontogenetic work spanning the past century that show evolutionarily lost structures are rarely entirely absent from earlier developmental stages. We discuss morphological and genetic insights from developmental studies reveal about the evolution of trait loss and regain.

Hand and foot musculature of Sooglossoidea: synapomorphies, convergences and hind limb digging behaviour in anurans

We describe the hand and foot musculature of the fossorial Indian purple frog, Nasikabatrachus sahyadrensis, and compare it to other members of Sooglossoidea: the Seychellean sooglossid genera Sechellophryne and Sooglossus. Due to the key phylogenetic position of Sooglossoidea, we compare its members with the diversity of Anura and define 52 characters from the hand and foot musculature, among which 26 are novel hypotheses of homology. We found several synapomorphies for Sooglossus, Sooglossidae, Nasikabatrachidae and Sooglossoidea. Additionally, we (1) propose synapomorphies for diverse anuran clades at different taxonomic levels, (2) re-evaluate the identity of some conflicting plantar and palmar muscles in the context of Batrachia and (3) discuss putative adaptations to hind limb digging behaviour resulting from morphological convergences. The lack of a clear pattern of convergences among hind limb digging species suggests the occurrence of a phenomenon of many-to-one mapping from form to function.

Rampant tooth loss across 200 million years of frog evolution

Teeth are present in most clades of vertebrates but have been lost completely several times in actinopterygian fishes and amniotes. Using phenotypic data collected from over 500 genera via micro-computed tomography, we provide the first rigorous assessment of the evolutionary history of dentition across all major lineages of amphibians. We demonstrate that dentition is invariably present in caecilians and salamanders, but teeth have been lost completely more than 20 times in frogs, a much higher occurrence of edentulism than in any other vertebrate group. The repeated loss of teeth in anurans is associated with a specialized diet of small invertebrate prey as well as shortening of the lower jaw, but it is not correlated with a reduction in body size. Frogs provide an unparalleled opportunity for investigating the molecular and developmental mechanisms of convergent tooth loss on a large phylogenetic scale.

Size, microhabitat, and loss of larval feeding drive cranial diversification in frogs

Habitat is one of the most important factors shaping organismal morphology, but it may vary across life history stages. Ontogenetic shifts in ecology may introduce antagonistic selection that constrains adult phenotype, particularly with ecologically distinct developmental phases such as the free-living, feeding larval stage of many frogs (Lissamphibia: Anura). We test the relative influences of developmental and ecological factors on the diversification of adult skull morphology with a detailed analysis of 15 individual cranial regions across 173 anuran species, representing every extant family. Skull size, adult microhabitat, larval feeding, and ossification timing are all significant factors shaping aspects of cranial evolution in frogs, with late-ossifying elements showing the greatest disparity and fastest evolutionary rates. Size and microhabitat show the strongest effects on cranial shape, and we identify a "large size-wide skull" pattern of anuran, and possibly amphibian, evolutionary allometry. Fossorial and aquatic microhabitats occupy distinct regions of morphospace and display fast evolution and high disparity. Taxa with and without feeding larvae do not notably differ in cranial morphology. However, loss of an actively feeding larval stage is associated with higher evolutionary rates and disparity, suggesting that functional pressures experienced earlier in ontogeny significantly impact adult morphological evolution.

Parallel diversification of the African tree toad genus Nectophryne (Bufonidae)

African amphibian diversity remains underestimated with many cryptic lineages awaiting formal description. An important hotspot of amphibian diversification is the Guineo-Congolian rainforest in Central Africa, its richness attributable to present day and ancestral range fragmentation through geological barriers, habitat expansion and contraction, and the presence of steep ecological gradients. The charismatic Nectophryne tree toads present an interesting case study for diversification in this region. The two formally described species comprising this genus show nearly identical geographic distributions extending across most of the Guineo-Congolian rainforest, but show little morphological disparity. Both species harbour extensive genetic diversity warranting taxonomic revisions, and interestingly, when comparing the subclades within each, the two species show remarkably parallel diversification histories, both in terms of timing of phylogenetic splits and their geographic distributions. This indicates that common processes may have shaped the evolutionary history of these lineages.

Whispers from vestigial nubbins: Arrested development provokes trait loss in toads

Despite the use of acoustic communication, many species of toads (family Bufonidae) have lost parts of the tympanic middle ear, representing at least 12 independent evolutionary occurrences of trait loss. The comparative development of the tympanic middle ear in toads is poorly understood. Here, we compared middle ear development among two pairs of closely related toad species in the genera Atelopus and Rhinella that have (eared) or lack (earless) middle ear structures. We bred toads in Peru and Ecuador, preserved developmental series from tadpoles to juveniles, and examined ontogenetic timing and volume of the otic capsule, oval window, operculum, opercularis muscle, columella (stapes), and extracolumella in three-dimensional histological reconstructions. All species had similar ontogenesis of the otic capsule, oval window, operculum, and opercularis muscle. Moreover, cell clusters of primordial columella in the oval window appeared just before metamorphosis in both eared and earless lineages. However, in earless lineages, the cell clusters either remained as small nubbins or cell buds in the location of the columella footplate within the oval window or disappeared by juvenile and adult stages. Thus, columella growth began around metamorphosis in all species but was truncated and/or degenerated after metamorphosis in earless species, leaving earless adults with morphology typical of metamorphic anurans. Shifts in the timing or expression of biochemical pathways that regulate the extension or differentiation of the columella after metamorphosis may be the developmental mechanism underlying convergent trait loss among toad lineages.

Larger Genomes Linked to Slower Development and Loss of Late-Developing Traits

Genome size varies widely among organisms and is known to affect vertebrate development, morphology, and physiology. In amphibians, genome size is hypothesized to contribute to loss of late-forming structures, although this hypothesis has mainly been discussed in salamanders. Here we estimated genome size for 22 anuran species and combined this novel data set with existing genome size data for an additional 234 anuran species to determine whether larger genome size is associated with loss of a late-forming anuran sensory structure, the tympanic middle ear. We established that genome size is negatively correlated with development rate across 90 anuran species and found that genome size evolution is correlated with evolutionary loss of the middle ear bone (columella) among 241 species (224 eared and 17 earless). We further tested whether the development of the tympanic middle ear could be constrained by large cell sizes and small body sizes during key stages of tympanic middle ear development (metamorphosis). Together, our evidence suggests that larger genomes, slower development rate, and smaller body sizes at metamorphosis may contribute to the loss of the anuran tympanic middle ear. We conclude that increases in anuran genome size, although less drastic than those in salamanders, may affect development of late-forming traits.

Braincase simplification and the origin of lissamphibians

Dissorophoidea, a group of temnospondyl tetrapods that first appear in the Late Carboniferous, is made up of two clades ⎼ Olsoniformes and Amphibamiformes (Branchiosauridae and Amphibamidae) ⎼ the latter of which is widely thought to have given rise to living amphibians (i.e., Lissamphibia). The lissamphibian braincase has a highly derived morphology with several secondarily lost elements; however, these losses have never been incorporated into phylogenetic analyses and thus the timing and nature of these evolutionary events remain unknown. Hindering research into this problem has been the lack of phylogenetic analyses of Dissorophoidea that includes both taxonomically dense sampling and specific characters to document changes in the braincase in the lineage leading to Lissamphibia. Here we build on a recent, broadly sampled dissorophoid phylogenetic analysis to visualize key events in the evolution of the lissamphibian braincase. Our ancestral character state reconstructions show a clear, step-wise trend towards reduction of braincase ossification leading to lissamphibians, including reduction of the sphenethmoid, loss of the basioccipital at the Amphibamiformes node, and further loss of both the basisphenoid and the hypoglossal nerve foramina at the Lissamphibia node. Our analysis confirms that the highly derived condition of the lissamphibian braincase is characterized by overall simplification in terms of the number and extent of chondrocranial ossifications.

Prolonged middle ear development in Rhinella horribilis

Despite the benefit of the tympanic middle ear to airborne hearing sensitivity, anurans range in how soon they develop functional middle ears after transitioning to life on land. Previous evidence suggested that bufonids had particularly slow middle ear developmental rates, but precise timelines have not yet been published for this family. Here, we provide the first age-verified middle ear development timeline for a true toad species (family Bufonidae). We find that although middle ear development begins during metamorphosis in Rhinella horribilis, the middle ear remains incomplete 15 weeks after the transition from aquatic tadpole to land-dwelling toadlet. Using this new middle ear timeline, we discuss commonalities and differences in middle ear development among bufonids, as well as among Anura.

A new earless species of Poyntonophrynus (Anura, Bufonidae) from the Serra da Neve Inselberg, Namibe Province, Angola

African pygmy toads of the genus Poyntonophrynus are some of the least known species of African toads. The genus comprises ten recognized species endemic to sub-Saharan Africa, five of which are restricted to southwestern Africa. Recent field research in Angola provided new material for three species of Poyntonophrynus, including a morphologically distinctive population from the Serra da Neve Inselberg. Based on a combination of external morphology, high-resolution computed tomography scanning, and molecular phylogenetic analysis, the Serra da Neve population is described as new species that is nested within the genus. The most striking character that differentiates the newly described species from its congeners is the lack of a tympanic middle ear, a condition common in the family Bufonidae, but so far not known for Poyntonophrynus. The description of this new species from southwestern Angola reinforces the biogeographic importance of the region and further suggests that southwestern Africa is the cradle of diversity for this genus.

Sensitive high-frequency hearing in earless and partially eared harlequin frogs (Atelopus)

Harlequin frogs, genus Atelopus, communicate at high frequencies despite most species lacking a complete tympanic middle ear that facilitates high-frequency hearing in most anurans and other tetrapods. Here, we tested whether Atelopus are better at sensing high-frequency acoustic sound compared with other eared and earless species in the Bufonidae family, determined whether middle ear variation within Atelopus affects hearing sensitivity and tested potential hearing mechanisms in Atelopus We determined that at high frequencies (2000-4000 Hz), Atelopus are 10-34 dB more sensitive than other earless bufonids but are relatively insensitive to mid-range frequencies (900-1500 Hz) compared with eared bufonids. Hearing among Atelopus species is fairly consistent, evidence that the partial middle ears present in a subset of Atelopus species do not convey a substantial hearing advantage. We further demonstrate that Atelopus hearing is probably not facilitated by vibration of the skin overlying the normal tympanic membrane region or the body lung wall, leaving the extratympanic hearing pathways in Atelopus enigmatic. Together, these results show Atelopus have sensitive high-frequency hearing without the aid of a tympanic middle ear and prompt further study of extratympanic hearing mechanisms in anurans.

Earless toads sense low frequencies but miss the high notes

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.

Evolutionary radiation of earless frogs in the Andes: molecular phylogenetics and habitat shifts in high-elevation terrestrial breeding frogs

The loss of hearing structures and loss of advertisement calls in many terrestrial breeding frogs (Strabomantidae) living at high elevations in South America are common and intriguing phenomena. The Andean frog genus Phrynopus Peters, 1873 has undergone an evolutionary radiation in which most species lack the tympanic membrane and tympanic annulus, yet the phylogenetic relationships among species in this group remain largely unknown. Here, we present an expanded molecular phylogeny of Phrynopus that includes 24 nominal species. Our phylogeny includes Phrynopus peruanus, the type species of the genus, and 10 other species for which genetic data were previously unavailable. We found strong support for monophyly of Phrynopus, and that two nominal species-Phrynopus curator and Phrynopus nicoleae-are junior synonyms of Phrynopus tribulosus. Using X-ray computed tomography (CT) imaging, we demonstrate that the absence of external hearing structures is associated with complete loss of the auditory skeletal elements (columella) in at least one member of the genus. We mapped the tympanum condition on to a species tree to infer whether the loss of hearing structures took place once or multiple times. We also assessed whether tympanum condition, body size, and body shape are associated with the elevational distribution and habitat use. We identified a single evolutionary transition that involved the loss of both the tympanic membrane and tympanic annulus, which in turn is correlated with the absence of advertisement calls. We also identified several species pairs where one species inhabits the Andean grassland and the other montane forest. When accounting for phylogenetic relatedness among species, we detected a significant pattern of increasing body size with increasing elevation. Additionally, species at higher elevations tend to develop shorter limbs, shorter head, and shorter snout than species living at lower elevations. Our findings strongly suggest a link between ecological divergence and morphological diversity of terrestrial breeding frogs living in montane gradients.

Trait independence primes convergent trait loss

The repeated, independent evolution of traits (convergent evolution) is often attributed to shared environmental selection pressures. However, developmental dependencies among traits can limit the phenotypic variation available to selection and bias evolutionary outcomes. Here, we determine how changes in developmentally correlated traits may impact convergent loss of the tympanic middle ear, a highly labile trait within toads that currently lack adaptive explanation. The middle ear's lability could reflect evolutionary trade-offs with other skull features under selection, or the middle ear may evolve independently of the rest of the skull, allowing it to be modified by active or passive processes without pleiotropic trade-offs with other skull features. We compare the skulls of 55 species (39 eared, 16 earless) within the family Bufonidae, spanning six hypothesized independent middle ear transitions. We test whether shared or lineage-specific changes in skull shape distinguish earless species from eared species and whether earless skulls lack other late-forming skull bones. We find no evidence for pleiotropic trade-offs between the middle ear and other skull structures. Instead, middle ear loss in anurans may provide a rare example of developmental independence contributing to evolutionary lability of a sensory system.

Evolution of Sound Source Localization Circuits in the Nonmammalian Vertebrate Brainstem

The earliest vertebrate ears likely subserved a gravistatic function for orientation in the aquatic environment. However, in addition to detecting acceleration created by the animal's own movements, the otolithic end organs that detect linear acceleration would have responded to particle movement created by external sources. The potential to identify and localize these external sources may have been a major selection force in the evolution of the early vertebrate ear and in the processing of sound in the central nervous system. The intrinsic physiological polarization of sensory hair cells on the otolith organs confers sensitivity to the direction of stimulation, including the direction of particle motion at auditory frequencies. In extant fishes, afferents from otolithic end organs encode the axis of particle motion, which is conveyed to the dorsal regions of first-order octaval nuclei. This directional information is further enhanced by bilateral computations in the medulla and the auditory midbrain. We propose that similar direction-sensitive neurons were present in the early aquatic tetrapods and that selection for sound localization in air acted upon preexisting brain stem circuits like those in fishes. With movement onto land, the early tetrapods may have retained some sensitivity to particle motion, transduced by bone conduction, and later acquired new auditory papillae and tympanic hearing. Tympanic hearing arose in parallel within each of the major tetrapod lineages and would have led to increased sensitivity to a broader frequency range and to modification of the preexisting circuitry for sound source localization.

Evidence of auditory insensitivity to vocalization frequencies in two frogs

The emergence and maintenance of animal communication systems requires the co-evolution of signal and receiver. Frogs and toads rely heavily on acoustic communication for coordinating reproduction and typically have ears tuned to the dominant frequency of their vocalizations, allowing discrimination from background noise and heterospecific calls. However, we present here evidence that two anurans, Brachycephalus ephippium and B. pitanga, are insensitive to the sound of their own calls. Both species produce advertisement calls outside their hearing sensitivity range and their inner ears are partly undeveloped, which accounts for their lack of high-frequency sensitivity. If unheard by the intended receivers, calls are not beneficial to the emitter and should be selected against because of the costs associated with signal production. We suggest that protection against predators conferred by their high toxicity might help to explain why calling has not yet disappeared, and that visual communication may have replaced auditory in these colourful, diurnal frogs.