Scaling of the marsupial middle ear and its functional significance

Structure and scaling of the middle ear in domestic dog breeds

Although domestic dogs vary considerably in both body size and skull morphology, behavioural audiograms have previously been found to be similar in breeds as distinct as a Chihuahua and a St Bernard. In this study, we created micro-CT reconstructions of the middle ears and bony labyrinths from the skulls of 17 dog breeds, including both Chihuahua and St Bernard, plus a mongrel and a wolf. From these reconstructions, we measured middle ear cavity and ossicular volumes, eardrum and stapes footplate areas and bony labyrinth volumes. All of these ear structures scaled with skull size with negative allometry and generally correlated better with condylobasal length than with maximum or interaural skull widths. Larger dogs have larger ear structures in absolute terms: the volume of the St Bernard's middle ear cavity was 14 times that of the Chihuahua. The middle and inner ears are otherwise very similar in morphology, the ossicular structure being particularly well-conserved across breeds. The expectation that larger ear structures in larger dogs would translate into hearing ranges shifted towards lower frequencies is not consistent with the existing audiogram data. Assuming that the audiograms accurately reflect the hearing of the breeds in question, oversimplifications in existing models of middle ear function or limitations imposed by other parts of the auditory system may be responsible for this paradox.

Comparative morphology of the avian bony columella

In birds, the columella is the only bony element of the sound conducting apparatus, conveying vibrations of the cartilaginous extracolumella to the fluid of the inner ear. Although avian columellar morphology has attracted some attention over the past century, it nonetheless remains poorly described in the literature. The few existing studies mostly focus on morphological descriptions in relatively few taxa, with no taxonomically broad surveys yet published. Here we use observations of columellae from 401 extant bird species to provide a comprehensive survey of columellar morphology in a phylogenetic context. We describe the columellae of several taxa for the first time and identify derived morphologies characterizing higher-level clades based on current phylogenies. In particular, we identify a derived columellar morphology diagnosing a major subclade of Accipitridae. Within Suliformes, we find that Fregatidae, Sulidae, and Phalacrocoracidae share a derived morphology that is absent in Anhingidae, suggesting a secondary reversal. Phylogenetically informed comparisons allow recognition of instances of homoplasy, including the distinctive bulbous columellae in suboscine passerines and taxa belonging to Eucavitaves, and bulging footplates that appear to have evolved at least twice independently in Strigiformes. We consider phylogenetic and functional factors influencing avian columellar morphology, finding that aquatic birds possess small footplates relative to columellar length, possibly related to hearing function in aquatic habitats. By contrast, the functional significance of the distinctive bulbous basal ends of the columellae of certain arboreal landbird taxa remains elusive.

Scaling of ear morphology across 127 bird species and its implications for hearing performance

The dimensions of auditory structures among animals of varying body size can have implications for hearing performance. Larger animals often have a hearing range focused on lower frequencies than smaller animals, which may be explained by several anatomical mechanisms in the ear and their scaling relationships. While the effect of size on ear morphology and hearing performance has been explored in some mammals, anurans and lizards, much less is known about the scaling relationships for the single-ossicle, internally-coupled ears of birds. Using micro- and nano-CT scans of the tympanic middle and inner ears of 127 ecologically and phylogenetically diverse bird species, spanning more than 400-fold in head mass (2.3 to 950 g), we undertook phylogenetically-informed scaling analyses to test whether 12 morphological traits, of functional importance to hearing, maintain their relative proportions with increasing head mass. We then extended our analysis by regressing these morphological traits with measures of hearing sensitivity and range to better understand morphological underpinnings of hearing performance. We find that most auditory structures scale together in equal proportions, whereas columella length increases disproportionately. We also find that the size of several auditory structures is associated with increased hearing sensitivity and frequency hearing limits, while head mass did not explain these measures. Although both birds and mammals demonstrate proportional scaling between auditory structures, the consequences for hearing in each group may diverge due to unique morphological predictors of auditory performance.

Jurassic mammaliaform petrosals from Western Siberia (Russia) and implications for early mammalian inner-ear anatomy

Five partially preserved mammaliaform petrosals recovered from Middle Jurassic sediments of the Berezovsk coal mine (Krasnoyarsk Krai, Russia) show similarities to other early mammaliaforms like the morganucodontan Morganucodon and the docodontan Haldanodon in having an inflated promontorium and a curved and apically inflated cochlear canal, but they are distinct from dryolestoid and derived mammalian petrosals by the weak coiling of the cochlear duct and the presence of a perilymphatic foramen with an open perilymphatic sulcus. The two larger and robust specimens exhibit striking similarities to docodontan petrosals. Inside the bone an intricate circumpromontorial venous plexus was discovered, as recently described for the docodontan Borealestes, confirming that this structure is consistently present in basal non-mammalian mammaliaforms. The three smaller and slender petrosals probably belong to haramiyidans and are unique in showing a septum-like structure medially along the cochlear nerve entrance. The protruding perforated bony bar, which is preserved in two of the three, is interpreted here to be a remnant of a bony septum with multiple foramina for cochlear nerve fibres, representing an autapomorphic feature of Haramiyida. This newly described passageway for nerve fibres shows that the formation of the osteological structure surrounding the nervous pathways of the cochlea is more plastic among the non-mammalian mammaliaforms than previously thought.

The external ear morphology and presence of tragi in Australian marsupials

Multiple studies have described the anatomy and function of the external ear (pinna) of bats, and other placental mammals, however, studies of marsupial pinna are largely absent. In bats, the tragus appears to be especially important for locating and capturing insect prey. In this study, we aimed to investigate the pinnae of Australian marsupials, with a focus on the presence/absence of tragi and how they may relate to diet. We investigated 23 Australian marsupial species with varying diets. The pinnae measurements (scapha width, scapha length) and tragi (where present) were measured. The interaural distance and body length were also recorded for each individual. Results indicated that all nectarivorous, carnivorous, and insectivorous species had tragi with the exception of the insectivorous striped possum (Dactylopsila trivirgata), numbat (Myrmecobius fasciatus), and nectarivorous sugar glider (Petaurus breviceps). No herbivorous or omnivorous species had tragi. Based on the findings in this study, and those conducted on placental mammals, we suggest marsupials use tragi in a similar way to placentals to locate and target insectivorous prey. The Tasmanian devil (Sarcophilus harrisii) displayed the largest interaural distance that likely aids in better localization and origin of noise associated with prey detection. In contrast, the smallest interaural distance was exhibited by a macropod. Previous studies have suggested the hearing of macropods is especially adapted to detect warnings of predators made by conspecifics. While the data in this study demonstrate a diversity in pinnae among marsupials, including presence and absence of tragi, it suggests that there is a correlation between pinna structure and diet choice among marsupials. A future study should investigate a larger number of individuals and species and include marsupials from Papua New Guinea, and Central and South America as a comparison.

The ossicular chain of Cainotheriidae (Mammalia, Artiodactyla)

This work describes an unparalleled sample of isolated fossil auditory ossicles of cainotheriid artiodactyls from the Paleogene karstic infillings of Dams (Tarn-et-Garonne, Quercy, France). This collection comprises a total of 18 mallei, 28 incudes and three stapedes. It allows the documentation of both intra- and interspecific variability of ossicular morphology within Cainotheriidae. We show that despite considerable intraspecific variability, the malleus, the incus and the stapes appear to be taxonomically informative at the Cainotheriidae scale. This work further provides the first description of a reconstructed ossicular chain of a terrestrial Paleogene artiodactyl species, found in a basicranium of the late Oligocene cainotheriine Caenomeryx filholi (Pech Desse locality).

Comparison of sheep and human middle-ear ossicles: anatomy and inertial properties

The sheep middle ear has been used in training to prepare physicians to perform surgeries and to test new ways of surgical access. This study aimed to (1) collect anatomical data and inertial properties of the sheep middle-ear ossicles and (2) explore effects of these features on sound transmission, in comparison to those of the human. Characteristic dimensions and inertial properties of the middle-ear ossicles of White-Alpine sheep (n = 11) were measured from high-resolution micro-CT data, and were assessed in comparison with the corresponding values of the human middle ear. The sheep middle-ear ossicles differed from those of human in several ways: anteroinferior orientation of the malleus handle, relatively small size of the incus with a relatively short distance to the lenticular process, a large area of the articular surfaces at the incudostapedial joint, and a relatively small moment of inertia along the anterior-posterior axis. Analysis in this study suggests that structure and orientation of the middle-ear ossicles in the sheep are conducive to an increase in the hinge-like ossicular-lever-action around the anterior-posterior axis. Considering the substantial anatomical differences, outcomes of middle-ear surgeries would presumably be difficult to assess from experiments using the sheep middle ear.

Scaling of the avian middle ear

This article presents a comparative study of morphology of the avian middle ear. The general morphology of the columella shows considerable variation across species, yet few studies have attempted to provide quantitative comparisons, and basic anatomical data has not been thoroughly reported. In this study, we examined the middle ear in 49 taxonomically diverse species of bird. We found significant correlations between measurements of several features (columellar length, mass, tympanic membrane area, footplate area) and interaural diameter. While scaling of columellar length with interaural diameter is consistent with isometry, masses and areas showed negative allometry, or a non-proportional scaling with interaural diameter. These observations remained true even for species with unusual middle ear morphology, such as Alcedinidae (Kingfishers) in which the basal struts of the columella form a structure almost resembling a mammalian stapes, or Tytonidae (Barn Owls) which have a highly bulbous footplate. It therefore appears that allometry cannot help explain the morphological variation in the columella.

Functional morphological adaptations of the bony labyrinth in marsupials (Mammalia, Theria)

Diprotodontia represents the largest and ecologically most distinct order of marsupials occurring in Australasian being highly divers in size, locomotion, habitat preferences, feeding, and activity pattern. The spatial orientation in the habitat and therefore the three-dimensional space is detected by the vestibular system of the inner ear, more precisely by the three semicircular canals. In this study, we investigated the bony labyrinth of diprotodontian and selected non-diprotodontian marsupial mammals of almost all genera with noninvasive micro-CT scanning and 3D-reconstructions. In principal component analyses, the subterranean taxon can be separated from gliding and saltatorial taxa, whereas arboreal species can be separated from saltatorial specimens. The highest PCA loadings of this functional distinction are clearly found in the diameter of the semicircular canals, whereas the overall shape (height, width, length) of the semicircular canals is less important. Additionally, the investigated arboreal and fossorial species of South America are nested in the morphospace of the Australasian taxa. Even if a phylogenetic signal in the anatomy of the bony labyrinth cannot be excluded entirely, the main functional morphological signal of the vestibular system is found in the diameter of the semicircular canals. With the large dataset of extant marsupial mammals analysed here, the locomotion mode of extinct taxa can be inferred in future studies independent of any evidence of postcranial material.

Mammalian development does not recapitulate suspected key transformations in the evolutionary detachment of the mammalian middle ear

The ectotympanic, malleus and incus of the developing mammalian middle ear (ME) are initially attached to the dentary via Meckel's cartilage, betraying their origins from the primary jaw joint of land vertebrates. This recapitulation has prompted mostly unquantified suggestions that several suspected--but similarly unquantified--key evolutionary transformations leading to the mammalian ME are recapitulated in development, through negative allometry and posterior/medial displacement of ME bones relative to the jaw joint. Here we show, using µCT reconstructions, that neither allometric nor topological change is quantifiable in the pre-detachment ME development of six marsupials and two monotremes. Also, differential ME positioning in the two monotreme species is not recapitulated. This challenges the developmental prerequisites of widely cited evolutionary scenarios of definitive mammalian middle ear (DMME) evolution, highlighting the requirement for further fossil evidence to test these hypotheses. Possible association between rear molar eruption, full ME ossification and ME detachment in marsupials suggests functional divergence between dentary and ME as a trigger for developmental, and possibly also evolutionary, ME detachment. The stable positioning of the dentary and ME supports suggestions that a 'partial mammalian middle ear' as found in many mammaliaforms--probably with a cartilaginous Meckel's cartilage--represents the only developmentally plausible evolutionary DMME precursor.

Structure and function of the mammalian middle ear. II: Inferring function from structure

Anatomists and zoologists who study middle ear morphology are often interested to know what the structure of an ear can reveal about the auditory acuity and hearing range of the animal in question. This paper represents an introduction to middle ear function targetted towards biological scientists with little experience in the field of auditory acoustics. Simple models of impedance matching are first described, based on the familiar concepts of the area and lever ratios of the middle ear. However, using the Mongolian gerbil Meriones unguiculatus as a test case, it is shown that the predictions made by such 'ideal transformer' models are generally not consistent with measurements derived from recent experimental studies. Electrical analogue models represent a better way to understand some of the complex, frequency-dependent responses of the middle ear: these have been used to model the effects of middle ear subcavities, and the possible function of the auditory ossicles as a transmission line. The concepts behind such models are explained here, again aimed at those with little background knowledge. Functional inferences based on middle ear anatomy are more likely to be valid at low frequencies. Acoustic impedance at low frequencies is dominated by compliance; expanded middle ear cavities, found in small desert mammals including gerbils, jerboas and the sengi Macroscelides, are expected to improve low-frequency sound transmission, as long as the ossicular system is not too stiff.

Exploring the mammalian sensory space: co-operations and trade-offs among senses

The evolution of a particular sensory organ is often discussed with no consideration of the roles played by other senses. Here, we treat mammalian vision, olfaction and hearing as an interconnected whole, a three-dimensional sensory space, evolving in response to ecological challenges. Until now, there has been no quantitative method for estimating how much a particular animal invests in its different senses. We propose an anatomical measure based on sensory organ sizes. Dimensions of functional importance are defined and measured, and normalized in relation to animal mass. For 119 taxonomically and ecologically diverse species, we can define the position of the species in a three-dimensional sensory space. Thus, we can ask questions related to possible trade-off vs. co-operation among senses. More generally, our method allows morphologists to identify sensory organ combinations that are characteristic of particular ecological niches. After normalization for animal size, we note that arboreal mammals tend to have larger eyes and smaller noses than terrestrial mammals. On the other hand, we observe a strong correlation between eyes and ears, indicating that co-operation between vision and hearing is a general mammalian feature. For some groups of mammals we note a correlation, and possible co-operation between olfaction and whiskers.

The inner ear of Diacodexis, the oldest artiodactyl mammal

We provide the first detailed description of the inner ear of the oldest artiodactyl, Diacodexis, based on a three-dimensional reconstruction extracted from computed tomography imagery of a skull of Diacodexis ilicis of earliest Wasatchian age (ca. 55 Ma). This description provides new anatomical data for the earliest artiodactyls, and reveals that the bony labyrinth of Diacodexis differs greatly from that of modern artiodactyls described so far. The bony labyrinth of Diacodexis presents a weakly coiled cochlea (720 °), a secondary common crus, a dorsal extension of the anterior semicircular canal more pronounced than that of the posterior one, and a small angle between the basal turn of the bony cochlear canal and the lateral semicircular canal. This suite of characters also occurs in basal eutherian mammals. Diacodexis strongly resembles small living tragulid ruminants in its overall body shape and hindlimb proportions. Comparison of the bony labyrinth of Diacodexis to that of the tragulid Moschiola meminna (Indian mouse deer) reveals great morphological difference in cochlear shape and semicircular canal disposition. The shape of the cochlea suggests that Diacodexis was a high-frequency hearing specialist, with a high low-frequency hearing limit (543 Hz at 60 dB). By comparison, the estimated low-frequency limit of Moschiola meminna is much lower (186.0 Hz at 60 dB). We also assess the locomotor agility of Diacodexis based on measurements of the semicircular canals. Locomotor agility estimates for Diacodexis range between 3.62 and 3.93, and suggest a degree of agility compatible with a nimble, fast running to jumping animal. These results are congruent with the postcranial functional analysis for this extinct taxon.

Environmental influences in the evolution of tetrapod hearing sensitivity and middle ear tuning

Vertebrates inhabit and communicate acoustically in most natural environments. We review the influence of environmental factors on the hearing sensitivity of terrestrial vertebrates, and on the anatomy and mechanics of the middle ears. Evidence suggests that both biotic and abiotic environmental factors affect the evolution of bandwidth and frequency of peak sensitivity of the hearing spectrum. Relevant abiotic factors include medium type, temperature, and noise produced by nonliving sources. Biotic factors include heterospecific, conspecific, or self-produced sounds that animals are selected to recognize, and acoustic interference by sounds that other animals generate. Within each class of tetrapods, the size of the middle ear structures correlates directly to body size and inversely to frequency of peak sensitivity. Adaptation to the underwater medium in cetaceans involved reorganization of the middle ear for novel acoustic pathways, whereas adaptation to subterranean life in several mammals resulted in hypertrophy of the middle ear ossicles to enhance their inertial mass for detection of seismic vibrations. The comparative approach has revealed a number of generalities about the effect of environmental factors on hearing performance and middle ear structure across species. The current taxonomic sampling of the major tetrapod groups is still highly unbalanced and incomplete. Future expansion of the comparative evidence should continue to reveal general patterns and novel mechanisms.

Impedance-Matching Hearing in Paleozoic Reptiles: Evidence of Advanced Sensory Perception at an Early Stage of Amniote Evolution

Background

Insights into the onset of evolutionary novelties are key to the understanding of amniote origins and diversification. The possession of an impedance-matching tympanic middle ear is characteristic of all terrestrial vertebrates with a sophisticated hearing sense and an adaptively important feature of many modern terrestrial vertebrates. Whereas tympanic ears seem to have evolved multiple times within tetrapods, especially among crown-group members such as frogs, mammals, squamates, turtles, crocodiles, and birds, the presence of true tympanic ears has never been recorded in a Paleozoic amniote, suggesting they evolved fairly recently in amniote history.

Methodology/Principal Findings

In the present study, we performed a morphological examination and a phylogenetic analysis of poorly known parareptiles from the Middle Permian of the Mezen River Basin in Russia. We recovered a well-supported clade that is characterized by a unique cheek morphology indicative of a tympanum stretching across large parts of the temporal region to an extent not seen in other amniotes, fossil or extant, and a braincase specialized in showing modifications clearly related to an increase in auditory function, unlike the braincase of any other Paleozoic tetrapod. In addition, we estimated the ratio of the tympanum area relative to the stapedial footplate for the basalmost taxon of the clade, which, at 23∶1, is in close correspondence to that of modern amniotes capable of efficient impedance-matching hearing.

Conclusions/Significance

Using modern amniotes as analogues, the possession of an impedance-matching middle ear in these parareptiles suggests unique ecological adaptations potentially related to living in dim-light environments. More importantly, our results demonstrate that already at an early stage of amniote diversification, and prior to the Permo-Triassic extinction event, the complexity of terrestrial vertebrate ecosystems had reached a level that proved advanced sensory perception to be of notable adaptive significance.