Primate auditory diversity and its influence on hearing performance

Anatomical, Histological, and Morphometrical Investigations of the Auditory Ossicles in Chlorocebus aethiops sabaeus from Saint Kitts Island

Otological studies rely on a lot of data drawn from animal studies. A lot of pathological or evolutionary questions may find answers in studies on primates, providing insights into the morphological, pathological, and physiological aspects of systematic biological studies. Our study on auditory ossicles moves from a pure morphological (macroscopic and microscopic) investigation of auditory ossicles to the morphometrical evaluation of several individuals as well as to some interpretative data regarding some functional aspects drawn from these investigations. Particularities from this perspective blend with metric data and point toward comparative elements that might also serve as an important reference in further morphologic and comparative studies.

The sensory ecology of primate food perception, revisited

Twenty years ago, Dominy and colleagues published "The sensory ecology of primate food perception," an impactful review that brought new perspectives to understanding primate foraging adaptations. Their review synthesized information on primate senses and explored how senses informed feeding behavior. Research on primate sensory ecology has seen explosive growth in the last two decades. Here, we revisit this important topic, focusing on the numerous new discoveries and lines of innovative research. We begin by reviewing each of the five traditionally recognized senses involved in foraging: audition, olfaction, vision, touch, and taste. For each sense, we provide an overview of sensory function and comparative ecology, comment on the state of knowledge at the time of the original review, and highlight advancements and lingering gaps in knowledge. Next, we provide an outline for creative, multidisciplinary, and innovative future research programs that we anticipate will generate exciting new discoveries in the next two decades.

Cortical adaptation of the night monkey to a nocturnal niche environment: a comparative non-invasive T1w/T2w myelin study

Night monkeys (Aotus) are the only genus of monkeys within the Simian lineage that successfully occupy a nocturnal environmental niche. Their behavior is supported by their sensory organs' distinctive morphological features; however, little is known about their evolutionary adaptations in sensory regions of the cerebral cortex. Here, we investigate this question by exploring the cortical organization of night monkeys using high-resolution in-vivo brain MRI and comparative cortical-surface T1w/T2w myeloarchitectonic mapping. Our results show that the night monkey cerebral cortex has a qualitatively similar but quantitatively different pattern of cortical myelin compared to the diurnal macaque and marmoset monkeys. T1w/T2w myelin and its gradient allowed us to parcellate high myelin areas, including the middle temporal complex (MT +) and auditory cortex, and a low-myelin area, Brodmann area 7 (BA7) in the three species, despite species differences in cortical convolutions. Relative to the total cortical-surface area, those of MT + and the auditory cortex are significantly larger in night monkeys than diurnal monkeys, whereas area BA7 occupies a similar fraction of the cortical sheet in all three species. We propose that the selective expansion of sensory areas dedicated to visual motion and auditory processing in night monkeys may reflect cortical adaptations to a nocturnal environment.

Obesity Animal Models for Acupuncture and Related Therapy Research Studies

Obesity and related diseases are considered as pandemic representing a worldwide threat for health. Animal models are critical to validate the effects and understand the mechanisms related to classical or innovative preventive and therapeutic strategies. It is, therefore, important to identify the best animal models for translational research, using different evaluation criteria such as the face, construct, and predictive validity. Because the pharmacological treatments and surgical interventions currently used for treating obesity often present many undesirable side effects, relatively high relapse probabilities, acupuncture, electroacupuncture (EA), and related therapies have gained more popularity and attention. Many kinds of experimental animal models have been used for obesity research studies, but in the context of acupuncture, most of the studies were performed in rodent obesity models. Though, are these obesity rodent models really the best for acupuncture or related therapies research studies? In this study, we review different obesity animal models that have been used over the past 10 years for acupuncture and EA research studies. We present their respective advantages, disadvantages, and specific constraints. With the development of research on acupuncture and EA and the increasing interest regarding these approaches, proper animal models are critical for preclinical studies aiming at developing future clinical trials in the human. The aim of the present study is to provide researchers with information and guidance related to the preclinical models that are currently available to investigate the outcomes of acupuncture and related therapies.

Hearing Sensitivity of Primates: Recurrent and Episodic Positive Selection in Hair Cells and Stereocilia Protein-Coding Genes

The large spectrum of hearing sensitivity observed in primates results from the impact of environmental and behavioral pressures to optimize sound perception and localization. Although evidence of positive selection in auditory genes has been detected in mammals including in Hominoids, selection has never been investigated in other primates. We analyzed 123 genes highly expressed in the inner ear of 27 primate species and tested to what extent positive selection may have shaped these genes in the order Primates tree. We combined both site and branch-site tests to obtain a comprehensive picture of the positively selected genes (PSGs) involved in hearing sensitivity, and drew a detailed description of the most affected branches in the tree. We chose a conservative approach, and thus focused on confounding factors potentially affecting PSG signals (alignment, GC-biased gene conversion, duplications, heterogeneous sequencing qualities). Using site tests, we showed that around 12% of these genes are PSGs, an α selection value consistent with average human genome estimates (10-15%). Using branch-site tests, we showed that the primate tree is heterogeneously affected by positive selection, with the black snub-nosed monkey, the bushbaby, and the orangutan, being the most impacted branches. A large proportion of these genes is inclined to shape hair cells and stereocilia, which are involved in the mechanotransduction process, known to influence frequency perception. Adaptive selection, and more specifically recurrent adaptive evolution, could have acted in parallel on a set of genes (ADGRV1, USH2A, PCDH15, PTPRQ, and ATP8A2) involved in stereocilia growth and the whole complex of bundle links connecting them, in species across different habitats, including high altitude and nocturnal environments.

HEARTBREAK Controls Post-translational Modification of INDEHISCENT to Regulate Fruit Morphology in Capsella

Morphological variation is the basis of natural diversity and adaptation. For example, angiosperms (flowering plants) evolved during the Cretaceous period more than 100 mya and quickly colonized terrestrial habitats [1]. A major reason for their astonishing success was the formation of fruits, which exist in a myriad of different shapes and sizes [2]. Evolution of organ shape is fueled by variation in expression patterns of regulatory genes causing changes in anisotropic cell expansion and division patterns [3, 4, 5]. However, the molecular mechanisms that alter the polarity of growth to generate novel shapes are largely unknown. The heart-shaped fruits produced by members of the Capsella genus comprise an anatomical novelty, making it particularly well suited for studies on morphological diversification [6, 7, 8]. Here, we show that post-translational modification of regulatory proteins provides a critical step in organ-shape formation. Our data reveal that the SUMO protease, HEARTBREAK (HTB), from Capsella rubella controls the activity of the key regulator of fruit development, INDEHISCENT (CrIND in C. rubella), via de-SUMOylation. This post-translational modification initiates a transduction pathway required to ensure precisely localized auxin biosynthesis, thereby facilitating anisotropic cell expansion to ultimately form the heart-shaped Capsella fruit. Therefore, although variation in the expression of key regulatory genes is known to be a primary driver in morphological evolution, our work demonstrates how other processes—such as post-translational modification of one such regulator—affects organ morphology.

•

HTB encodes a SUMO protease required for fruit shape in Capsella

•

Anisotropic cell growth is suppressed in the fruit valves of the htb mutant

•

HTB stabilizes CrIND through de-SUMOylation to facilitate local auxin biosynthesis

The acoustic near-field measurement of aye-ayes' biological auditory system utilizing a biomimetic robotic tap-scanning

The aye-aye (Daubentonia madagascariensis) is best known for its unique acoustic-based foraging behavior called 'tap-scanning' or 'percussive foraging'. The tap-scanning is a unique behavior allowing aye-aye to locate small cavities beneath tree bark and extract wood-boring larvae from it. The tap-scanning requires the animal auditory system to have exceptional acoustic near-field sensitivity. This paper has experimentally investigated the effects of external pinna in the acoustic sensing and detection capabilities of aye-ayes. To experimentally evaluate the effects of external ear (pinna) of the aye-aye, the tap-scanning process was simulated using a robotic arm. A pinna was 3D printed using a CT scan obtained from a carcass. The pinna's effect on the acoustic near-field has been evaluated in time and frequency domains for simulated tap-scanning with the pinna in upright and cupped positions. This idea originates from behavioral observations of the aye-aye using its ears in this way. The results suggest that the aye-aye can substantially enhance its acoustic near-field sensitivity through a cupped conformation during tap-scanning. Three phenomena contribute to this substantial enhancement of the acoustic near-field: (i) a considerable increase in the signal-to-noise ratio, (ii) the creation of a focal area and potentially a focal point to increase the spatial resolution, and (iii) an increase in the receiver peak frequency by changing near-field beam pattern for higher frequencies that can result in greater sensitivity due to a smaller wavelength.

Vestigial auriculomotor activity indicates the direction of auditory attention in humans

Unlike dogs and cats, people do not point their ears as they focus attention on novel, salient, or task-relevant stimuli. Our species may nevertheless have retained a vestigial pinna-orienting system that has persisted as a 'neural fossil’ within in the brain for about 25 million years. Consistent with this hypothesis, we demonstrate that the direction of auditory attention is reflected in sustained electrical activity of muscles within the vestigial auriculomotor system. Surface electromyograms (EMGs) were taken from muscles that either move the pinna or alter its shape. To assess reflexive, stimulus-driven attention we presented novel sounds from speakers at four different lateral locations while the participants silently read a boring text in front of them. To test voluntary, goal-directed attention we instructed participants to listen to a short story coming from one of these speakers, while ignoring a competing story from the corresponding speaker on the opposite side. In both experiments, EMG recordings showed larger activity at the ear on the side of the attended stimulus, but with slightly different patterns. Upward movement (perking) differed according to the lateral focus of attention only during voluntary orienting; rearward folding of the pinna’s upper-lateral edge exhibited such differences only during reflexive orienting. The existence of a pinna-orienting system in humans, one that is experimentally accessible, offers opportunities for basic as well as applied science.

Dogs, cats, monkeys and other animals perk their ears in the direction of sounds they are interested in. Humans and their closest ape relatives, however, appear to have lost this ability. Some humans are able to wiggle their ears, suggesting that some of the brain circuits and muscles that allow automatic ear movements towards sounds are still present. This may be a ‘vestigial feature’, an ability that is maintained even though it no longer serves its original purpose.

Now, Strauss et al. show that vestigial movements of muscles around the ear indicate the direction of sounds a person is paying attention to. In the experiments, human volunteers tried to read a boring text while surprising sounds like a traffic jam, a baby crying, or footsteps played. During this exercise, Strauss et al. recorded the electrical activity in the muscles of their ears to see if they moved in response to the direction the sound came from. In a second set of experiments, the same electrical recordings were made as participants listened to a podcast while a second podcast was playing from a different direction. The individuals’ ears were also recorded using high resolution video.

Both sets of experiments revealed tiny involuntary movements in muscles surrounding the ear closest to the direction of a sound the person is listening to. When the participants tried to listen to one podcast and tune out another, they also made ear ‘perking’ movements in the direction of their preferred podcast.

The results suggest that movements of the vestigial muscles in the human ear indicate the direction of sounds a person is paying attention to. These tiny movements could be used to develop better hearing aids that sense the electrical activity in the ear muscles and amplify sounds the person is trying to focus on, while minimizing other sounds.

The relative efficacy of the cranium and os coxa for taxonomic assessment in macaques

OBJECTIVES

The cranium is generally considered more reliable than the postcranium for assessing primate taxonomy, although recent research suggests that pelvic shape may be equally reliable. However, little research has focused on intrageneric taxonomic discrimination. Here, we test the relative taxonomic efficacy of the cranium and os coxa for differentiating two macaque species, with and without considering sexual dimorphism.

MATERIALS AND METHODS

Geometric morphometric analyses were performed on cranial and os coxa landmarks for 119 adult Macaca fascicularis, M. mulatta, and Chlorocebus pygerythrus. Among-group shape variation was examined using canonical variates analyses. Cross-validated discriminant function analysis provided rates of correct group classification. Additionally, average morphological distances were compared with neutral genetic distances.

RESULTS

Macaque species were clearly differentiated, both cranially and pelvically, when sex was not considered. Males were more often correctly classified based on the os coxa, while female classification rates were high for both morphologies. Female crania and male os coxa were differentiated approximately the same as genetic distance, while male crania were more similar (convergent), and female os coxa were more divergent than expected based on genetic distance.

DISCUSSION

The hypothesis that cranial and os coxal shape can be used to discriminate among macaque species was supported. The cranium was better at differentiating females, while the os coxa was better at differentiating male macaques. Hence, there is no a priori reason for preferring the cranium when assessing intragenetic taxonomic relationships, but the effects of high levels of sexual dimorphism must be corrected for to accurately assess taxonomic signatures.

Functional anatomy of the middle and inner ears of the red fox, in comparison to domestic dogs and cats

Anatomical middle and inner ear parameters are often used to predict hearing sensitivities of mammalian species. Given that ear morphology is substantially affected both by phylogeny and body size, it is interesting to consider whether the relatively small anatomical differences expected in related species of similar size have a noticeable impact on hearing. We present a detailed anatomical description of the middle and inner ears of the red fox Vulpes vulpes, a widespread, wild carnivore for which a behavioural audiogram is available. We compare fox ears to those of the well‐studied and similarly sized domestic dog and cat, taking data for dogs and cats from the literature as well as providing new measurements of basilar membrane (BM) length and hair cell numbers and densities in these animals. Our results show that the middle ear of the red fox is very similar to that of dogs. The most obvious difference from that of the cat is the lack of a fully formed bony septum in the bulla tympanica of the fox. The cochlear structures of the fox, however, are very like those of the cat, whereas dogs have a broader BM in the basal cochlea. We further report that the mass of the middle ear ossicles and the bulla volume increase with age in foxes. Overall, the ear structures of foxes, dogs and cats are anatomically very similar, and their behavioural audiograms overlap. However, the results of several published models and correlations that use middle and inner ear measurements to predict aspects of hearing were not always found to match well with audiogram data, especially when it came to the sharper tuning in the fox audiogram. This highlights that, although there is evidently a broad correspondence between structure and function, it is not always possible to draw direct links when considering more subtle differences between related species.

Gene expression dataset for whole cochlea of Macaca fascicularis

Macaca fascicularis is a highly advantageous model in which to study human cochlea with regard to both evolutionary proximity and physiological similarity of the auditory system. To better understand the properties of primate cochlear function, we analyzed the genes predominantly expressed in M. fascicularis cochlea. We compared the cochlear transcripts obtained from an adult male M. fascicularis by macaque and human GeneChip microarrays with those in multiple macaque and human tissues or cells and identified 344 genes with expression levels more than 2-fold greater than in the other tissues. These “cochlear signature genes” included 35 genes responsible for syndromic or nonsyndromic hereditary hearing loss. Gene set enrichment analysis revealed groups of genes categorized as “ear development” and “ear morphogenesis” in the top 20 gene ontology categories in the macaque and human arrays, respectively. This dataset will facilitate both the study of genes that contribute to primate cochlear function and provide insight to discover novel genes associated with hereditary hearing loss that have yet to be established using animal models.

Description and analysis of three Homo naledi incudes from the Dinaledi Chamber, Rising Star cave (South Africa)

This study describes three incudes recovered from the Dinaledi Chamber in the Rising Star cave system in South Africa. All three bones were recovered during sieving of excavated sediments and likely represent three Homo naledi individuals. Morphologically and metrically, the Dinaledi ossicles resemble those of chimpanzees and Paranthropus robustus more than they do later members of the genus Homo, and fall outside of the modern human range of variation in several dimensions. Despite this, when overall size is considered, the functional lengths in H. naledi and P. robustus are very similar to those predicted for a human with a similar-sized incus. In this sense, both taxa seem to show a relatively elongated functional length, distinguishing them from chimpanzees. The functional length in H. naledi is slightly longer in absolute terms than in P. robustus, suggesting H. naledi may already show a slight increase in functional length compared with early hominins. While H. naledi lacks the more open angle between the long and short processes found in modern humans, considered a derived feature within the genus Homo, the value in H. naledi is similar to that predicted for a hominoid with a similar-sized incus. Principal components analysis of size-standardized variables shows H. naledi falling outside of the recent human range of variation, but within the confidence ellipse for gorillas. Phylogenetic polarity is complicated by the absence of incus data from early members of the genus Homo, but the generally primitive nature of the H. naledi incudes is consistent with other primitive features of the species, such as the very small cranial capacity. These ossicles add significantly to the understanding of incus variation in hominins and provide important new data on the morphology and taxonomic affinities of H. naledi.

Testing Convergent Evolution in Auditory Processing Genes between Echolocating Mammals and the Aye-Aye, a Percussive-Foraging Primate

Several taxonomically distinct mammalian groups-certain microbats and cetaceans (e.g., dolphins)-share both morphological adaptations related to echolocation behavior and strong signatures of convergent evolution at the amino acid level across seven genes related to auditory processing. Aye-ayes (Daubentonia madagascariensis) are nocturnal lemurs with a specialized auditory processing system. Aye-ayes tap rapidly along the surfaces of trees, listening to reverberations to identify the mines of wood-boring insect larvae; this behavior has been hypothesized to functionally mimic echolocation. Here we investigated whether there are signals of convergence in auditory processing genes between aye-ayes and known mammalian echolocators. We developed a computational pipeline (Basic Exon Assembly Tool) that produces consensus sequences for regions of interest from shotgun genomic sequencing data for nonmodel organisms without requiring de novo genome assembly. We reconstructed complete coding region sequences for the seven convergent echolocating bat-dolphin genes for aye-ayes and another lemur. We compared sequences from these two lemurs in a phylogenetic framework with those of bat and dolphin echolocators and appropriate nonecholocating outgroups. Our analysis reaffirms the existence of amino acid convergence at these loci among echolocating bats and dolphins; some methods also detected signals of convergence between echolocating bats and both mice and elephants. However, we observed no significant signal of amino acid convergence between aye-ayes and echolocating bats and dolphins, suggesting that aye-aye tap-foraging auditory adaptations represent distinct evolutionary innovations. These results are also consistent with a developing consensus that convergent behavioral ecology does not reliably predict convergent molecular evolution.

Morphology and function of Neandertal and modern human ear ossicles

The diminutive middle ear ossicles (malleus, incus, stapes) housed in the tympanic cavity of the temporal bone play an important role in audition. The few known ossicles of Neandertals are distinctly different from those of anatomically modern humans (AMHs), despite the close relationship between both human species. Although not mutually exclusive, these differences may affect hearing capacity or could reflect covariation with the surrounding temporal bone. Until now, detailed comparisons were hampered by the small sample of Neandertal ossicles and the unavailability of methods combining analyses of ossicles with surrounding structures. Here, we present an analysis of the largest sample of Neandertal ossicles to date, including many previously unknown specimens, covering a wide geographic and temporal range. Microcomputed tomography scans and 3D geometric morphometrics were used to quantify shape and functional properties of the ossicles and the tympanic cavity and make comparisons with recent and extinct AMHs as well as African apes. We find striking morphological differences between ossicles of AMHs and Neandertals. Ossicles of both Neandertals and AMHs appear derived compared with the inferred ancestral morphology, albeit in different ways. Brain size increase evolved separately in AMHs and Neandertals, leading to differences in the tympanic cavity and, consequently, the shape and spatial configuration of the ossicles. Despite these different evolutionary trajectories, functional properties of the middle ear of AMHs and Neandertals are largely similar. The relevance of these functionally equivalent solutions is likely to conserve a similar auditory sensitivity level inherited from their last common ancestor.

Comparative anatomy of the middle ear ossicles of extant hominids--Introducing a geometric morphometric protocol

The presence of three interconnected auditory ossicles in the middle ear is a defining characteristic of mammals, and aspects of ossicle morphology are related to hearing sensitivity. However, analysis and comparison of ossicles are complicated by their minute size and complex three-dimensional shapes. Here we introduce a geometric morphometric measurement protocol for 3D shape analysis based on landmarks and semilandmarks obtained from μCT images and apply it to ossicles of extant hominids (great apes and humans). We show that the protocol is reliable and reproducible over a range of voxel resolutions, and captures even subtle shape differences. Using this approach it is possible to distinguish the hominid taxa by mean shapes of their malleus and incus (p < 0.01). The stapes appears less diagnostic, although this may in part be related to the small sample size available. Using ancestral state estimation, we show that, within hominids, Homo sapiens is derived with respect to its malleus (short manubrium, long corpus, head anterior-posterior flattened, articular facet shape), incus (wide intercrural curvature, long incudal processes, articular facet shape) and stapes (high stapes with kidney-shaped footplate). H. sapiens also shows a number of plesiomorphic shape traits whereas Gorilla and Pan possess a number of autapomorphic characteristics. The Pongo ossicles appear to be close to the plesiomorphic hominid condition. The malleus shows little difference in size among hominids, and allometry is thus of little importance. In contrast, the incus and stapes are more variable in size, and their shape is more strongly related to size differences. Although the form-function relationships in the middle ear are not fully understood, some aspects of ossicle morphology suggest that interspecific differences in hearing capacities are present among hominids. Finally, the results of this study provide a comparative framework for morphometric studies analyzing ossicles of extinct hominids, with a bearing on taxonomy, phylogeny and auditory function.

Evidence for a vestigial pinna-orienting system in humans

Although some people can voluntarily move their ears, overt reflexive control of the pinnae has been lost during the course of primate evolution. Humans and apes do not move their ears to express emotion, they do not defensively retract them when startled, and they do not point them at novel, salient, or task-relevant stimuli. Nevertheless, it is the thesis of this review that neural circuits for pinna orienting have survived in a purely vestigial state for over 25 million years. There are three lines of evidence: (1) Shifting the eyes hard to one side is accompanied by electromyographic (EMG) activity in certain ear muscles and by a barely visible (2-3 mm) curling of the dorsal edge of the pinna. (2) The capture of attention by a novel, unexpected sound emanating from behind and to one side has been found to trigger a weak EMG response in the muscle behind the corresponding ear. (3) Reflexive EMG bursts recorded during a selective attention task suggested that subjects were unconsciously attempting to orient their ears toward the relevant sounds. In addition to pinna orienting, the possibility that pinna startle might have survived in a vestigial state is also considered. It is suggested that the postauricular reflex to sudden, intense sounds constitutes a vestigial startle response, but that the reflex arc is dominated by a pathway that bypasses the main organizing center for startle.

A genome-wide association study identifies multiple loci for variation in human ear morphology

Here we report a genome-wide association study for non-pathological pinna morphology in over 5,000 Latin Americans. We find genome-wide significant association at seven genomic regions affecting: lobe size and attachment, folding of antihelix, helix rolling, ear protrusion and antitragus size (linear regression P values 2 × 10(-8) to 3 × 10(-14)). Four traits are associated with a functional variant in the Ectodysplasin A receptor (EDAR) gene, a key regulator of embryonic skin appendage development. We confirm expression of Edar in the developing mouse ear and that Edar-deficient mice have an abnormally shaped pinna. Two traits are associated with SNPs in a region overlapping the T-Box Protein 15 (TBX15) gene, a major determinant of mouse skeletal development. Strongest association in this region is observed for SNP rs17023457 located in an evolutionarily conserved binding site for the transcription factor Cartilage paired-class homeoprotein 1 (CART1), and we confirm that rs17023457 alters in vitro binding of CART1.

Evolution of the auditory ossicles in extant hominids: metric variation in African apes and humans

The auditory ossicles in primates have proven to be a reliable source of phylogenetic information. Nevertheless, to date, very little data have been published on the metric dimensions of the ear ossicles in African apes and humans. The present study relies on the largest samples of African ape ear ossicles studied to date to address questions of taxonomic differences and the evolutionary transformation of the ossicles in gorillas, chimpanzees and humans. Both African ape taxa show a malleus that is characterized by a long and slender manubrium and relatively short corpus, whereas humans show the opposite constellation of a short and thick manubrium and relatively long corpus. These changes in the manubrium are plausibly linked with changes in the size of the tympanic membrane. The main difference between the incus in African apes and humans seems to be related to changes in the functional length. Compared with chimpanzees, human incudes are larger in nearly all dimensions, except articular facet height, and show a more open angle between the axes. The gorilla incus resembles humans more closely in its metric dimensions, including functional length, perhaps as a result of the dramatically larger body size compared with chimpanzees. The differences between the stapedes of humans and African apes are primarily size-related, with humans being larger in nearly all dimensions. Nevertheless, some distinctions between the African apes were found in the obturator foramen and head height. Although correlations between metric variables in different ossicles were generally lower than those between variables in the same bone, variables of the malleus/incus complex appear to be more strongly correlated than those of the incus/stapes complex, perhaps reflecting the different embryological and evolutionary origins of the ossicles. The middle ear lever ratio for the African apes is similar to other haplorhines, but humans show the lowest lever ratio within primates. Very low levels of sexual dimorphism were found in the ossicles within each taxon, but some relationship with body size and several dimensions of the ear bones was found. Several of the metric distinctions in the incus and stapes imply a slightly different articulation of the ossicular chain within the tympanic cavity in African apes compared with humans. The limited auditory implications of these metric differences in the ossicles are also discussed. Finally, the results of this study suggest that several plesiomorphic features for apes may be retained in the ear bones of the early hominin taxa Australopithecus and Paranthropus as well as in the Neandertals.

Early hominin auditory ossicles from South Africa

The middle ear ossicles are only rarely preserved in fossil hominins. Here, we report the discovery of a complete ossicular chain (malleus, incus, and stapes) of Paranthropus robustus as well as additional ear ossicles from Australopithecus africanus. The malleus in both early hominin taxa is clearly human-like in the proportions of the manubrium and corpus, whereas the incus and stapes resemble African and Asian great apes more closely. A deep phylogenetic origin is proposed for the derived malleus morphology, and this may represent one of the earliest human-like features to appear in the fossil record. The anatomical differences found in the early hominin incus and stapes, along with other aspects of the outer, middle, and inner ear, are consistent with the suggestion of different auditory capacities in these early hominin taxa compared with modern humans.

Reassessment of the La Ferrassie 3 Neandertal ossicular chain

The ossicular chain in La Ferrassie 3 was briefly described in the monograph on the La Ferrassie Neandertal children, but to date has not been the subject of detailed study. We provide new data on these important fossils and re-examine some previous suggestions of derived Neandertal features in the middle ear ossicles based on more limited evidence. The malleus shows a curved lateral margin of the manubrium and a relatively large head. The incus shows a tall articular facet, a depressed area on the medial surface of the body, a straight anterior border of the long process and a more closed angle between the processes. The stapes shows an asymmetrical configuration of the crura, with an anteriorly skewed head, and generally small dimensions, including a smaller and relatively wider stapedial footplate. These same features can also be seen in the few other Neandertal ear ossicles known, suggesting that a consistent anatomical pattern characterizes the Neandertal ossicular chain. While the phylogenetic polarity of many of these features remains to be clarified, the asymmetrical stapes and anteriorly skewed stapedial head appear to be derived Neandertal features. In addition, while the larger malleus head and incus articular facet in La Ferrassie 3 might reflect larger body mass in Neandertals, the larger stapes footplates in Homo sapiens cannot be explained by changes in body mass. Indeed, H. sapiens seems to depart from the general mammalian pattern in combining an increase in stapes footplate size with a decrease in body mass. Although the malleus/incus lever ratio in La Ferrassie 3 is similar to that in H. sapiens, Neandertals appear to be characterized by a slightly different spatial relationship and articulation of the ossicular chain within the tympanic cavity. While only limited inferences can be drawn regarding hearing ability based on the ossicles, the few physiologically relevant dimensions in the La Ferrassie 3 ear bones are similar to H. sapiens.

Receiver bias and the acoustic ecology of aye-ayes (Daubentonia madagascariensis)

The aye-aye is a rare lemur from Madagascar that uses its highly specialized middle digit for percussive foraging. This acoustic behavior, also termed tap-scanning, produces dominant frequencies between 6 and 15 kHz. An enhanced auditory sensitivity to these frequencies raises the possibility that the acoustic and auditory specializations of aye-ayes have imposed constraints on the evolution of their vocal signals, especially their primary long-distance vocalization, the screech. Here we explore this concept, termed receiver bias, and suggest that the dominant frequency of the screech call (~2.7 kHz) represents an evolutionary compromise between the opposing adaptive advantages of long-distance sound propagation and enhanced detection by conspecific receivers.

Social drive and the evolution of primate hearing

The structure and function of primate communication have attracted much attention, and vocal signals, in particular, have been studied in detail. As a general rule, larger social groups emit more types of vocal signals, including those conveying the presence of specific types of predators. The adaptive advantages of receiving and responding to alarm calls are expected to exert a selective pressure on the auditory system. Yet, the comparative biology of primate hearing is limited to select species, and little attention has been paid to the effects of social and vocal complexity on hearing. Here, we use the auditory brainstem response method to generate the largest number of standardized audiograms available for any primate radiation. We compared the auditory sensitivities of 11 strepsirrhine species with and without independent contrasts and show that social complexity explains a significant amount of variation in two audiometric parameters-overall sensitivity and high-frequency limit. We verified the generality of this latter result by augmenting our analysis with published data from nine species spanning the primate order. To account for these findings, we develop and test a model of social drive. We hypothesize that social complexity has favoured enhanced hearing sensitivities, especially at higher frequencies.

Whispering to the deaf: communication by a frog without external vocal sac or tympanum in noisy environments

Atelopus franciscus is a diurnal bufonid frog that lives in South-American tropical rain forests. As in many other frogs, males produce calls to defend their territories and attract females. However, this species is a so-called "earless" frog lacking an external tympanum and is thus anatomically deaf. Moreover, A. franciscus has no external vocal sac and lives in a sound constraining environment along river banks where it competes with other calling frogs. Despite these constraints, male A. franciscus reply acoustically to the calls of conspecifics in the field. To resolve this apparent paradox, we studied the vocal apparatus and middle-ear, analysed signal content of the calls, examined sound and signal content propagation in its natural habitat, and performed playback experiments. We show that A. franciscus males can produce only low intensity calls that propagate a short distance (<8 m) as a result of the lack of an external vocal sac. The species-specific coding of the signal is based on the pulse duration, providing a simple coding that is efficient as it allows discrimination from calls of sympatric frogs. Moreover, the signal is redundant and consequently adapted to noisy environments. As such a coding system can be efficient only at short-range, territory holders established themselves at short distances from each other. Finally, we show that the middle-ear of A. franciscus does not present any particular adaptations to compensate for the lack of an external tympanum, suggesting the existence of extra-tympanic pathways for sound propagation.

Technical note: a landmark-based approach to the study of the ear ossicles using ultra-high-resolution X-ray computed tomography data

Previous study of the ear ossicles in Primates has demonstrated that they vary on both functional and phylogenetic bases. Such studies have generally employed two-dimensional linear measurements rather than three-dimensional data. The availability of Ultra- high-resolution X-ray computed tomography (UhrCT) has made it possible to accurately image the ossicles so that broadly accepted methodologies for acquiring and studying morphometric data can be applied. Using UhrCT data also allows for the ossicular chain to be studied in anatomical position, so that it is possible to consider the spatial and size relationships of all three bones. One issue impeding the morphometric study of the ear ossicles is a lack of broadly recognized landmarks. Distinguishing landmarks on the ossicles is difficult in part because there are only two areas of articulation in the ossicular chain, one of which (the malleus/incus articulation) has a complex three-dimensional form. A measurement error study is presented demonstrating that a suite of 16 landmarks can be precisely located on reconstructions of the ossicles from UhrCT data. Estimates of measurement error showed that most landmarks were highly replicable, with an average CV for associated interlandmark distances of less than 3%. The positions of these landmarks are chosen to reflect not only the overall shape of the bones in the chain and their relative positions, but also functional parameters. This study should provide a basis for further examination of the smallest bones in the body in three dimensions.

Correlations between auditory structures and hearing sensitivity in non-human primates

Primates show distinctions in hearing sensitivity and auditory morphology that generally follow phylogenetic patterns. However, few previous studies have attempted to investigate how differences in primate hearing are directly related to differences in ear morphology. This research helps fill this void by exploring the form-to-function relationships of the auditory system in a phylogenetically broad sample of non-human primates. Numerous structures from the outer, middle, and inner ears were measured in taxa with known hearing capabilities. The structures investigated include the overall size and shape of the pinna, the areas of the tympanic membrane and stapedial footplate, the masses and lever arm lengths of the ossicles, the volumes of the middle ear cavities, and the length of the cochlea. The results demonstrate that a variety of auditory structures show significant correlations with certain aspects of hearing (particularly low-frequency sensitivity). Although the majority of these relationships agree with expectations from auditory theory, some traditional (and possibly outdated) ideas were not supported. For example, the common misconception that higher middle ear transformer ratios (e.g., impedance transformer ratio) result in increased hearing sensitivity was not supported. Although simple correlations between form and function do not necessarily imply causality, the relationships defined in this study not only increase our understanding of auditory patterns in extant taxa but also lay the foundation to begin investigating the hearing in fossil primates.

Relief index of second mandibular molars is a correlate of diet among prosimian primates and other euarchontan mammals

This study describes and tests a new method of calculating a shape metric known as the relief index (RFI) on lower second molars of extant euarchontan mammals, including scandentians (treeshrews), dermopterans (flying lemurs), and prosimian primates (strepsirhines and tarsiers). RFI is the ratio of the tooth crown three-dimensional area to two-dimensional planar area. It essentially expresses hypsodonty and complexity of tooth shape. Like other measurements of complexity, RFI ignores taxon-specific features, such as certain cusps and crests, which are usually considered in more traditional studies of tooth function. Traditional statistical analyses of the study sample show that RFI distinguishes taxa with differing amounts of structural carbohydrates in their diets, with frugivore/gramnivores being significantly lower in RFI than omnivores, and insectivores/folivores being significantly higher in RFI than the other two. Information on absolute size, or body mass, is needed to reliably parse out insectivores and folivores; however, if the study sample is limited to Primates, RFI alone distinguishes many folivores (lower) from insectivores (higher). Finally, phylogenetically independent contrasts of RFI and dietary preference are strongly correlated with one another, indicating that variance in RFI is better explained by dietary diversity than phylogenetic affinity in this sample. Because of the accuracy and phylogenetic insensitivity of the RFI among Euarchonta, this method can be applied to fossil primates and stem-primates (plesiadapiforms) and used to elucidate and compare their dietary preferences. Such comparisons are important for developing a more detailed view of primate evolution.

Cranial anatomy of the Paleocene plesiadapiform Carpolestes simpsoni (Mammalia, Primates) using ultra high-resolution X-ray computed tomography, and the relationships of plesiadapiforms to Euprimates

Central to issues surrounding the origin of euprimates, affinities of Paleocene Carpolestidae have been controversial. Carpolestids have been classified as plesiadapoid primates, tarsiiform euprimates, dermopterans, or the sister taxon of euprimates to the exclusion of other plesiadapiforms, based exclusively on dental or postcranial data. Newly discovered crania of Carpolestes simpsoni from the latest Paleocene of the Clarks Fork Basin, Wyoming, are the first described for the family Carpolestidae. The two best preserved skulls were studied using ultra high-resolution X-ray computed tomography. Comparison of these specimens to those of other stem primates (Plesiadapiformes) demonstrates that the diversity of cranial morphology in this group is greater than previously thought. Carpolestes differs from euprimates and is similar to other plesiadapiforms (Ignacius and Plesiadapis) in lacking a postorbital bar and having a relatively long rostrum. Carpolestes is similar to fossil euprimates and Plesiadapis in having a bullar morphology consistent with a petrosal origin, and differs from Ignacius, in which the bulla is composed of the entotympanic. Carpolestes differs from primitive euprimates and all other known plesiadapiforms in possessing a two-chambered auditory bulla, similar to that of modern Tarsius. However, Carpolestes had an internal carotid artery (ICA) that took a transpromontorial route from a posteromedially positioned posterior carotid foramen (pcf), unlike Tarsius, in which this artery takes a perbullar route from an anterolaterally positioned pcf. Carpolestes has clear grooves on the promontorium for both the promontorial and stapedial arteries, indicating that it had an unreduced internal carotid circulation, similar to that of early euprimates. Carpolestes differs from primitive euprimates and some specimens of Ignacius in not having bony tubes surrounding the branches of the ICA. Cladistic analysis of cranial data fails to support a close relationship of Carpolestidae to either tarsiiform euprimates or extant Dermoptera, but suggests a close relationship between Carpolestidae, Plesiadapidae, and Euprimates.

Evolution of the special senses in primates: past, present, and future

The present special issue of The Anatomical Record is the result of a symposium entitled Evolution of the Special Senses in Primates. Considered together, the special senses of primates are remarkable because they constitute a singular and definitive suite of mammalian characteristics. Examining their evolution is pivotal for understanding the origin and present-day variation of primate behavior and ecology. Accordingly, the 14 articles assembled here consider the different constraints and opportunities associated with the uptake and use of physical and chemical stimuli. The present issue brings together experts on different primate sensory modalities and stresses events at the sensory periphery, where the organism is exposed to and comes into contact with its environment. Key topics include color vision, the genetics of olfaction, the morphological basis and significance of chemical communication, and the neural organization and scaling of primate sensory systems. The result is a special issue that both reflects our current understanding of primate sensory modalities and challenges certain fundamental assumptions concerning their evolution.