Independent origins of a novel atympanic middle ear system within Chamaeleonidae

The evolution of the vertebrate ear is a complicated story of convergence, co-option, loss of function, and occasional regaining of said function. An incredible variety of structures have been adopted as sound receptors, but only chameleons are known to have a bony airborne sound receiver. In some chameleons, the pterygoid bone captures sound vibrations and relays them to the inner ear via a connection to the extracolumella. The distribution of this unique hearing system has not been examined across Chamaeleonidae. Here, I report on dissections on 12 species across four genera and describe their middle ear anatomy for the first time. Half of these species were found to have a link between their extracolumella and pterygoid, and ancestral state reconstruction supports four independent acquisitions of this novel sound conduction pathway. Species with this pathway tend to have a gular pouch, which seems to produce biotremors and possibly airborne sound, suggesting that this hearing system plays some role in intraspecific communication. Three species were also μ-CT scanned using enhanced contrast to investigate differences in the musculature surrounding the middle ear cavity. In species with a middle ear connected to the pterygoid, the muscles directly lateral to the pterygoid insert farther anterior onto the mandible, which may serve to minimize dampening of vibrations on the pterygoid. Together, these data suggest that the ear plays a more significant role in the lives of some chameleons than has been recognized, and that parallelism is common in the evolution of the ear.

The internal cranial anatomy of Champsosaurus (Choristodera: Champsosauridae): Implications for neurosensory function

Although isolated Champsosaurus remains are common in Upper Cretaceous sediments of North America, the braincase of these animals is enigmatic due to the fragility of their skulls. Here, two well-preserved specimens of Champsosaurus (CMN 8920 and CMN 8919) are CT scanned to describe their neurosensory structures and infer sensory capability. The anterior portion of the braincase was poorly ossified and thus does not permit visualization of a complete endocast; however, impressions of the olfactory stalks indicate that they were elongate and likely facilitated good olfaction. The posterior portion of the braincase is ossified and morphologically similar to that of other extinct diapsids. The absence of an otic notch and an expansion of the pars inferior of the inner ear suggests Champsosaurus was limited to detecting low frequency sounds. Comparison of the shapes of semicircular canals with lepidosaurs and archosauromorphs demonstrates that the semicircular canals of Champsosaurus are most similar to those of aquatic reptiles, suggesting that Champsosaurus was well adapted for sensing movement in an aquatic environment. This analysis also demonstrates that birds, non-avian archosauromorphs, and lepidosaurs possess significantly different canal morphologies, and represents the first morphometric analysis of semicircular canals across Diapsida.

Bony labyrinth morphometry reveals hidden diversity in lungless salamanders (Family Plethodontidae): Structural correlates of ecology, development, and vision in the inner ear

Lungless salamanders (Family Plethodontidae) form a highly speciose group that has undergone spectacular adaptive radiation to colonize a multitude of habitats. Substantial morphological variation in the otic region coupled with great ecological diversity within this clade make plethodontids an excellent model for exploring the ecomorphology of the amphibian ear. We examined the influence of habitat, development, and vision on inner ear morphology in 52 plethodontid species. We collected traditional and 3D geometric morphometric measurements to characterize variation in size and shape of the otic endocast and peripheral structures of the salamander ear. Phylogenetic comparative analyses demonstrate structural convergence in the inner ear across ecologically similar species. Species that dwell in spatially complex microhabitats exhibit robust, highly curved semicircular canals suggesting enhanced vestibular sense, whereas species with reduced visual systems demonstrate reduced canal curvature indicative of relaxed selection on the vestibulo-ocular reflex. Cave specialists show parallel enlargement of auditory-associated structures. The morphological correlates of ecology among diverse species reveal underlying evidence of habitat specialization in the inner ear and suggest that there exists physiological variation in the function of the salamander ear even in the apparent absence of selective pressures on the auditory system to support acoustic behavior.

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.

Silent sounds in the Andes: underwater vocalizations of three frog species with reduced tympanic middle ears (Anura: Telmatobiidae: Telmatobius)

Underwater vocalization in anurans is restricted to a few, distantly related species. In some of them, sound is transmitted through tympanic and extra-tympanic pathways. Members of the Andean genus Telmatobius Wiegmann, 1834 lack a tympanic membrane, and earlier reports assumed the absence of vocalizations in the genus. We recorded underwater vocalizations and examined the middle-ear morphology in three species of Telmatobius with different lifestyles: Telmatobius oxycephalus Vellard, 1946 (semiaquatic, riverine); Telmatobius hintoni Parker, 1940 (markedly aquatic, riverine); Telmatobius culeus (Garman 1876) (fully aquatic, lacustrine). Males emit underwater calls, which in the three species are simple and stereotyped; they consist of a repeated train of notes, with a low fundamental frequency (309–941 Hz). In each of the three species, the tympanic membrane is absent and the tympanic cavity is extremely reduced or absent, whereas the opercular system is well developed. Our data, along with prior knowledge in other species of anurans, suggest that the species examined here probably perceived sound through extra-tympanic pathways. Given the limited knowledge about underwater calling in anurans, Telmatobius seems a logical candidate to study the functional and evolutionary bases of underwater hearing and tympanic middle-ear reduction in anurans.

Acoustic orientation in the great crested newt (Triturus cristatus)

We carried out laboratory experiments to determine whether orientation during migration in the great crested newt (Triturus cristatus) is influenced by acoustic information. Newts retrieved during the aquatic breeding seasons (adults), as well as during the terrestrial phase after breeding (adults and juveniles), were subjected to calls from sympatric (Rana temporariaandBufo bufo) and allopatric (Lithobates catesbeianus) anurans. In addition to natural stimuli, we also used modified anuran calls (continuous sound with inter-note intervals removed), white noise with and without envelop, and a heterochthonous sound (pile driving). In a circular arena, adult newts retrieved both during their aquatic and terrestrial phase orientated towards theB. bufostimulus, and migrated at random directions when exposed to the other calls; the lack of orientation towards the sympatricR. temporariaparallels a largely non-overlapping breeding season. Inexperienced juveniles did not orientate towards anuran calls, suggesting that phonotactic responses could be learned. Both aquatic as well as terrestrial adults significantly moved away from a white noise envelop. The results suggest different degrees of heterospecific call attraction across life stages, and provide evidence that unnatural sound might have an adverse effect on breeding migrations.

Comparative and developmental patterns of amphibious auditory function in salamanders

Early amphibious tetrapods may have detected aquatic sound pressure using sound-induced lung vibrations, but their lack of tympanic middle ears would have restricted aerial sensitivity. Sharing these characteristics, salamanders could be models for the carryover of auditory function across an aquatic-terrestrial boundary without tympanic middle ears. We measured amphibious auditory evoked potential audiograms in five phylogenetically and ecologically distinct salamanders (Amphiuma means, Notophthalmus viridescens, Ambystoma talpoideum, Eurycea spp., and Plethodon glutinosus) and tested whether metamorphosis and terrestrial niche were linked to aerial sensitivity. Threshold differences between media varied between species. A. means' relative aerial sensitivity was greatest at 100 Hz and decreased with increasing frequency. In contrast, all other salamanders retained greater sensitivity up to 500 Hz, and in A. talpoideum and Eurycea, relative sensitivity at 500 Hz was higher than at 100 Hz. Aerial thresholds of terrestrial P. glutinosus above 200 Hz were similar to A. talpoideum and Eurycea, but lower than N. viridescens and A. means. Metamorphosis did not affect aerial sensitivity in N. viridescens or A. talpoideum. These results fail to support a hypothesis of terrestrial hearing specialization across ontogeny or phylogeny. We discuss methodological limitations to our amphibious comparisons and factors affecting variation in amphibious performance.