How does the tympanic membrane function in frogs

The lateral-most portion of the stapedial footplate elongates to complete the formation of the shaft of the pars media plectri, which extends laterally towards the outside of the head. Meanwhile, the tympanic annulus and pars externa plectri develop as cartilaginous condensations associated with the posterior margin of the palatoquadrate. As the palatoquadrate swings posteriorly during metamorphosis, so too do the tympanic annulus and pars externa plectri.

As the ontogenetic sequence of development of these structures progresses, they are positioned in the same medial-lateral plane. At this point, the partes media and externa plectri connect synchondrotically to each other and the tympanic annulus induces the differentiation of the tympanic membrane 27 , The sequences of losses and gains appear to be related to the relative timing of the development of structures heterochronies and tissue differentiation phenomena.

For example, Helff 69 demonstrated the inductive effects of the tympanic annulus on the tegument to produce the differentiation of the tympanic membrane, which explains why the tympanic membrane never occurs in the absence of a tympanic annulus. Hetherington 27 and Smirnov 71 also observed that several species undergo post-metamorphic development of previously absent or undeveloped structures e. Meanwhile, Smirnov 72 pointed out that developmental heterochronies progenesis, neoteny and post-displacement seem to play a major role in the post-metamorphic development of these structures.

All these events occur in specific sequences and their disruption in particular stages could produce the observed patterns of losses in the subsequent stages of development of the TME.

Therefore, research into the genetic basis for the absence of induction of lateral elements promises to be a fruitful line of investigation. Additionally, genetic mechanisms that directly regulate the expression of these ear structures might be involved.

Knowledge of the origin of the components of the vertebrate auditory system is incipient generally and for anurans particularly. However, recent studies of Xenopus laevis support a model in which the cartilaginous elements of the TME are derived from three neural crest cell streams see 73 : 1 the mandibular stream forms the tympanic annulus, 2 the hyoid stream gives rise to the partes media and externa plectri and 3 the branchial stream forms the pars interna plectri.

Also, it is likely that the development of the tympanic annulus and pars externa plectri in the margins of the palatoquadrate and the partes interna and media plectri in the otic capsule results from the initiation of a common developmental module, as in the morphogenesis of many other structures 74 , Unfortunately, information on the developmental control genes that lead to the formation of elements in the amphibian middle ear is unavailable.

However, some genetic pathways involved in this differentiation process have been identified in other vertebrates and could be examined in frogs The lateral—medial dependency between the presence and absence of tympanic middle ear structures appears also to be related to functional constraints: a tympanic membrane without a tympanic annulus or columella would have no acoustic function, as would a tympanic annulus without a columella.

In contrast, the tympanic annulus retains its acoustic function in the absence of a tympanic membrane and the columella remains acoustically functional even in the absence of both structures, as evidenced by the middle ears of salamanders 1.

This asymmetric functional dependency appears to have allowed these three structures to evolve sequentially rather than as a single transformation series i.

The losses, regains and re-losses of TME structures in Bufonidae make true toads an excellent model to study the behavioural correlates of TME morphology.

Previous studies have hypothesized a relationship between earlessness and aquatic or fossorial habitats and lack of acoustic communication or production of low-frequency calls Additionally, based on the limited evidence presently available see section S5 of the Supplementary Information , the loss of TME structures in Bufonidae appears to be coincident with the origin of a scramble competition mating system in which males in dense aggregations attempt amplexus indiscriminately and struggle for possession of females In this mating system, acoustic territorial defence is absent and reliance on hearing for mate choice is greatly reduced or eliminated, as is the effectiveness of prezygotic isolating barriers like advertisement calls 78 , which presumably results in the natural interspecific hybridization observed in many bufonid species see 79 and references therein.

Nevertheless, although most of the species of early diverging clades of Bufonidae for which the mating system is known exhibit scramble competition see section S5 of the Supplementary Information , the reproductive behaviour of most species is unknown, making the character state reconstruction of this behaviour at the root node of Bufonidae ambiguous. Indeed, despite the absence of a TME and the occurrence of a scramble competition mating strategy, interspecific acoustic diversity is maintained in most genera e.

The maintenance of call diversity and widespread production of advertisement calls may be explained by extratympanic hearing pathways in earless frogs. Multiple extratympanic pathways, including a lung pathway e. Given that in at least some anurans airborne sounds are transferred via both tympanic and extratympanic pathways reviewed by 3 , 25 , anurans may experience relaxed selective pressures on the TME if TME plasticity does not greatly affect acoustic acuity.

If the generality of alternative sound transfer pathways for aerial sounds is corroborated across anuran diversity, then the pre-existence of alternative pathways for airborne sound transmission might explain the high rate of TME loss in anurans. Nevertheless, currently proposed sound localization pathways in anurans all require middle ear coupling 93 , leaving an alternative mechanism by which earless species localize audible sounds unknown.

Since the tympanic annulus and membrane first arose in combination with the plesiomorphically present columella, either prior to the origin of Anura or in Lalagobatrachia the clade formed by all anurans except Ascaphidae and Leiopelmatidae , our analysis indicates that the TME was completely lost at least 38 times in anurans, usually in small clades within diverse families.

Bufonidae is exceptional within both Anura and among all tetrapods in that the loss of all TME structures preceded a radiation of more than earless species followed by independent regains and many additional losses in most derived clades. In contrast, among the approximately species of amniotes the TME was completely lost only three times and was never regained.

The complex pattern of TME evolution, extensive morphological and reproductive diversity and maintenance of bioacoustic diversity despite the loss of TME structures make Bufonidae a promising model to study extratympanic pathways of sound transmission, the physiological and behavioural consequences of middle ear loss and the underlying genetic and developmental mechanisms that shaped its remarkable TME diversity.

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Amphibians like frogs, some reptiles and many insects use this protective circular patch of skin stretched over a ring of cartilage just like a drum to transmit sound waves to the middle and inner ear for interpretation by the brain. For a frog, the tympanum allows it to hear both in the air and below the water. How do frog ears work? Their eardrum works like a regular eardrum with one very special adaptation…it is actually connected to their lungs.

The lungs vibrate and are almost as sensitive to hearing as the eardrum. This allows frogs to make really loud sounds without hurting their own eardrums! It separates the outer ear from the middle ear. When sound waves reach the tympanic membrane they cause it to vibrate. The vibrations are then transferred to the tiny bones in the middle ear. The middle ear bones then transfer the vibrating signals to the inner ear. A ruptured eardrum can result in hearing loss.

It can also make your middle ear vulnerable to infections. A ruptured eardrum usually heals within a few weeks without treatment. But sometimes it requires a patch or surgical repair to heal.

It is approximately 0. It functions much like our eardrum does —the tympanum transmits sound waves to the middle and inner ear, allowing a frog to hear both in the air and below water. Naturally Curious is supported by donations. This entry was posted on August 15, by Mary Holland. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account.

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