The earless toad project
Original drawing by Staci Amburgey
This project aims to understand why so many species of frogs and toads have lost (and potentially regained!) their ears. To do this we are investigating toads (family Bufonidae) and integrating neurophysiology (hearing data), morphology (histology, 3D reconstructions, microCT scans) and development (tracking and comparing 3 eared and 3 earless species from egg to 1 year post metamorphosis) to get an unprecedented, holistic idea of why ear loss is occurring at such a large scale across frogs and toads.
It is important to note that ear loss in frogs and toads does not mean that all ear structures are missing. The inner ear structures (where hair cells sense incoming sound, just like your inner ear) are still fully formed and functional. However, the tympanic middle ear components (the tympanic membrane, columella or stapes, middle ear cavity and eustachian tube). These structures are similar to the outer and middle ears used by almost every species of land-living tetrapod (including you and your dog) to bring sound from the outside world to the inner ear, where it can be sensed by hair cells.
There are many insights into evolution that ear loss can provide.
Above is a clip from a Taylor Swift song edited to remove frequencies earless toads do not hear as well as eared toads to imitate "earless toad hearing".
We have auditory brainstem recordings for all 13 focal bufonid species (5 earless, 8 eared) and have found VERY interesting results both between eared and earless species and amongst eared and earless species. Stay tuned for new hearing data news.
The video above displays a 3D reconstruction of the inner, middle and outer ear structures of Bufo margaritifer. This model also shows the opercularis system. Scale bar is 1 mm.
I have created 3D models of 13 focal toad species to compare the hearing structures and surrounding tissue between 5 earless and 8 eared toad species.
Check back for updates on our results or look for incoming data on my blog.
We have microCT scans for our 13 focal species plus an additional 48 toad species loaned from the Smithsonian to analyze differences in the skull between closely related eared and earless species that will help us understand the developmental and selection pressures that may be acting on ear loss in anurans.
One of the most interesting facts about toads (and frogs in general) is that many species undergo a process of metamorphosis that changes them from an underwater, legless tadpole to a land-lovin', 4-legged toad. It is right after a toad grows its 4 legs that the middle ear structures begin to develop but in some species this can take up to 1 whole year! Looking for differences in tadpole skull morphology and the timing/rate of skull development will provide key insights to the evolution of ear loss in toads. This part of the project is in the hands of Jenny Stynoski. You can find her updates on the development of ear loss in toads here.
So far, I have looked at the development of one toad species, Rhinella marina, and found that their ear development is a lot faster than the other documented toad species. It will be interesting to see the variation across our other 6 eared focal species.
In collaboration with the Center for Anchored Phylogenomics I am embarking on new genetic work for the earless toads!
I am investigating the coding changes and selection pressures in genes related to middle ear development across ear loss events in toads.