The inner ears of all bat species initially develop in a way that is required for echolocation, even in those species that are incapable of echolocation, according to research published in Nature Ecology & Evolution this week. The study suggests that the ability to echolocate evolved once and has been lost in some species, rather than evolving several times.
Although bats are known for their ability to navigate using echoes of sonar signals produced in their voice boxes and detected in their inner ears (an ability known as laryngeal echolocation), some bat species cannot echolocate in this way. Both fossil and genetic evidence suggest that the bats that use laryngeal echolocation do not form a single evolutionary group. This means that echolocation has either evolved more than once, or has evolved and then been lost again in some species.
Shuyi Zhang, Emma Teeling, Zhe Wang and colleagues tackle this question by looking not at the inner ear of adult bats, but at how the inner ear develops in foetal and early postnatal bats. The adult cochlea in the inner ear of species with laryngeal echolocation is much bigger than that of other species. The authors examine cochlear development in seven species of bats (five that have laryngeal echolocation and two that do not) and five species of other non-echolocating mammals. They observe rapid early development of the cochleae in all the bat species, but not in the other mammals. However, the non-echolocating bats show dramatic slowing of cochlear development after the early expansion, forming adult cochleae that resemble those of other mammals.
By looking at embryonic development rather than either genes or fossils, the authors have shed light on a long-standing evolutionary puzzle. Their results suggest a single evolutionary origin for laryngeal echolocation in bats.
In an accompanying News & Views article, Brock Fenton and John Ratcliffe explore the results in the context of the evolution of other bat characteristics.