The Origin for Modern Echolocation

 by Hannah Watts

Bats are among the most recognisable and widespread of all mammals being present in every continent of the world. But much of their fossil record is missing, and bat origins remain poorly understood, as do the relationships of early to modern bats. Representing one-fifth of all mammals alive today (>1,460 species1). They are the only mammals to use powered flight, and many bats (but not flying-foxes or Old World fruit bats) use echolocation calls produced in the larynx to orientate in darkness and find food. These two key biological innovations make bats morphologically highly distinctive in their skeletons and skulls and it has long been debated how flight and echolocation evolved in bats. Some think that they coevolved or that one came first with the other following due to evolutionary advantage. One thing that is known is that all bats fly but not all bats echolocate. Old World fruit bats and flying-foxes (Pteropodidae, suborder Yinpterochiroptera) do not use laryngeal echolocation, unlike the other ∼86% of extant bats, which utilise high-frequency sounds produced by the larynx to echolocate. There is debate about when and how laryngeal echolocation evolved in bats, with two specific hypotheses debated:  that laryngeal echolocation evolved independently within the two bat suborders Yinpterochiroptera and Yangochiroptera or that laryngeal echolocation evolved once in the common ancestor of extant bats but was subsequently lost in some Yinpterochiropterans. 

The find of a new, three-dimensionally preserved fossil bat recovered from 50-million-year-old limestone cave sediments at Vielase, in the Quercy Phosphorites, southwestern France is biocorrelated as late early Eocene in age and the phylogenetic analyses places the new Taxon outside modern bats. The species was named as Veilasia and it is being used by scientists to aid the information surrounding the evolution of echolocation. Vielasia’s petrosal is relatively isolated from the rest of the skull, with the enlarged pars cochlearis contacting the basicranium via four tiny bony splints and otherwise presumably loosely attached to the basicranium via ligaments, as in all extant bats known to echolocate laryngeally. In non-echolocating pteropodidae and Onychonycteris, the petrosal is instead fused or sutured to the basicranium.

Whilst this evidence suggests that echolocation developed after flight, there is some evidence that shows bat communication may also have been influenced by interactions with other taxa. For example, the arms race between some bat species and their tympanate moth prey (which can hear ultrasound). Some bats have evolved the use of low echolocation frequencies that are below the range of moth hearing, and this enables them to prey on moths seen in the Mexican free-tailed bat which calls as low as 11 kHz. Similarly, we may be able to gain insight into the influence that flowering, fruiting and nectar-producing plants have had on the bat echolocation call design and vice versa - around 1,000 plant species in the Neotropics are bat-pollinated by bats in the family Phyllostomidae. Adaptations of the plants to the bats include outward-facing flowers; large, sturdy petals with exposed stamens and strong-smelling, night-opening flowers.

Therefore despite evidence for both convolution and the separate evolution, it is clear that the echolocation of bats is complex relying on physiological elements whilst also being influenced by their external environment.