Postnatal ontogeny of the cochlea and flight ability in Jamaican fruit bats (Phyllostomidae) with implications for the evolution of echolocation.

Recent evidence has shown that the developmental emergence of echolocation calls in young bats follow an independent developmental pathway from other vocalizations and that adult-like echolocation call structure significantly precedes flight ability. These data in combination with new insights into the echolocation ability of some shrews suggest that the evolution of echolocation in bats may involve inheritance of a primitive sonar system that was modified to its current state, rather than the ad hoc evolution of echolocation in the earliest bats. Because the cochlea is crucial in the sensation of echoes returning from sonar pulses, we tracked changes in cochlear morphology during development that included the basilar membrane (BM) and secondary spiral lamina (SSL) along the length of the cochlea in relation to stages of flight ability in young bats. Our data show that the morphological prerequisite for sonar sensitivity of the cochlea significantly precedes the onset of flight in young bats and, in fact, development of this prerequisite is complete before parturition. In addition, there were no discernible changes in cochlear morphology with stages of flight development, demonstrating temporal asymmetry between the development of morphology associated with echo-pulse return sensitivity and volancy. These data further corroborate and support the hypothesis that adaptations for sonar and echolocation evolved before flight in mammals.

Ontogeny of the larynx and flight ability in Jamaican fruit bats (Phyllostomidae) with considerations for the evolution of echolocation.

Echolocating bats have adaptations of the larynx such as hypertrophied intrinsic musculature and calcified or ossified cartilages to support sonar emission. We examined growth and development of the larynx relative to developing flight ability in Jamaican fruit bats to assess how changes in sonar production are coordinated with the onset of flight during ontogeny as a window for understanding the evolutionary relationships between these systems. In addition, we compare the extent of laryngeal calcification in an echolocating shrew species (Sorex vagrans) and the house mouse (Mus musculus), to assess what laryngeal chiropteran adaptations are associated with flight versus echolocation. Individuals were categorized into one of five developmental flight stages (flop, flutter, flap, flight, and adult) determined by drop-tests. Larynges were cleared and stained with alcian blue and alizarin red, or sectioned and stained with hematoxylin and eosin. Our results showed calcification of the cricoid cartilage in bats, represented during the flap stage and this increased significantly in individuals at the flight stage. Thyroid and arytenoid cartilages showed no evidence of calcification and neither cricoid nor thyroid showed significant increases in rate of growth relative to the larynx as a whole. The physiological cross-sectional area of the cricothyroid muscles increased significantly at the flap stage. Shrew larynges showed signs of calcification along the margins of the cricoid and thyroid cartilages, while the mouse larynx did not. These data suggest the larynx of echolocating bats becomes stronger and sturdier in tandem with flight development, indicating possible developmental integration between flight and echolocation.