Bones and echolocation

Echolocation is an active form of orientation in which animals emit sounds and then listen to reflected echoes of those sounds to form images of their surroundings in their brains. Although echolocation is usually associated with bats, it is not characteristic of all bats. Most echolocating bats produce signals in the larynx, but within one family of mainly non-echolocating species (Pteropodidae), a few species use echolocation sounds produced by tongue clicks. Here we demonstrate, using data obtained from micro-computed tomography scans of 26 species (n = 35 fluid-preserved bats), that proximal articulation of the stylohyal bone (part of the mammalian hyoid apparatus) with the tympanic bone always distinguishes laryngeally echolocating bats from all other bats (that is, non-echolocating pteropodids and those that echolocate with tongue clicks). In laryngeally echolocating bats, the proximal end of the stylohyal bone directly articulates with the tympanic bone and is often fused with it. Previous research on the morphology of the stylohyal bone in the oldest known fossil bat (Onychonycteris finneyi) suggested that it did not echolocate, but our findings suggest that O. finneyi may have used laryngeal echolocation because its stylohyal bones may have articulated with its tympanic bones. The present findings reopen basic questions about the timing and the origin of flight and echolocation in the early evolution of bats. Our data also provide an independent anatomical character by which to distinguish laryngeally echolocating bats from other bats.

Fig.1. Tomografia di pipistrello con il sistema di ossa caratteristico per quelle specie che producono gli ultrasuoni nella laringe (Fonte: Robarts Research Institute).

Tra i pipistrelli non tutti usano l´ecolocazione, ma tra quelli che la usano si possono distinguere due tipi: quelli che producono gli ultrasuoni tramite lo schiocco della lingua, e quelli che usano la laringe. La ricerca condotta su 26 specie ha mostrato che in tutte le specie che usano il secondo metodo esiste un caratteristico ossicino che collega la laringe con il timpano dell´orecchio. Grazie a questa scoperta in futuro si potrà distinguere meglio anche il tipo di ecolocazione nelle forme fossili di pipistrelli, in più questo dettaglio potrebbe riaprire la discussione sul più antico pipistrello conosciuto -Onychonycteris finney- e se era capace di usare il sistema di orientamento tramite ultrasuoni.

BIBLIOGRAFIA:
VESELKA et al. (2010): A bony connection signals laryngeal echolocation in bats. Nature online publication 24 January 2010; doi:10.1038/nature08737