Frequency hopping by big brown bats is coupled with sonar sound grouping in response to pulse-echo ambiguity

Abstract

When flying in clutter, big brown bats actively modify their biosonar broadcasts by successively alternating the lowest frequencies in the first harmonic of their FM broadcasts in a phenomenon called “frequency hopping;” and by grouping broadcasts into “sonar sound groups,” with short time intervals within a group and longer intervals between groups. These modifications can minimize pulse-echo ambiguity. We analyzed the relationship between frequency hopping and emission of sonar sound groups while bats flew through chain corridors with different clutter densities and through circular hoop tunnels. In more difficult flight tasks, bats emit more sounds in groups of triplets and quadruplets, often with frequency shifts of 1–6 kHz between each sound in the group, creating not only sound but frequency triplets and quadruplets. Echoes of successive sounds remain highly correlated and not separable by conventional receivers according to which broadcast causes them. The consistent pairing of frequency shifting with sonar sound grouping may reflect an auditory streaming mechanism whereby coupled shifts in both frequency and timing can link streams of received echoes to their corresponding emitted calls to disambiguate overlapping echo streams for closely spaced broadcasts. [Work supported by ONR.]