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Vesper bats (family Vespertilionidae) experienced a rapid adaptive radiation beginning around 36 Ma that resulted in the second most species-rich mammalian family (>400 species). Coincident with that radiation was an initial burst of DNA transposon activity that has continued into the present in some species. Such extensive and recent DNA transposon activity has not been seen in any other extant mammal. Indeed, retrotransposon activity is much more common in all other sequenced mammal genomes. Deep sequencing of the small RNA fraction from a vespertilionid bat, Eptesicus fuscus, as well as a dog and horse revealed large numbers of 17–24 bp putative miRNAs (p/miRNAs). Although the origination rate of p/miRNAs is similar in all three taxa, 61.1% of postdivergence p/miRNAs in Eptesicus are derived from transposable elements (TEs) compared with only 23.9% and 16.5% in the dog and horse …