The number of miRNAs annotated in a species is significantly correlated with the percentage of young miRNAs in the species. Over 146 species, we observed a Spearman’s correlation of 0.68 (P = 4.1e–06) between the number of annotated miRNAs and the percentage of young miRNAs in the species. This relationship is likely the result of limitations in current knowledge of miRNA sequences and annotations. In this analysis, young miRNAs were defined as those younger than 25 % of the overall age range. See Additional file 1 for the list of all species considered and miRNA counts. (PDF 15 kb)