Since the speciation from platypus, the frequency of His has apparently increased, whereas those of Thr and Gln have decreased in all other mammals (Supplemental Table S1; http://dx.doi.org/10.3168/jds.2013-7317). Arginine is also strongly elevated in horse, showing a rapid shift from 1.6 to 4.0% since its divergence from its common ancestor with cow and dog.