Some suggest that the sound of "Ru" gradually disappeared in later years; removing "Ru" from 'Hiruko' leaves "Hiko" ("彦" (a kanji representing a man) and removing "Ru" from 'Hirume' leaves "Hime" ("姫" (meaning princess)).
後世になって『る』の音が消滅していったと見るむきもあり、「ヒルコ」の『る』を除けばヒコ（彦）となり、「ひるめ」の『る』を除けばヒメ（姫）となる。