However, the all-trans-retinol and retinal forms, which occur in animal-based foods, are several times as effective in humans, per microgram, than are the plant-based forms. All vitamers, in case of "vitamin A"1 mediate the same biological functions serving as a precursor for retinal, for the visual cycle, or for all-trans-retinoic acid, functioning as a hormone-like substance and a ligand for the nuclear hormone receptors, RARs. In some cases these differences are extreme: for example the carotenoid forms of vitamin A1 cannot be absorbed by cats or ferrets at all, and therefore do not have vitamin A activity in these species.