The
difference in lmax value for this visual pigment type
may well reflect phylogenetic relatedness in additionto aspects ofvisual ecology. Traditionally, visual
pigments in this cone type have been called violetsensitive
(VS) iftheir lmax value is above 400 nm
and ultraviolet-sensitive (UVS) ifit is below 400 nm.
Nevertheless, all avian UVS/VS visual pigments are
thought to be derived from the same ancestral
vertebrate opsin type (SWS1) with lmax values
between 360 and 430 nm (Yokoyama, 2000).