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THEY live in the deep blue sea, but whales and dolphins can鈥檛 actually see
the colour blue. German and Swedish biologists have discovered they lack the
visual pigment needed to see blue light.

The finding is puzzling because blue is the colour that penetrates farthest
into the deep ocean, yet whales and seals seem to have abandoned blue vision
independently.

Most mammals have two types of cells in their eyes, called cones, that pick
out blue and green light. Humans and other primates have a third cone for red
light. Earlier studies had indicated that some dolphins and seals lack colour
vision, so Leo Peichl of the Max Planck Institute for Brain Research in
Frankfurt and his colleagues decided to systematically test many species.
Antibody tests on 14 species of toothed whales and seals showed that all lacked
the pigments found in blue colour receptors.

However, they found the pigments in river otters and wolves, close relatives
of seals, and in the hippopotamus, which is closely related to whales. This
suggests whales and seals lost their colour vision after they split off from a
common ancestor.

Why this happened is a mystery, says Peichl. He thinks that the ancestors of
modern whales and seals may have lost their blue vision shortly after returning
to the sea. Palaeontologists believe early whales and seals lived in coastal
waters, where debris tends to blot out the blue light. Evolution may then have
favoured the selection of other visual traits. 鈥淭hose species which have moved
to deeper water might want their blue cones back, but they can鈥檛 undo their
genetic deletions,鈥� he told New Scientist. 鈥淭hat鈥檚 very speculative,
but it鈥檚 the only idea we could come up with.鈥�

Other specialists in colour vision are also puzzled. 鈥淭here must be something
functional going on, but I don鈥檛 know what,鈥� says Mickey Rowe of the University
of California at Santa Barbara.