Study of birds' mating habits means dinosaurs may have seen red
A gene discovered in birds may provide evidence that dinosaurs had enhanced red colour vision, academics at the University of Cambridge said.
This red spectrum vision has helped female birds to select a mate based on external redness, which was seen as a sign of high quality.
The "red gene" was pinpointed in zebra finches earlier this year and a new study shows it originated in the reptile lineage around 250 million years ago.
It has resulted in the bright red bird feathers and painted turtles that exist today.
The gene, called CYP2J19, allows birds and turtles to convert the yellow pigments in their diets into red, which they then use to heighten colour vision in the red spectrum through droplets of red oil in their retinas.
In some birds and a few turtle species, red pigment produced by the gene is also used for external display such as red beaks and feathers, or the red neck patches and rims of shells seen in species such as the painted turtle.
Scientists traced the gene from various bird and reptile species back to the ancient archelosaur genetic line - the ancestral lineage of turtles, birds and dinosaurs.
The findings, published in the journal Proceedings of the Royal Society B, provide evidence dinosaurs would have carried the gene and had enhanced red vision.
This may have resulted in some dinosaurs producing bright red pigment for display purposes, although researchers say this is more speculative.
"These findings are evidence that the red gene originated in the archelosaur lineage to produce red for colour vision, and was much later independently deployed in both birds and turtles to be displayed in the red feathers and shells of some species, going from seeing red to being red," says senior author Dr Nick Mundy, from the University of Cambridge's Department of Zoology.
"This external redness was often sexually selected as an 'honest signal' of genuine high quality in a mate."
Previous research in zebra finches showed a possible link between red beaks and the ability to break down toxins in the body, suggesting external redness signals physiological quality.
"The excellent red spectrum vision provided by the CYP2J19 gene would help female birds and turtles pick the brightest red males," says Hanlu Twyman, the PhD student who is lead author on the work.
Unlike mammals, avian and turtle retinal cones contain a range of brightly-coloured oil droplets, including green, yellow and red.
These oil droplets function in a similar way to filters on a camera lens.
"By filtering the incoming light, the oil droplets lead to greater separation of the range of wavelengths that each cone responds to, creating much better colour sensitivity," explained Dr Mundy. "Humans can distinguish between some shades of red such as scarlet and crimson.
"However, birds and turtles can see a host of intermediate reds between these two shades, for example.
"Our work suggests that dinosaurs would have also had this ability to see a wide spectrum of redness."
Crocodiles appear to have lost the gene, and Dr Mundy says there is evidence that oil droplets were lost from the retinas of species that were nocturnal for long periods of their genetic past, and that this hypothesis fits for mammals and snakes, and may also be the case with crocodiles.