Lately, some images have been jiggling around in my mind, trying to make a connection. It started last week when I looked into the red eyes of the male Box turtle in the pic above, and was reminded of something…. As a visual artist, I never stop marveling at the Box turtle’s immensely varied, seemingly random patterns. But male turtles alone have bright red eyes and the deepest orange pigments (usually). There’s only one other class of land animals that regularly sports such brilliant pigments, red eyes, and sexual dichromatism.
They’re birds, of course. And the image in my mind was from our summer trip to the west coast.
This Spotted Towhee looks a lot like its eastern counterpart, in fact they both used to be called the Rufous-sided Towhee. A very attractive bird, east or west. But it was the red eye that really stuck with me – after all, animals with red eyes are not all that common. When I did a Google search to find out if red eyes in turtles and birds were more than mere coincidence, I found something surprising. A whole host of research papers in fact, illuminating the new evolutionary position of turtles in relation to birds.
Apparently turtles have never fit neatly into any branch of the evolutionary tree that paleontologists have reconstructed. To quote from one of the scientific articles I waded through to write this post:
“Consensus on the evolutionary position of turtles within the amniote phylogeny has eluded evolutionary biologists for more than a century. This phylogenetic problem has remained unsolved partly because turtles have such a unique morphology that only few characters can be used to link them with any other group of amniotes.”
Understandably this would be a maddening problem for turtle scientists, but advances in genetic research are providing some answers. In particular this study from 2016, which focuses on the particular gene CYP2J19.
“Seeing red to being red: conserved genetic mechanism for red cone oil droplets and co-option for red coloration in birds and turtles”
This was a difficult study for me to digest, not being a scientist by training. However, the conclusions are so fascinating I have applied myself to it, and will try to condense what I learned. (But I encourage you to read it anyway.)
CYP2J19 is also referred to as the “redness gene”, since it was recently discovered to be responsible for the production of ketocarotenoid pigments (red coloration) in birds. But – this is where it really gets interesting – that apparently wasn’t it’s original purpose. Gene CYP219 is also found in the red retinal oil droplets in bird’s eyes, that help confer excellent tetrachromatic color vision. This was likely where it first evolved, in bird’s dinosaur ancestors. Only later was it co-opted for the coloration of scales and plumage.
This makes sense, since what would be the the point of having fine degrees of bright pigmentation, if you couldn’t visually discriminate between them? Hence, the catchy title of the research paper, “Seeing Red to Being Red…” Biologists refer to colorful male bird plumage as “sexual signalling”; and it’s considered an “honest” signal because it truthfully indicates a male’s fitness to female birds.
But what does all this have to do with the colorful male boxies I see in the forest? Turns out, gene CYP2J19 is found in one other group of tetrapods besides birds – you guessed it – turtles! This genetic evidence helps clinch the chelonian’s new evolutionary position as the sister group of birds and crocodiles (Archosauria). In fact, a new name, “Archelosauria“, has been proposed for the grouping of turtles, birds, crocodilians, and their fossil relatives.
Again a short quote, stated better than I can translate:
“We show that turtles, but not crocodiles or lepidosaurs, possess a CYP2J19 orthologue, which arose via gene duplication before turtles and archosaurs split, and which is strongly and specifically expressed in the ketocarotenoid-containing retina and red integument (skin, scales, etc).”
Even more amazingly:
“We infer that CYP2J19 initially functioned in colour vision in archelosaurs and conclude that red ketocarotenoid-based coloration evolved independently in birds and turtles via gene regulatory changes of CYP2J19.”
Though it could be viewed as a chance occurence that both turtles and birds evolved red eyes and other colorful pigmentation, the driver for both was likely sexual selection.
“Honest sexual signaling in turtles: experimental evidence of a trade-off between immune response and coloration in red-eared sliders Trachemys scripta elegans.”
The very long title of this 2014 paper suggests this may be true for particularly colorful turtle species like the Red-eared Slider. So it’s not off base to infer that bright coloration may provide honest sexual signalling in Box turtles as well. I’ve wondered about the reasons for Box turtle coloration for awhile, so it’s neat to see some scientific evidence supporting my suspicions. This paper is interesting for another reason, because the researchers reached their conclusions by temporarily challenging the Slider’s immune systems with a bacterial antigen. A decline in bright coloration resulted, which would be consistant with honest sexual signalling. That is, faded pigmentation may send a message to other turtles that one’s health is compromised.
So, is it possible that’s what happened to my acquaintance from the previous post who lost a lot of his facial color? His health was obviously challenged, and the significant decrease in his bright pigmentation may now be sending the signal he’s not the choice mate he once was.
Fascinating to learn and share all this new research, but unfortunately it keeps me hunkered down in my shell reading articles online. I’d rather be outside where it’s all happening!
Speaking of which, oneforestfragment is excited to be leading a new series of interpretive walks in Beargrass Creek SNP on the 3rd Saturday of the month, starting in Sept. More details TBA…