I have been reading an article discussing a study by Tiffany Slater and Maria McNamara in which they identified molecular evidence of phaeomelanin in the fossil record. Slater is quoted as saying "scientists still don’t know how – or why – phaeomelanin evolved because it is toxic to animals".
I am curious what toxicity is being referred to here, since looking into it has only turned up the fact that phaeomelanin is responsible for red haired pigments in humans as well as reddish coloration in other animals, and the only mentions I've seen of toxicity are either phototoxicity that might occur when exposed to certain kinds of light, or increased susceptibility to skin damage from sunlight.
I did find a few studies in the search results whose titles were completely impenetrable to a layman such as myself, so maybe there is more information there that I wasn't able to dig out.
Is there a little more context around this statement that phaeomelanin is toxic that I am missing? I'm especially interested in information in layman's terms as I am not deeply familiar with the field of biology, but I am always curious!