Watch where you go: see where you went

Just a quick one… I just read Leo Peichl’s excellent 2005 review of the diversity of mammalian photoreceptors, and it’s a goldmine of fascinating detail, full of examples and interesting speculation. It summarises his and many other people’s findings over the years, covering classes of photoreceptor, their distribution on the retina, possible evolutionary pathways, and so on. One item stood out though. Apparently, most rodents can see ultraviolet (UV) light (as, incidentally, can plenty of non-mammalian species). It’s not clear why though, given that plenty of rodents are nocturnal and plenty are diurnal: why should they see UV? One suggestion is that rodent urine is highly UV-reflective. As Peichl says, “…rodents might profit from seeing their scent marks in addition to smelling them.”

Which is perhaps bizarre, but makes sense. But how did this evolve? Wouldn’t it be easier to develop say, coloured urine, if seeing it really helps? I think some birds can see UV, so it can’t be a camouflage issue. And what about the first rat to see his own pee?

“Look everyone! I can see where I peed yesterday!”

“You been eating that fermented cheese again, Barry?”

Peichl, L. (2005). Diversity of mammalian photoreceptor properties: Adaptations to habitat and lifestyle?. The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology, 287A(1), 1001-1012. DOI: 10.1002/ar.a.20262