blue smurf pee, rabbits, and buckthorn

This past week I participated in one of several Christmas Bird Counts that I do annually. I’ve done counts here in Michigan on sunny, cloudy, snowy, rainy, raw, and balmy days, but never on a day that included all of the above, plus thunder, lightning, and a rainbow (all with the temperature under 40 degree F). Very strange.

We had a nice diversity of species, including a Gray Catbird, which is pretty rare in winter around here.  It was camping out in a buckthorn tree, which harbor about the only berries left at this point (fruit being the main food of a winter catbird in these parts).  This brings us to the most unusual sighting of the day.

As we tromped through the woods, we came across a bright blue stain in the snow, as if someone had spilled windshield washer fluid.  A rabbit turd was next to it.  We thought this was weird, but figured there must have been some litter that had leached blue ink beneath the snow, and that the rabbit poop was coincidental.  Then we found another identical stain and turd.  And another.  I quit counting after twenty or so.  Not every pile of bunny pellets had a blue stain, but all blue stains had some poop. Obviously, this was rabbit urine. As usual, we were in an urban area, next door to a chemical plant, in fact. Was there a family of toxic rabbits running around, voiding blue pee?

I came to learn that this neon cerulean tinkle is a result of rabbits consuming buckthorn (Rhamnus cathartica), a highly invasive tree in North America.  One of the chemical constituents is excreted in the urine, and within a short time turns blue when exposed to sunlight.

The only credible mention of this was an anecdotal account on the Ontario Woodlot Association web site. The author observed this blue pee and experimented by feeding domestic rabbits and goats buckthorn and other shrubs.  Blue pee resulted only after buckthorn consumption. Blue urine has not escaped the notice of other folks, but nearly all other explanations I found on the Internet were remarkably asinine, including people spitting out mouthwash (I know I often take advantage of a nice stroll in the woods to bust out the Listerine) and Bigfoot piss.

Because buckthorn is such an important invasive species and so common in urban areas, I am especially interested in the often unexpected ways it impacts other organisms (like the ecosystem-altering way it interacts with non-native earthworms). I have done quite a bit of research on buckthorn, but mostly as it relates to birds consuming the fruits, not mammals eating bark and shoots.  I did a literature search, and found nothing on this phenomena, which must, of course, occur in the native Eurasian range of buckthorn as well.

I’m surprised that I’ve not found blue urine stains at my own study site, which is jammed with buckthorn and has no shortage of rabbits.  I figure it must have to do with the fact that buckthorn is not a preferred forage for mammals (due to the many phytochemicals it contains, one of which apparently causes this phenomena).  If the rabbits aren’t eating it, they aren’t peeing it. It’s also possible that not all rabbits would void blue urine after eating buckthorn.  Perhaps only certain individuals are unable to metabolize some chemical constituent. Or the reaction may require consumption of a certain amount, or the combination of buckthorn and some other food.

I’d be really interested in any research results on this, if anybody has some references.