Pee is yellow. This much you know. But why is it yellow? Do you know? Do you even care? Quickly then a bit of science (cue the echoing "science, science, science" like from an 80's educational show). Urine is yellow because of the presence of urobilin. Urobilin is a breakdown product of bilirubin, which also gives bile its yellowish colour. And bilirubin in turn is a breakdown product of hemoglobin. As red blood cells are constantly being turned over (in the average human 100 million red cells die each day, but fortunately 100 million are born each day as well), there is a constant stream of urobilin waste the body needs to get rid of.
Urine is full of all sorts of other waste products as well, most notably urea, which is a byproduct of protein metabolism. These other waste products are colourless though and the urobilin is excreted at a more or less constant rate, so the only variable in how yellow the pee is is how much water is being excreted. More water means more dilute urobilin and less yellow and less water means more concentrated urobilin and more yellow. Logical, yes?
So now that you know this, what can you do with this information? The first thing to understand is that urine concentration will vary from day to day, so one really clear pee or one really dark yellow pee doesn't mean much. If however your dog (I'll get to cats later) is producing very clear pee day after day, there may be something wrong. There may be. It may also be that he just loves to drink water and his body is getting rid of the excess. But definitely get it checked out to rule out diabetes, kidney disease, adrenal gland disease etc.. If your dog is producing very dark yellow pee day after day he may be dehydrated. This is a decent discussion of how to tell:
That's all well and good for dogs, but what about cats? You'll only see the colour of your cat's pee if you are invading their privacy much too closely or if you are unlucky enough to have the pee appear on a white towel or bed-sheet. However, if you use clumping litter you can use the size of the clumps as a way to guess at concentration, because as volume goes up, concentration tends to go down, and vice versa. If the clumps start getting much larger, the urine is possibly becoming more dilute and you should contact your veterinarian. By the same token, if the clumps are getting smaller make sure dehydration is not an issue.
What about other colours? Red is the only one worth talking about. Any redness or pinkness in the urine could indicate a problem such as an infection or inflammation or stones and needs to be brought to your veterinarian's attention. Also, if it is April 1, collect a normal sample, put some blue food colouring in it and drop it off at your clinic...
Finally, a few random facts about pee:
- Many people assume that a pet in kidney failure will stop producing urine. The opposite is in fact true. Up until very close to the end kidney failure patients produce a lot of dilute urine. The kidneys are failing to concentrate the urine, not failing to make it.
- Urine kills grass because the urea being excreted is high in nitrogen. It's like dumping a bunch of nitrogen fertilizer in one spot.
- Stinkier dog pee usually just means more concentrated pee (unless you've fed your dog asparagus or something strange). I actually get that question a lot. Infection is a possible cause too, but generally there are other symptoms such as accidents, urgency or straining.
- Dogs and cats can tell large numbers of other specific dogs and cats apart by their urine scent, so all that sniffing on the walk is about figuring out who was there and do they know them. A longer deeper sniff usually means that it was an unfamiliar animal. It's a pretty exciting day for Orbit, my dog, when I come home from work after being peed on...
Helpful sign posted above a urinal in Bali. Hard to imagine you'd still be standing if you scored a "7".