I don't think theres a reserve tank, I just think they use sheer will power to dreg the tanksl.
Being a girl, obviously zoey doesn't have this problem. But the other dog in my life, Dan's family's dog Boo, does the obsessive marking thing too. When we take him on long walks, he will mark up a strom. He'll have little bits the first couple trees, but then he will completely run out and will comtinue to lift his leg every other minute. What I have always wondered is when he is out of material, does he think "mission accomplished: that tree is marked!" or does he think "oh man! if only I'd drinken more water earlier - now no one will know I was here!"