The end product of purine breakdown in most mammals, birds, terrestrial reptiles, and insects and also (except in mammals and elasmobranch fishes; see urea) the major form in which metabolic nitrogen is excreted. Being fairly insoluble, uric acid can be expelled in solid form, which conserves valuable water in arid environments. Bird droppings are a mixture of faeces and white uric acid. In humans small quantities of uric acid are formed as the end product of nucleic acid metabolism. The accumulation of uric acid in the synovial fluid of joints causes gout.