Mammals, amphibians, cartilaginous fishes (sharks and rays), and hagfishes excrete urea as their principal nitrogenous waste product and are therefore called ureotelic. Urea is quite soluble in water, but its excretion can result in a large loss of water that many animals can ill afford. Mammals have somewhat solved this problem through the evolution of excretory systems that can produce urine with a high concentration of urea, thereby decreasing the water loss associated with excretion of urea. Also, recall from Key Concept 51.1 that sharks and rays solve the water loss problem of living in sea water by retaining high concentrations of urea and TMAO in their extracellular fluid, so that it is hyperosmotic to seawater.