text: This is to do with the length of the Loops of Henle in the medulla of mammalian kidneys.\nThe loops work as countercurrent multipliers providing a relatively high concentration of solutes deep in the medulla. Long loops generate a very high solute concentration relative to general body tissues and this promotes the reabsorption of water from the glomerular filtrate.\nA desert rat has very long Loops of Henle.\nI'll let you work out the rest!