Updated: Apr 16, 2022
Yep, you read that right. This little animal excretes kidney stones for urine.
A pair of Notomys rodents. Credit: Stephen Michael Barnett, Wikimedia Commons, CC-BY 2.0.
That's what you get for living in one of the driest places on earth - the middle of the Australian desert (literally). Notomys alexis, or the Spinifex hopping mouse, takes up residence in shrubland, eucalyptus woods, sand dunes, and spinifex hummocks (hence the name) in central and western Australia. It lives in groups of 10, with groups living in a single burrow.
Because of its super-dry home, the mouse has to do everything it can to conserve water. Most of these adaptations are what you'd expect, such as sleeping together in deep burrows where the temperature is more stable. But their kidney function is almost unheard of in the animal kingdom.
First, a little bit about kidneys so you understand the craziness this animal deals with during its appointments with the white throne. Just like you, it has two kidneys. The kidneys contain two major sections: the outer renal cortex and the inner renal medulla. Within the medulla are nephrons, little units which contain a convoluted system of tubes and capillaries (mini blood vessels). This system's sole function is to filter and reabsorb substances such as water and salts as needed. Depending on how much water the organism needs, varying amounts of water will be reabsorbed as regulated by antidiuretic hormone, or ADH.
Cartoon of a nephron. Credit: Sheldahl, Wikimedia Commons, CC-BY-SA 4.0.
At the end of all this filtration and resorption, the fluid enters the end of the tube system - the collecting duct. This tube goes down, down into the medulla like a hollow rope into a bottomless pit, where the waste fluid reaches its final concentration. This concentration is measured in mili-osmoles, or mOsm.
This is where N. alexis' kidneys make yours look pathetic. your kidneys will reabsorb from the waste fluid what you need and pee out the rest as urine. As more water is reabsorbed, the concentration of urine increases, becoming more solute (salts & other compounds) than water. By the time urine reaches the ureters, the tubes that take urine to your bladder, your urine concentration will be anywhere from 50 mOsm/kg to 1200 mOsm/kg, depending on how much fluids you've been consuming. If you're in the Australian desert without supplies, your urine should be around 1200 mOsm/kg, which means it's highly concentrated.
And the N. alexis hiding under a nearby rock is laughing at your pathetic ability to survive dry conditions while it pees out urine of around 9400 mOsm/kg.
Yep, basically rock.
The little sucker laughing at your pathetically wet urine and inferior survival abilities :)