This is a stickleback male guarding its nest. Notice the fact that it’s still alive? That fact is both a biological marvel unique among creatures, and a way to test if athletes have been taking steroids, because why not? What else you got, reality?
When the male stickleback matures and decides it’s time to make a family, he builds a nest of debris, sticking it together with a special secretion that he makes in his kidneys. The problem is, he already had a use for his kidneys. The kidneys clear waste products from the blood, and they’re the first stop urine takes on its way out of the body. Failure to clear waste products from the blood will eventually lead to death, but failing to get liquid out of the body will do so as well—possibly faster. The stickleback, when he begins making a nest, has lost the ability to pee.
Instead of swelling up like a little urine-filled water balloon and dying, the fish goes a little weird. It dumps salt into its intestines. When cell membranes have a great deal of salt on one side, and liquid with only a little salt on the other side, the liquid transfers to the “salty” side via osmosis. The stickleback basically reroutes its digestive system, and uses osmosis to dump waste water through its intestines—a system unique among animals.
But then the world had to take it one step farther. All of this happens to fish for one reason only—they get an infusion of androgens. Androgens are hormones, most notably testosterone, that bring on male puberty. As part of that they build muscle, which is why they are so often taken by athletes. An infusion of hormones will make any immature stickleback, including females, turn their kidneys into nest-glue-producers, and the change will be visible to the naked eye. These fish are very sensitive, so converting their kidneys may be an easy, quick way to test for steroid use in human athletes.
[Source: Sexual maturation and changes in water and salt transport components in the kidney and intestine of three-spined stickleback}