Whether it's lion manes, peacock tails, or narwhal horns, secondary sexual characteristics are generally there to set males apart and, theoretically, make them more impressive to potential mates. For red-spotted newts, there's nothing sexier than thick, oozing kidneys. Yes, really.
That's the finding of Southeast Missouri State University researcher Dustin Siegel. These particular newts reproduce by having the males produce a sperm-filled jelly called spermatophore, which the females then store until it's actually time to start reproducing. This already atypical method of reproduction has just gotten weirder, as Siegel has found the male newts' kidneys play a part in all this as well. New Scientist's Zoologger explains:
During the mating season, the rear portions of the kidneys secreted an unidentified liquid, but didn't do so for the rest of the year. The tubes running through the kidneys also changed shape for the mating season, developing thicker walls. The changes in the kidneys mirrored those in other secondary sexual structures – for instance, the males' tails became thicker for the mating season. That is good evidence that the kidneys are indeed involved in sex, but what they are doing remains a mystery. The liquid they make is a glycoprotein – a protein bonded to a carbohydrate – but it has not been specifically identified. Siegel points out that the liquid from the kidneys drains into the Wolffian ducts that house the sperm, so he suspects that it helps to boost the sperm's motility or lifespan.
It may not just be the red-spotted newts that enjoy the sexual benefits of kidneys and their mysterious ooze. Siegel speculates that salamanders likely have the same secondary sexual characteristics, since they reproduce in much the same way, and this may help explain the role of similarly unidentified liquids produced by snakes and lizards during sex. For more on this lovely, lovely topic, check out New Scientist.