Male sea lampreys are hot to trot—literally. New research reveals that swollen tissue on the back of the eel-like creatures (Petromyzon marinus, pictured), known as rope tissue, heats up when a male encounters a female. Previous studies have found that the males rub this rope tissue against a female's stomach to seduce them. If the female is pleased with her partner, the couple will reproduce by releasing their sperm and eggs simultaneously. But scientists didn't know whether the male rope tissue stimulates a female to release her eggs or whether it serves another role in the mating behavior of these jawless fish. Now, after observing the tissue under the microscope, researchers discovered that it resembles brown fat, which is found in newborn babies and hibernating mammals to help them keep warm. To test whether the male lampreys' rope tissue could also generate heat, the team inserted miniature thermometers into the tissue. The bump on the back of the male lampreys rises by up to 0.3°C when they meet their mates, the scientists report online today in The Journal of Experimental Biology. And in the company of certain females, some males turn up the heat more than others. However, more research is needed to say whether a "hot" female makes a male lamprey grow hotter.