After dancing seductively for their potential mates, male sea lampreys (Petromyzon marinus) crank up the heat, literally, using a ridge of tissue on their backs.

Courtship behaviour of lampreys — eel-like, bloodsucking, jawless fishes — includes the male rubbing his ridge against the belly of an interested female. Researchers had assumed that this simply aroused females mechanically, but when Weiming Li and his colleagues at Michigan State University in East Lansing dissected the tissue, they found that ridges from mature males were full of cells packed with oil droplets and cells primed for energy production, a hallmark of heat-producing tissue. The ridge temperature in males jumped by up to 0.3 °C in the presence of sexually mature females.

The authors say that the ridge is the first example of a heat-generating sexual trait.

J. Exp. Biol. 216, 2702–2712 ( 2013)