A shark's head contains a number of sensors known as ampullae of Lorenzini that can help the animal detect electric fields emitted by the earth's magnetic field or by other sea-dwellers. The canals connecting these sensors to the shark's skin contain a clear, protein-based gel. Brandon R. Brown of the University of San Francisco analyzed samples of this gel from a black-tip reef shark and a white shark, and found that temperature changes as small as one tenth of a degree Celsius produced a small but significant voltage. He posits that in the wild, the gel could enable sharks to sense temperature fluctuations as small as 0.001 degree. The findings reveal yet another way in which sharks and their dogfish and ray relatives differ from mammals, which use ion channels to detect temperature changes.