Like most fish, sharks are largely ectothermic, or cold-blooded. This means their body temperature depends on the water around them. To avoid freezing, most shark species remain close to the ocean's surface, where the water temperatures are warmer. However, scalloped hammerheads have long been known to dive thousands of feet into the ocean's cold depths in search of prey. Researchers from the University of Hawaii at Mānoa have now uncovered the shark's long-kept secret. The smart fish shut their gills and "hold their breath" to stay warm.