during the hottest part of the day in the afternoon, and dispersing by 19:00. Individual sharks return to the aggregation site every one to six days. These female sharks are speculated to be taking advantage of the warmer water to speed their growth or that of their embryos. The shallow waters may also enable them to avoid unwanted attention by males. Off Enewetak, grey reef sharks exhibit different social behaviors on different parts of the reef. Sharks tend to be solitary on shallower reefs and pinnacles. Near reef drop-offs, loose aggregations of five to 20 sharks form in the morning and grow in number throughout the day before dispersing at night. In level areas, sharks form polarized schools (all swimming in the same direction) of around 30 individuals near the sea bottom, arranging themselves parallel to each other or slowly swimming in circles. Most individuals within polarized schools are females, and the formation of these schools has been theorized to relate to mating or pupping. Threat display The "hunch" threat display of the grey reef shark is the most pronounced and well-known agonistic display (a display directed toward competitors or threats) of any shark. Investigations of this behavior have been focused on the reaction of sharks to approaching divers, some of which have culminated in attacks. The display consists of the shark raising its snout, dropping its pectoral fins, arching its back, and curving its body laterally. While holding this posture, the shark swims with a stiff, exaggerated side-to-side motion, sometimes combined with rolls or figure-8 loops. The intensity of the display increases if the shark is more closely approached or if obstacles are blocking its escape routes, such as landmarks or other sharks. If the diver persists, the shark may either retreat or launch a rapid, open-mouthed attack, slashing with its upper teeth. Most observed displays by grey reef sharks have been in response to a diver (or submersible) approaching and following it from a few meters behind and above. They also perform the display toward moray eels, and in one instance toward a much larger great hammerhead (which subsequently withdrew). However, they have never been seen performing threat displays toward each other. This suggests the display is primarily a response to potential threats (i.e. predators) rather than competitors. As grey reef sharks are not territorial, they are thought to be defending a critical volume of "personal space" around