to July in the Northern Hemisphere. However, females with "full-term embryos" have also been reported in the fall off Enewetak. The newborns measure long. Sexual maturation occurs around seven years of age, when the males are long and females are long. Females on the Great Barrier Reef mature at 11 years of age, later than at other locations, and at a slightly larger size. The lifespan is at least 25 years. Behavior Grey reef sharks are active at all times of the day, with activity levels peaking at night. At Rangiroa, groups of around 30 sharks spend the day together in a small part of their collective home range, dispersing at night into shallower water to forage for food. Their home range is about . At Enewetak in the Marshall Islands, grey reef sharks from different parts of the reef exhibit different social and ranging behaviors. Sharks on the outer ocean reefs tend to be nomadic, swimming long distances along the reef, while those around lagoon reefs and underwater pinnacles stay within defined daytime and nighttime home ranges. Where strong tidal currents occur, grey reef sharks move against the water, toward the shore with the ebbing tide and back out to sea with the rising tide. This may allow them to better detect the scent of their prey, or afford them the cover of turbid water in which to hunt. Little evidence of territoriality is seen in the grey reef shark; individuals tolerate others of their species entering and feeding within their home ranges. Off Hawaii, individuals may stay around the same part of the reef up to three years, while at Rangiroa, they regularly shift their locations up to . Individual grey reef sharks at Enewetak become highly aggressive at specific locations, suggesting they may exhibit dominant behavior over other sharks in their home areas. Sociality Social aggregation is well documented in grey reef sharks. In the northwestern Hawaiian Islands, large numbers of pregnant females have been observed slowly swimming in circles in shallow water, occasionally exposing their dorsal fins or backs. These groups last from 11:00 to 15:00, corresponding to peak daylight hours. Similarly, at Sand Island off Johnston Atoll, females form aggregations in shallow water from March to June. The number of sharks per group differs from year to year. Each day, the sharks begin arriving at the aggregation area at 09:00, reaching a peak in numbers