<P> Because krill move, blue whales typically feed at depths of more than 100 metres (330 ft) during the day and only surface - feed at night . Dive times are typically 10 minutes when feeding, though dives of up to 21 minutes are possible . The whale feeds by lunging forward at groups of krill, taking the animals and a large quantity of water into its mouth . The water is then squeezed out through the baleen plates by pressure from the ventral pouch and tongue . Once the mouth is clear of water, the remaining krill, unable to pass through the plates, are swallowed . The blue whale also incidentally consumes small fish, crustaceans and squid caught up with krill . </P> <P> Mating starts in late autumn and continues to the end of winter . Little is known about mating behaviour or breeding grounds . In the fall, males will follow females for prolonged periods of time . Occasionally, a second male will attempt to displace the first, and the whales will race each other at high speed, ranging from 17 miles per hour (27 km / h) to 20 miles per hour (32 km / h) in New Zealand . This often causes the racing whales to breach, which is rare in blue whales . This racing behavior may even escalate to physical violence between the males . Scientists have observed this behavior in multiple parts of the world, including the Gulf of St. Lawrence in Canada and the South Taranaki Bight in New Zealand . </P> <P> Females typically give birth once every two to three years at the start of the winter after a gestation period of 10 to 12 months . The calf weighs about 2.5 tonnes (2.8 short tons) and is around 7 metres (23 ft) in length . Blue whale calves drink 380--570 litres (100--150 U.S. gallons) of milk a day . Blue whale milk has an energy content of about 18,300 kJ / kg (4,370 kcal / kg). The calf is weaned after six months, by which time it has doubled in length . The first video of a calf thought to be nursing was made 5 February 2016 . </P> <P> Sexual maturity is typically reached at five to ten years of age . In the Northern Hemisphere, whaling records show that males averaged 20--21 m (66--69 ft) and females 21--23 m (69--75 ft) at sexual maturity, while in the Southern Hemisphere it was 22.6 and 24 m (74 and 79 ft), respectively . In the Northern Hemisphere, as adults, males averaged 24 m (79 ft) and females 25 m (82 ft) with average calculated weights of 90.5 and 101.5 tonnes (100 and 112 tons), respectively . Blue whales in the eastern North Pacific population were found to be on average 0.91 meters (3 ft) shorter, therefore with males averaging 23.3 meters (76 ft) and 80.5 tonnes (88.5 tons) and females 24 meters (79 ft) and 90.5 tonnes (100 tons). Antarctic males averaged 25 m (82 ft) and females 26.2 m (86 ft), averaging 101.5 and 118 tonnes (112 and 130 tons). Pygmy blue whales average 19.2 meters (63 feet) at sexual maturity, with males averaging 21 meters and females 22 meters (69 and 72 feet) when fully grown, averaging 76 and 90 tonnes (83.5 and 99 tons). </P>

How long does it take for a blue whale to give birth