Blue whale
Reproduction and birth

Blue whales generally reach sexual maturity at 8–10 years. In the Northern Hemisphere, the length at which they reach maturity is 21–23 meters (69–75 ft) for females and 20–21 meters (66–69 ft) for males. In the Southern Hemisphere, the length of maturity is 23–24 meters (75–79 ft) and 22 meters (72 ft) for females and males respectively. Male pygmy blue whales average 18.7 meters (61.4 ft) at sexual maturity. Female pygmy blue whales are 21.0–21.7 meters (68.9–71.2 ft) in length and roughly 10 years old at the age of sexual maturity. Little is known about mating behavior, or breeding and birthing areas. Blue whales appear to be polygynous, with males competing for females. A male blue whale typically trails a female and will fight off potential rivals. The species mates from fall to winter.

Pregnant females eat roughly four percent of their body weight daily, amounting to 60% of their overall body weight throughout summer foraging periods. Gestation may last 10–12 months with calves being 6–7 meters (20–23 ft) long and weighing 2–3 metric tons (2.0–3.0 long tons; 2.2–3.3 short tons) at birth. Estimates suggest that because calves require 2–4 kilograms (4.4–8.8 lb) milk per kg of mass gain, blue whales likely produce 220 kilograms (490 lb) of milk per day (ranging from 110 to 320 kilograms (240 to 710 lb) of milk per day). The first video of a calf thought to be nursing was filmed in New Zealand in 2016. Calves may be weaned when they reach 6–8 months old at a length of 16 meters (53 ft). They gain roughly 37,500 pounds (17,000 kg) during the weaning period. Interbirth periods last two to three years; they average 2.6 years in pygmy blue whales.
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