Blue whales have patches with gradual transitions between several shades of grey. In good light, they look bluish in clear water. The dorsal fin which is located at the far end of the body, is relatively small compared with the size of the body in general. The upper mandible has between 270 and 395 black baleen on each side. The longest baleen may be a metre in length.
When blue whales come to the surface to breathe, they emit a column of steam (spout) that can be 10–12 metres high and be visible for more than a minute. After the spout, their back continues to stick out of the water for several seconds until the small dorsal fin and the powerful tailfin become visible and the whale begins a new dive.
Blue whales become sxually mature at around 8–10 years old. By then, the females are 21–24 metres long: a bit bigger in the southern hemisphere than in the northern hemisphere. The males are 20–22 meters long when they become sexually mature. Mating takes place in the autumn and winter, and the females have a gestation period of between 10 and 12 months before giving birth to a 6–7 metre long calf, weighing from 2 to 3 tonnes. The suckling period is 6–8 months. The calf will have become about 16 metres long by the time it is weaned and must start to catch its own food. The females have a calf about every three years.
It is difficult to determine the age of baleen whales, but we assume that blue whales become at least 80–90 years old. Animals that have been recognised due to their pattern of patches have reached more than 40 years of age.
Although blue whale populations were greatly reduced by whaling, they can still be found in the North Atlantic, the North Pacific and circumpolar in the southern hemisphere. They graze in cold waters, preferably right at the edge of the ice in the summer and migrate to warm areas in the winter. Krill is their most important prey. During their migration between their breeding and grazing areas, blue whales swim at a speed of 5–30 km/h. When they are eating, they often swim at between 3 and 6 km/h. Their dive times are 8–15 minutes, but dive times of up to 36 minutes have been recorded.