Blue whale  (Balaenoptera musculus)

Blue whales usually occur alone or in groups numbering between two and three individuals, but occasionally large groups of up to 60 individuals may form in areas of high food abundance (7). They feed mainly on shrimp-like krill, which are filtered through the baleen plates (7). Whales tend to feed at less than 100 metres deep, and make dives lasting between 5 and 20 minutes (7). Most blue whales are thought to spend the summer feeding in the colder waters of high latitudes, migrating to warm waters in the winter where females give birth (5); although some may be resident in the same area year round (3). No feeding occurs on the breeding grounds. The two main populations (north and south) remain separated as the seasons are reversed in the two hemispheres.

A single calf is produced after a gestation period of 10 to 11 months. The inter-birth period is probably two to three years, although this may have decreased recently in response to the low population densities (7). At birth, a calf measures about 7 m in length (3) and may consume up to 50 gallons of milk a day in its first year of life, leading to a weight gain of 90 kilograms a day (6). Communication seems to occur via a variety of low frequency sounds and clicks (7).