The mothers are the sole providers of care, with lactation lasting 24 days.[12] Researchers have found males gather under water, turn on their backs, put their heads together, and vocalize to attract females ready for breeding.[13] The single pups are born well developed, capable of swimming and diving within hours. Suckling for three to four weeks, pups feed on the mother's rich, fatty milk and grow rapidly; born weighing up to 16 kilograms, the pups may double their weight by the time of weaning.