They have a four-chambered heart, although modified for their ectothermic nature due to an elongated cardiac septum, physiologically similar to the heart of a bird, which is especially efficient at oxygenating their blood. They normally dive for only a few minutes, but will stay underwater for up to 30 minutes if threatened, and if they remain inactive they can hold their breath for up to two hours. They have an ectothermic metabolism, so can survive for long periods between meals—though when they do eat, they can eat up to half their body weight at a time.