Eurasian otters (Lutra lutra) are carnivores and belong to the family of mustelids (Mustelidae). They are related to pine marten, beech marten, badger, polecat, mink and stoat. Worldwide there are 13 different otter species. Eurasian otters are well-adapted to water habitats despite being land predators. This relates to adaptions of their body that improve their abilities in water – a streamlined body, their eyes and ears placed in one line on top of their head, webbed feet and long, strong vibrissae on their snout. Male otters can grow up to 1.30 m long and weigh up to 12 kg. Females are approximately 20 cm shorter and weigh about 10 kg. The energy budget of otters is adapted according to their lifestyle: They lose 25 times more heat in water compared to when they are on land. To retain body temperature, otters eat 1 kg food per day. Furthermore, the fur of otters is very dense with up to 75,000 hairs per cm2 (compared to humans with 150-200 hairs per cm2), and single hairs interlock like a zip and keep warm air within the fur, so that the skin has no contact with water. While diving, trapped air is released from the fur and small air bubbles form in the water.