Eurasian otters are well-adapted to water as semi-aquatic carnivores. Water is their main source of food and it also serves as protection from enemies. For a diving otter is protected against lead, spears, or chasing dogs. And food can be found in water throughout the whole year. But living close to water does not just mean plenty of food available; it is also a major strain on their energy budget. Staying in water withdraws approximately 25 times more heat compared to staying dry on land. That means keeping warm to retain the high body temperature necessary for mammals. This is especially high for Eurasian otters, where the body temperature has to be 39° - 40° Celsius when active. The high energy use demands plenty of food. Otters eat about 1kg per day, with a body weight of 6-12kg. Thus, calorie-rich fish such as fatty eel and carp quickly become an otters’ favourite food.
Fish are caught mainly at night. Only sometimes are Eurasian otters active during the day. Their active behaviour is characterised by 4-6 active phases per day. Therefore, several resting periods occur during the night.
There are only fragmented research findings available on space-time-behaviour of otters. It is very difficult to observe otters in the wild or to equip them with data transmitters. It is assumed that Eurasian otters stay in large territories. These can cover around 10 – 20 km river length, depending on food availability, number of hiding opportunities, and population density. Otters of the same sex do not tolerate each other in one territory. Between sexes living areas overlap completely. The larger territories of males can cover 1 – 3 female territories.
As semi-aquatic carnivores, Eurasian otters stay close to water. Here they can find food and protection. But otters can also run long distances over dry land, especially to travel between pond areas or to reach other waterbodies.