Sharks cannot swim upside down. In fact, when flipped upside down, they enter a state called tonic immobility, a type of paralysis that some animals go into when threatened. Sharks remain in a state of tonic immobility for about 15 minutes before they can recover. Some predators, such as orca whales, take advantage of this fact and will flip sharks over when hunting them. Some sharks go into tonic immobility more easily than others; for instance, tiger sharks seem to be more responsive than great whites.
More facts about sharks and immobility:
- Sharks aren't the only animals to go into tonic immobility. Beetles go into a tonic state as well, and it is said that you can put a chicken into a state of tonic immobility by placing its head to the ground and drawing a line back and forth outward from its beak.
- Sharks can lose as many as 30,000 teeth during their lifetime, and a shark's liver can make up about 30 percent of its body mass.
- Both lizards and crocodiles "shut down" when flipped on their backs. In lizards, this is because the position makes it hard for them to get oxygen, so they go into a trance-like state to conserve energy. In crocodiles, it's more of a panic response.