Sharks have existed on Earth for over 400 million years. They live in every ocean on the planet, and in some rivers, and are essential to a balanced ocean ecosystem—sharks keep the food chain in check and encourage species diversity. Sharks even help preserve carbon-capturing sea meadows by preying on the sea turtles that eat seagrasses.
Some shark behaviors may look a little odd to us, but there are good reasons why they have evolved to perform them. Here are eight of the most unusual acts and how they help sharks thrive.
1. Sharks begin hunting before they’re born.
Not every shark breeds in the same way; some lay eggs and others give birth to live young. The live-bearers, including great white, mako, tiger, and bull sharks, have an advantage over their egg-laying cousins: They begin to hone their hunting skills even before they’re born.
A female sand tiger shark has two uteruses and will produce multiple eggs. Research has shown that around 10 will be fertilized, while the rest serve as food for their stronger siblings. The growing embryos will begin hunting when they reach lengths of about 2 inches, using the unfertilized eggs as a means of subsistence. After that, they turn on their smaller and weaker siblings. This form of intrauterine cannibalism is known as embryophagy. Baby sharks will swim from one uterus to another in search of their prey until only the strongest two are born.
Embryophagy gives the pups several advantages. It sustains them over their nine to 12 months in the womb, making them bigger and healthier at birth, which gives them a better chance at survival.
It also clears the gene pool of the weaker sharks. The female sand tiger shark mates with several males, so her embryos have multiple fathers, but research has shown that in 60 percent of the litters the two pups came from the same father. The weaker fathers‘ genes go no further, while the two pups that are eventually born carry the stronger fathers‘ genes into the world. It also means that the baby sharks enter the world as a fully-fledged hunters.
2. Adult sharks can be cannibals, too.
Many sharks engage in cannibalism, especially when one of them is injured. A 2016 study found that the extinct Orthocanthus, which lived 300 million years ago, ate their own young when other resources ran out. Today’s lemon sharks seem to choose the tangled roots of shallow mangrove forests as a nursery, not only to protect their young from larger predators, but also from other lemon sharks.
Cannibalism is an unusual behavior for an animal that wants to survive, but it’s a fundamental way to preserve their species. Not only is it a means to a meal, but it also allows the survival of the fittest individuals. Good thing sharks aren’t sentimental.
3. Unborn sharks can sense danger.
Pre-birth survival is no easier for those sharks born from eggs—and some species have developed ways of protecting themselves even before they are born, as a 2013 study of bamboo sharks found.
Egg-bearing sharks deposit their offspring in casings (sometimes called mermaid’s purses) that attach themselves to plants by long tendrils. These pouches allow the embryos to hide their movements and scent as they grow. In the final stages of the gestation period, the cases open just enough to introduce the embryos to the salt waters of the ocean. At the same time, the baby sharks’ electrosensory systems reach maturity and the animals begin to sense the world around them, which may include the electronic signals of their predators. Juvenile bamboo sharks are pre-programmed to read these signals and take action—namely, to hold their breath, screw themselves into balls, and freeze to avoid detection.
The study’s authors hint that this behavior could be used to develop electronic shark-repelling devices. Fishing nets accidentally kill millions of sharks (and other marine animals) every year, so equipping the nets with such devices might protect not just bamboo sharks, but all elasmobranchs.
4. Sharks will spy on you.
Spy-hopping, a practice in which a sea creature raises its head out of the water, is most often associated with whales. While it’s rare in sharks, a few species are known to pop their head out of the water and take a look around.
Some sharks will lift their head vertically above the water, while others swim along the surface with just an eye clear of it. In both cases, spy-hopping is about gathering information, perhaps for the location of prey, but the action itself is non-aggressive. A shark comes up to the surface gently, and may do it several times without ever lashing out.
Great white sharks are masters of spy-hopping, particularly around boats that are using chum to entice them closer. Research on oceanic whitetip sharks concluded that the fish can use their renowned sense of smell above the waterline to detect food at a distance. It’s also possible that the sharks are just curious.
5. Some sharks leap out of the ocean.
Breaching is another behavior that might seem illogical for an animal that needs water to survive, but several species, like great white, basking, and bull sharks, do leap out of the water at high speeds. It can be a spectacular sight, with some sharks able to completely clear the water for several seconds due to the power they generate as they swim. The record is held by a great white, affectionately known as Rocket, who jumped 15 feet above the sea.
Breaching usually derives from hunting. Capturing an agile seal is no easy task, and sharks have learned that they need to ambush their prey if they want to be successful. Using the darker waters below as cover, a great white is able to sneak up on a seal, launch itself upwards at burst speeds of around 36 feet per second, and reach the seal before it can escape. The breach is an explosive result of this upward trajectory.
But hunting isn’t the only reason sharks breach. Basking sharks have no need to launch themselves out of the water to catch their prey, but they still engage in breaching. It’s possible that they do it as part of their courtship or in an aggressive display by male sharks for dominance. The splash the breach makes may be used as a signal to other sharks either in warning or as a form of communication. Marine pollution, water temperature changes, increased salinity, and the need to rid themselves of parasites may also make sharks jump.
6. They will walk away from danger.
The epaulette shark has evolved a survival strategy that larger sharks can only imagine: calmly walking away from trouble.
The behavior was first observed in 1995 by researcher Peter Pridmore, but a 2015 BBC program filmed at the Great Barrier Reef drew the world’s attention. The epaulette shark lives and hunts on the seafloor around Australia and New Guinea, but as the tide goes out, many find themselves stranded on the reef in temperatures upward of 85°F. They have developed the ability to reduce their oxygen intake and to use their pectoral and pelvic fins to walk distances of 100 feet or more. Crawling across the sun-drenched coral, they can find small pools of water that allow them to replenish their oxygen until the sea returns.
Since 1995, eight more walking sharks have been identified, including the Halmahera epaulette shark—discovered in Indonesia in 2013—and four during a study of the genus Hemiscyllium in 2020. A 2022 study concluded that this ability is becoming even more useful amid climate change, suggesting “that this species has adaptations to tolerate some, but perhaps not all, of the challenging conditions predicted for the 21st century.”
7. Sharks can spit out their stomachs.
Unfortunately, sharks have been found with their stomachs containing all sorts of non-edible objects, including tires, a chicken coop, and even an unexploded bomb. The problem is growing worse as the world’s oceans continue to fill with trash.
One way that a shark copes is through stomach eversion—basically, the ability to upchuck their stomachs, wash them out, and then swallow them again, all within a fraction of a second. A 2005 study of a Caribbean reef shark recorded two eversions lasting 0.28 and 0.40 seconds each with a gap of 1.52 seconds between them. The study concluded that “its function may be related to removal of indigestible food particles and mucus from the inner surface.”
A 1990 study of captive sharks showed that they can also evert their lower intestine through their cloaca, though this cleansing regime seems to last far longer than the stomach variety, with a recent paper in the Journal of Ethology reporting that a shark will swim for over two minutes with its gut hanging from its body (as seen on this recording of a captive tiger shark).
Eversion comes with risks in the wild. As part of the Journal of Ethology’s study, researchers witnessed an oceanic whitetip shark being chased by smaller predators both “exploiting ejected digested material” and trying to take bites out of the protruding organ.
8. Sharks choose to live inside volcanoes.
Sharks can be discovered in nearly every environment on Earth, but one place that few people would expect to find them is in the middle of an active volcano.
The Kavachi volcano is located under the Pacific Ocean near the Solomon Islands. It regularly erupts, spewing sulfur, carbon dioxide, ash, and rock out into the water, turning it orange. The caldera is too toxic for humans to enter, and the water is superheated—yet two species of shark have made this hostile environment their home. During a non-shark-related study of the volcano in 2015, a robotic submersible discovered hammerhead and silky sharks swimming and hunting in the acidic waters.
Scientists are not exactly sure how they survive, but their evolutionary adaptations have allowed them to outlive the ”big five” mass extinction events that killed off a staggering number of other species. They have also developed early-warning systems that give them an innate ability to escape the area before an eruption.
Surprisingly, the live volcano offers the sharks considerable protection from larger predators—and is a far safer place to exist than in an ocean near humans.