Shark Behavior: Intelligence, Sleep and Where They Hang out

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Shark brain
Sharks can be diurnal (active during the daytime), nocturnal (active at night), motile (move around as opposed to being stationary), sedentary (remain in the same area), territorial (defend an area within the home range), and social (associates with others of its species; forms social groups). [Source: Animal Diversity Web (ADW)]

Sharks flipped on the back go into a sleeplike state called tonic immobility. Some species can expel unwanted objects they have swallowed by ejecting their stomachs and intestines through their anus. Groups of sharks tend to segregate by sex.

Aggressive sharks arch their back, lower their pectoral fins straight down, raises their snout, and swim with an exaggerated rolling motion. This display means an attack is coming. Scientists using "bite meters" baited with fish have found that some sharks bite with a pressure of 18 tons per square inch. Explosion from dynamites kills bony fish by injuring their swimming bladders. This does not happen with sharks, and explosions in fact seem to attract them.[Source: Nathaniel Kenney, National Geographic, February 1968]

Some shark species swim in schools; some are mostly solitary. They often move around slowly during the day and become more active at night. They have sensitive perception channels and constantly surveying their surroundings. When food is sensed they speed up and become more active very quickly. When some species feel the vibrations of a fish dying they become very aggressive.

Shark behavior at night was the subject of the 2018 French research documentary called “700 Requins Dans La Nuit. “ (National Geographic later aired a shorter version called “700 Sharks.”). It was shot in southern channel of Fakarava Atoll — in French Polynesia in the South Pacific — where 500 gray reef sharks gather to hunt. [Source: Michelle Marchante, Miami Herald, June 16, 2021]

Websites and Resources: Shark Foundation ; International Shark Attack Files, Florida Museum of Natural History, University of Florida ; Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio

Shark Intelligence

Porbeagle shark brain
For along time it was thought that sharks were one-dimensional, stupid swimming eating machines, but it turns they are one of the more intelligent set of creatures in the sea.

The brain-size-to-body-mass ratio of cartilaginous fish such as sharks and rays is about ten times that of bony fish and equal to many "advanced” mammals. A 440-pound tiger shark has a brain that weighs nearly three ounces and is the size of two golf balls, which is about the same size as the brain of wolf or mountain lion and half the size of a brain of a 350-pound tiger. By contrast a fish of similar size has a brain the size of only three marbles.

In the 1950s, Eugenie Clark demonstrated that lemon sharks could learn to ring a bell and differentiate between shapes. Sharks in the laboratory perform tasks such as pushing a target or ringing a bell as well as white rats.

Most sharks are known to use body language to signal aggression, but there is little data available on whether sharks utilize other forms of communication between individuals. Sharks can learn how to negotiate mazes ten times faster than rabbits and remember the route a year later, while even humans forget after a few weeks. Some sharks in fact like doing mazes so much they circle the door of the maze waiting to start.

Sharks Become More Aggressive During Full Moons

Shark brain compared to human brain
Sharks appear to be more aggressive during full moons and some have even so as far as calling them marine werewolves. Researcher Stephen Midway of the Florida Museum of Natural History coauthored a study, published in Frontiers in Marine Science that examined how the moon and lunar cycles affect sharks. “We do know that the Moon influences the oceans — whether through forces on tides or the Earth’s geomagnetic field,” he said. “Sharks — like many marine organisms — will respond to their environment, so the chance for a lunar influence on sharks is there, but more research is needed.” [Source: Elizabeth Rayne, SYFY, January 19, 2022]

Elizabeth Rayne wrote in SYFY: Midway and his team looked through 50 years of data on shark attacks from all over the world and found that they are more likely to get vicious when lunar illumination is over 50 percent. Sharks can be triggered by lunar phases, which doesn’t necessarily mean a full Moon, either. ““Different environments and conditions are more suitable for different shark species (although some occur in both), which is why different communities of sharks can be found along different coasts, such as Florida and California,” said Midway.

“It is possible that sharks have a greater advantage at snapping up prey when tides are stronger and this may influence the behavior of what they eat. They could also be in extra-predatory mode when they know an easy meal is around. Lunar illumination also has some sort of influence, making it easier for sharks to see the silhouettes of their prey at night. Moonlight can’t explain daytime attacks on humans. ““Because many sharks are similar to each other (when compared to other fishes), it is plausible that many or all sharks could have common responses to Moon effects,” Midway said in the study.

Sleeping and Resting Sharks

As we all know many sharks are “negatively buoyant.”and have to keep moving so that water flows through their gills to breathe and keep themselves from sinking. Sleeping sharks have been observed in Japan, the Yucatan and elsewhere. They extract oxygen from water carried through the gills by currents or use bubbling freshwater from fissures to remove parasites from their skin.

In waters off Aldabra Atoll in the Seychelles, blacktip reef sharks position themselves in water only a few centimeters deep and wait for the tide to refill the lagoon. According to National Geographic: With their bellies touching the sand, they point their snouts into the current to keep water flowing over their gills.

When gray reef sharks need a rest they “surf” currents. According to Florida International University marine scientist and assistant professor Yannis Papastamatiou, who did a study on the topi published in June 2021 in the Journal of Animal Ecology it is similar to the way birds soar on wind currents, except they do it underwater.

Michelle Marchante wrote in the Miami Herald: “The discovery was made during a visit to the southern channel of Fakarava Atoll in French Polynesia, where more than 500 gray reef sharks gather to hunt. During the daytime dives that Papastamatiou noticed that many of the sharks remained in the small channel, even though they weren’t hunting. Then he noticed something else: The sharks had developed a “conveyor belt” like system. When one shark reached the end of the line, it allowed the current to carry it back to the beginning, he said. So did another shark. And another. And another. Many barely moved their tails. They looked almost motionless, like they were floating. [Source: Michelle Marchante, Miami Herald, June 16, 2021]

“But they weren’t sleeping. To figure out what was happening, the team used a variety of tools, including animal-borne cameras, special tags to gather data on the sharks activity and swimming depths, and a detailed map to predict and model where possible updrafts might appear, depending on the direction of the tide. Data confirmed what researchers noticed during their underwater observations: “The sharks were using the updrafts to “surf the slope” and cut their energy usage by at least 15 percent, which is significant for a species that can never stop swimming, said Papastamatiou.

“But, wait, do sharks actually sleep? “It’s a bit complicated. “Sharks do not sleep like humans do, but instead have active and restful periods,” according to the Florida Museum of Natural History. Some species, like nurse sharks, don’t have to swim all the time, which lets them have “stationary rest,” according to the museum. What is going on with grey reef sharks is still not completely understood.

Sharks Like to Stay Close to Coastal Cities, Study Says

Helen Ray of CBS News wrote: Animals, in general, are categorized as either urban adapters, such as rats, raccoons, pigeons and opossums who thrive and even depend on humans to survive, or urban avoiders, typically seen in land predators like wolfs, cougars and grizzly bears. Based on studies of land predators, scientists predicted that sharks would avoid the densely populated coastline, or appear only during periods when there are fewer people, such as late hours and weekdays. But scientists discovered the opposite. [Source: Helen Ray, CBS News, June 19, 2022]

When looking at the data, scientists found that "space use patterns of tracked sharks were consistent with that of 'urban adapters.'" They said the "modeling also revealed that an unmeasured spatial variable was driving considerable shark residency in areas exposed to high urbanization."

The scientists found three reasons why the sharks could be more attracted to populated coastlines, including nutrient runoff and sewage discharge going into the marine to create bottom-up food webs that attract sharks; marine food waste, such as fish carcasses being dumped into the bay; and leftover fish from the Miami Seaquarium being discarded into the water. Neil Hammerschlag, one of the study's authors, told the University of Miami that if sharks spend a lot of time close to the shore, they could be at risk of "exposure to toxic pollutants as well as fishing, which could impact their health and survival."

Lots of Sharks Living in and Around Active Volcanoes

In 2015 large numbers of sharks were found at Kavachi volcano, a very active volcano in the Solomon, Islands by National Geographic grantee Dr Brennan Phillips. 9 News reported: As the camera dropped into the roiling orange waters of one of the world's most active submarine volcanoes, scientists were expecting to see plenty of activity; what they weren't expecting to find was hordes of sharks thriving in the explosive crater. Reef sharks, hammerheads and scalloped hammerheads all swam up to the lens, unbothered by the fact an eruption had taken place shortly before. [Source: Raffaella Ciccarelli, 9 News, August 9, 2020]

NASA’s Earth Observatory reports that Kavachi erupts almost continuously. Residents of islands near it say that they often see steam and ash near the volcano’s location. Occasionally Kavachi eruptions create ephemeral islands. The summit of the “sharkcano” is 20 meters (65 feet) below the surface. level. The base of the volcano connects to the seafloor at a depth of 1.2 kilometers (.75 miles). [Source: Joshua Hawkins, BGR, May 25, 2022]

According to 9 News: Further studies into the sharks have been hampered by the fact Kavachi is so active, which begs the question: how do the sharks survive in an active crater, here temperatures reach near-boiling point? "I thought it sounded like a sci-fi movie. It's an amazing find," Professor Heithaus at Florida International University said. "It just demonstrates how adaptable sharks are. Extreme environments are something they can clearly handle; whether it's a volcano or surviving thousands of meters underwater. It's really not yet known why they are there. It could be something to do with reproduction, or who knows what else is living in there... maybe they're just sniffing out a meal."

Sharks have a cluster of pores on their snout called the ampullae of Lorenzini, which Heithaus believes can detect changes in the earth's magnetic field – allowing them to swim to safety before an eruption occurs. This mysterious "sixth sense" may also help them hunt and sniff out other volcanic islands. "It looked like the sharks in the volcano were used to dealing with eruptions," he said. "You would think it's dangerous but studies have shown us they can detect approaching hurricanes and cyclones, so they may be able to detect when something bad is about to happen and move out of the way."

While his research is still in progress, Professor Heithaus said the correlation between sharks and volcanoes is undeniable. His studies have taken him to the rugged shores of Réunion Island, which is home to an active volcano called Piton de la Fournaise. Sharks are so abundant there, swimming was made illegal in 2013. Eleven people have lost their lives in attacks since 2011, mostly by bull sharks. In this instance, Professor Heithaus believes the bull sharks are taking advantage of the fact sediment washes down from the volcano's slope. The cloudy waters make an ideal hunting ground for the "smart" predators.

However sharks use volcanoes, it's welcome news they do, as a recent study found sharks are "functionally extinct" on nearly one in five coral reefs. The extreme habitat ironically provides a safe-haven for them, Professor Heithaus said. "You're not going to go fishing around a volcano and probably some of the bigger sharks, who are predators, will be less inclined to go in there.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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