11 Facts About Bull Sharks
Compared to sharks like the hammerhead (with its oddly shaped skull) and the goblin (with its projectile mouth), the bull shark (Carcharhinus leucas) looks rather generic—but this predator can swim thousands of miles upriver, hunt baby hippos, and take over the occasional golf course.
1. BULL SHARKS HAVE A VERY STRONG BITE.
In 2012, scientists compared the bite strengths of 13 different sharks and shark-like fish and found that an adult bull shark can theoretically close its jaws with just under 6000 newtons of force at the back of its mouth and over 2000 newtons at the front. (Humans chomp with a maximum force of around 900 to 1300 newtons.) So proportionately, the bull shark has the strongest bite of any shark whose jaw strength has been measured. Nobody knows exactly why the shark evolved such a powerful mouth, but it may have something to do with its diet: Bull sharks eat everything from oysters to turtles to other bull sharks, and their herculean jaws might make it easier to tear through a wide range of shells, scales, and skins.
2. THEY THRIVE IN FRESH WATER.
All organisms need a specific salt-to-water ratio within their bodies. This presents a challenge to aquatic life forms—when they absorb too much salt, their cells get dehydrated, but if they take in too much fresh water, the cells get bloated and some of the chemicals inside their bodies may become fatally diluted.
Fish have evolved a couple of tricks to cope with the problem, one of which involves pee. Excess salt is removed from the bloodstreams of oceangoing fish by the kidneys and then flushed out during the urination process. Freshwater fish do the opposite: Their pee contains very little salt and their bodies retain a higher concentration of the substance.
Most fish have no control over how much salt gets removed by their kidneys, which is one reason why many species can only survive in either fresh or salt water. But bull sharks can actually regulate the amount of salt that goes into their pee, which means they can live in both environments. When they swim in the ocean, their urine is highly saline. And as they venture into fresh water, the kidneys work hard to retain salt—thus producing watery, diluted urine. (Bull sharks also use their rectal glands and livers to maintain the proper balance between salt and water.)
3. A BULL SHARK WAS ONCE CAUGHT NEAR ALTON, ILLINOIS.
In 1937, two fishermen captured a 5-foot bull shark more than a thousand miles up the Mississippi River—as far north as this species is known to travel up that river. Given their high tolerance for fresh water, it’s no wonder that bull sharks do well in river systems. One specimen was found swimming in the Amazon River about 2485 miles away from the ocean; they also frequent Africa’s Zambezi River and the Ganges River in south Asia.
4. NOT EVEN HIPPOS ARE SAFE FROM BULL SHARK ATTACKS.
In their trips upriver, bull sharks encounter some of the fiercest predators on earth: In Australia, an 18-foot-long saltwater crocodile named Brutus was once photographed devouring a small bull shark, and in African waterways the sharks have run-ins with hippos. The mammals generate a lot of poop, which attracts small fish that bull sharks eat—so it’s not at all uncommon to see a bull shark or two creeping around a pod of hippos.
The two species don’t always get along. Bull sharks do prey on young hippos on occasion, and adults are sometimes bitten as well—though these attacks might be accidental (in murky, poop-filled water, bull sharks probably mistake hippo legs for fish). Whatever the case may be, large hippopotamuses are adept at fending off bull sharks.
5. UNLIKE GREAT WHITES, BULLS CAN DO WELL IN AQUARIUMS.
No one has been able to keep a great white in captivity for much longer than six months. Bull sharks, however, can live in captivity for 25 years or more. In North America, you can see them at the Oklahoma Aquarium, which houses 10 bull sharks in a nearly 500,000-gallon tank.
6. LAKE NICARAGUA’S RESIDENT BULL SHARKS WERE ONCE THOUGHT TO CONSTITUTE A NEW SPECIES.
Scientists used to think that the large-bodied sharks that periodically attack locals in Lake Nicaragua—the biggest lake in Central America—represented their own, distinct species. A popular hypothesis claimed that Lake Nicaragua must have started out as a bay in the Pacific Ocean that closed up as time went by, trapping some sharks inside that evolved into a brand new species—the so-called “Lake Nicaragua shark,” which ichthyologists called Carcharhinus nicaraguensis.
But Lake Nicaragua isn't cut off from the ocean at all, and scientists eventually determined that “Lake Nicaragua Sharks” are really just bull sharks that were entering the Rio San Juan from the Caribbean and making their way to the lake [PDF].
7. AN AUSTRALIAN GOLF COURSE IS INFESTED WITH THEM.
At the Carbrook Golf Club, located near Brisbane, Australia, tee time sometimes comes with teeth. The course is next to the Logan River, which flooded several times in the 1990s. At some point, newborn bull sharks were probably swept into the brackish, landlocked lake that sits beside tees 12 through 15, and were trapped there when the Logan receded. The lake is spacious (about 50 acres, or the size of Grand Central Terminal) and stocked with fish—good news for the sharks.
The first shark fin sightings were quickly dismissed. “Our members have a tendency to drink a little bit while they play so we really just put that down to too much alcohol,” general manager Scott Wagstaff told National Geographic. But by 2003, there were photos, and a 2011 YouTube video taken by Wagstaff went viral. According to the club’s website, “there are between 6 and 12 sharks” in the lake; the biggest is roughly 9 feet long. It’s also likely that they are breeding.
Carbrook Golf Club celebrates their cartilaginous guests: “Shark Lake Challenge” tournaments are held on the last Wednesday of every month and a stylized shark fin adorns the club logo. For obvious reasons, ball retrieval has been prohibited.
8. BITING IS A PART OF MATING.
Male bull sharks become sexually active at 14 or 15, but females don’t start reproducing until they're 18. Female bull sharks of reproductive age have scars behind their skulls: A male bites his partner to hang on during coitus (a behavior that appears to be common), then uses one of his claspers—a pair of fin extensions located under the pelvic area—to insert sperm into her cloaca.
Following a 10- to 11-month gestation period, the female will give birth to a litter of pups. Newborns typically range between 22 and 32 inches long.
9. ONLY TWO OTHER SHARK SPECIES BITE HUMANS MORE OFTEN.
All types of shark-on-human attacks are ultra-rare—you’re more likely to get struck by lightning. But the three shark species that are most commonly implicated in attacks on people are, in order, the great white shark, the tiger shark, and the bull shark. Bull sharks have been involved in at least 100 documented cases, and there's an explanation for that: They frequent shallow, coastal waters in tropical regions and often stray into brackish and fresh water—all places where humans can be found. In areas where the sharks can’t see very well, they may mistake humans for other, meatier prey.
Since bull sharks lack eye-catching markings or facial features, it can be difficult to identify them. This is why some experts wonder if bull sharks might have been responsible for certain attacks that were blamed on great whites, Ganges river sharks (Glyphis gangeticus), or other species.
10. A BULL SHARK MAY HAVE PARTICIPATED IN THE FAMOUS NEW JERSEY SHARK ATTACKS OF 1916.
On July 1, 1916, a 25-year-old swimmer was bitten by a shark off the coast of New Jersey's Long Beach Island. Though a lifeguard managed to rescue him, the man died of blood loss. Five days later, a bell captain lost both legs in a shark attack near Spring Lake. He too died. Farther north, a boy and an adult man were killed by a shark in Matawan Creek on July 12. Within 30 minutes, a teenager was bitten less than a mile downstream, but he survived.
All five attacks have historically been pinned on a single 8-foot-long great white that was captured and killed on July 14 in Raritan Bay, just a few miles away from the Matawan attacks. Inside its stomach were human remains. No subsequent shark attacks were reported in New Jersey that summer.
But the situation might not be so clear-cut: The two Matawan Creek incidents occurred 11 miles from the ocean, and great whites don't travel to shallow inland waters. Bulls are also more abundant off the Jersey shore than great white sharks are. That leads some experts, including explorer Fabien Cousteau, to conclude that a bull shark was the real culprit. Shark attack expert George Burgess still thinks the evidence points to the great white with a bellyful of body parts. It’s also possible that more than one shark committed the 1916 attacks.
11. NO, BULL SHARKS DON’T HAVE THE ANIMAL KINGDOM’S HIGHEST TESTOSTERONE LEVELS.
According to the internet, some books, and Grand Theft Auto, bull sharks are extra-aggressive because they have more testosterone than any other animal. In reality, very little research has been done on the subject of bull shark hormones. One study compared the testosterone levels in three bull sharks: The female had a low testosterone level (0.1 nanograms per milliliter), one of the males had only 2.7 nanograms per milliliter, and the other male had a ludicrously high testosterone level of 358 nanograms per milliliter—so the results were inconclusive. Bonnethead sharks and rainbow trout both have extreme testosterone levels, so in that regard, bull sharks aren’t special. At least they can still brag about their jaws.