How Bull Sharks Work

Bull sharks got their name for a reason. For one, their body is shorter and more stout than that of their famous cousin, the great white. They also have a blunt, rounded nose that's wider than it is long, giving them a bullish appearance. The name might also come from their aggressive demeanor.

You can tell bulls apart from other species of shark by their lack of an interdorsal ridge. You know the large fin on a shark's back that sticks out of the water and scares the daylights out of ocean swimmers? That's called the dorsal fin. The bull shark has a second dorsal fin farther back and much smaller than the first. The area between the two dorsal fins can either be flat, or have a small, raised ridge of skin -- this is the interdorsal ridge. The fins of a young bull shark typically have dark tips, but they grow out of this awkward stage in adulthood.

Bulls are dark to light gray with a white underbelly and have smaller eyes than many of their shark cousins. Their small eyes indicate that they have limited vision, which could account for the fact that they like to swim in murky waters near the shoreline where eyesight isn't as important and prey is abundant. Their jaw is stuffed with hundreds of wide, triangular teeth that are about 1.5 inches long, heavily serrated and perfect for tearing apart the flesh of their prey.

Aside from being wider than great whites and having a rounder nose, bulls are also shorter. A large great white can grow up to 20 feet (6 m) long, but a full-grown adult bull shark is usually no more than 11 feet (3.3 m) long, topping the scales at about 500 pounds (226 kg). This is the largest they'll ever get, though. A typical female, which grows larger than the male, is about 7.8 feet (2.3 m) and weighs around 285 pounds (129 kg). The male only grows to about 7.3 feet (2.2 m) and weighs a paltry 210 pounds (95 kg) on average. Why are the females larger? Simply because they live about four years longer and continue to grow throughout their lifespan.

The reproductive habits of bull sharks aren't as different from humans as you might think. Females grow their young in the womb like us, instead of depositing massive amounts of eggs outside of their body like many fish do. However, they're pregnant for about a month or two longer than humans, and can carry anywhere between one and 13 baby sharks, or pups, at a time. Females can't give birth until the ripe old age of 10 years old, which would be about 50 years old in human terms based on an 80-year lifespan. Mating generally happens in the summer, and their free-swimming pups are born the following April to June. The pups are only a couple of feet long at birth, and their chances of surviving the dangerous ocean waters are slim. For this reason, the expectant mother often travels to the safer freshwaters to give birth and nurse her pups.

But how can these sharks survive in freshwater? Swim on over to the next page to find out.

Picture the scene: It's a breezy, sunny day and you're wading in the cool, shallow waters of the Mississippi River. You're somewhere inland -- way inland. Like Missouri. You look upstream and a 7-foot bull shark swims your way with, what looks like to you, a smile on his face. While not highly likely, it can, and has happened. Bull sharks are one of only two species of shark that can survive in freshwater.

Bulls can actually survive in just about any kind of water, from freshwater rivers and lakes to the saltiest of water bodies like St. Lucia Estuary in South Africa. After Hurricane Katrina swept through New Orleans, La., large numbers of bulls were seen swimming inland in Lake Pontchartrain. In 1955, there was a report of a bull-shark sighting all the way up the Mississippi River near Chicago, Ill., in Lake Michigan.

Bulls make their way into freshwaters to keep their newborn pups safe from harm. They don't mate or actually give birth in freshwater -- they do this in nearby tidal creeks that have a mix of salinity. The freshwater lakes and rivers provide safety for their pups, which would be hunted by larger predators in the ocean. Freshwater also provides a nice food buffet for mom, dad and the kids. The bull shark is king during its freshwater exploits, so pretty much everything is on the menu.

Bulls have a physiological feature that separates them from other sharks and allows them to process freshwater and saltwater by manipulating the amount of sodium and urea that their bodies need. Urea is the waste produced by the body when proteins are metabolized. When bull sharks reduce the amount of salt and urea in their bodies, they are able to adapt to various levels of salinity in water bodies.

How do they accomplish this? By peeing a lot -- roughly 20 times as much as when they're in the ocean. When bulls swim into freshwaters, they need to achieve a balance of saltwater to freshwater that still allows them to function normally. They're able to reduce the concentration of salt and urea in their blood by increasing their urine production, which is expelled in a much more diluted concentration than it normally would be. However, this overworks the kidneys of the bull shark, forcing it to travel back and forth between the ocean and inland waters.

The most notable place where this happens is in Lake Nicaragua, the largest lake in Central America. Lake Nicaragua connects to the Caribbean Sea by way of the San Juan River. The bull shark makes its way up the San Juan, sometimes leaping from the water like salmon, through eight different sets of rapids to find a temporary home in the lake. In the late 1960s, researchers from the University of Nebraska tagged more than 2,000 bull sharks in the lake to study their traveling habits. What they found was pretty remarkable. Roughly 10 percent of the tagged sharks were spotted in the ocean and then back in the lake. This means that the bulls routinely make the 118-mile (189 km) journey up the San Juan -- a trip they complete in a speedy seven to 11 days.

In the next section, we'll tell you about the bull shark's feeding habits.

If we're going to talk about the bull shark's diet, we need to break it up into the freshwater menu and the saltwater menu. When ocean-bound, bulls eat bony fish, stingrays, dolphins, sea turtles, other sharks, mollusks, crustaceans and all kinds of other schooling fish. They hunt alone, instead of in groups, and although they appear slow as they troll the ocean floor, they're known for their agility and quick bursts of speed -- up to 11 miles per hour. This may not sound too quick, but when you see a 10-foot shark coming your way at that speed, you may rethink your definition of fast. They're also not afraid to pick on someone their own size or even larger, something most other sharks won't do.

When the bulls hit freshwater, their diet gets a little more interesting. They feast on tarpon fish, crabs, shrimp, squid, sea urchins, turtles and sawfishes. Not interesting? How about birds, sloths, dogs, rats, cows and antelope? There have been reports in Australia of river-dwelling bulls eating horses, and in Mozambique, they feed on hippos. It's not exactly what you'd expect to find on a shark's menu, but bull sharks are opportunists -- they even eat their own kind if necessary.

The bull's hunting technique is known as the "bump and bite." This means that they will typically head-butt their prey before sinking their teeth into it. The bump is an investigative technique used to help the nearly blind shark figure out what it's about to bite. If you're scuba diving and get bumped by a bull shark, you're in trouble. Another technique the bull uses to make up for its poor vision is to hunt in murky water. Like other sharks, they're able to find their prey with their keen sense of smell. In shadowy waters, bulls can sniff out prey that can't see its approaching predator.

For more information on shark and other sea life, please see the links on the following page.