Great white sharks certainly don't suffer from a lack of self-esteem. After all, they're the No. 1 predator in the sea. Tiger and bull sharks aren't too shabby either -- they're stealth hunters that look great on television and scare the daylights out of beachcombers. White tips and threshers look pretty tough, too, even though they aren't as feared as their great white and bullish cousins. Hammerheads, on the other hand, might feel a little left out. They have little mouths with little teeth, and they aren't known to bother humans much. They don't exactly strike fear into the hearts of scuba divers or surfers. Then there's the head. The wide, flat, hammer-shaped head with bug eyes on either side -- they're just kind of funny-looking.
Despite how they may feel about themselves, hammerheads are one of the most intriguing species of shark. There are nine different classifications of hammerhead sharks, but only four are common and abundant: the great hammerhead, scalloped hammerhead, smooth hammerhead and bonnethead. Out of these four, all but the bonnethead are considered to be dangerous to humans, but you probably shouldn't worry much about them. There have only been 38 recorded hammerhead attacks on humans since 1580, only 16 of which were unprovoked, and no one has ever been killed by a hammerhead [source: ISAF]. They aren't generally aggressive toward people, so the unprovoked attacks were likely due to surprise or fear.
You can find hammerheads in temperate and tropical waters all over the world. Some, (like the great hammerhead), stay in deeper waters, while other species hang out closer to the shore. They like cooler water and will migrate in schools toward the poles in the warm summer months. They're fished commercially and recreationally for their hides and meat, and like most sharks, their fins are commonly cut off in a process called finning and used as an ingredient in shark fin soup in Asia. Shark finning is condemned as cruel throughout much of the world and is outlawed in U.S. waters.
We'll get into the unusual physical characteristics of the hammerhead shark in this article and explore the different theories about what their odd-shaped head is used for. We'll also learn about their mating habits, how they hunt, what they eat and which ones travel in schools.
The Life of a Hammerhead Shark
Hammerheads have quite a range in size depending on the species. Scalloped hammerheads are generally 5 to 10 feet long (1.5 to 3m) and weigh about 175 to 225 pounds (80 to 100 kg). Smooth hammerheads can grow a couple of feet longer, but the great hammerhead is by far the largest. These carnivores can grow up to 18 feet long (5 m) and weigh as much as 800 pounds (360 kg). The bonnethead is the smallest of the four major species of hammerhead, at an average of 3.5 feet long (1 m) and about 20 pounds (9 kg) [source: Florida Museum of Natural History].
Aside from the unusual shape of its head, hammerheads have another distinctive physical feature -- their tall dorsal fin, which protrudes high above the surface in shallow waters. Hammerheads are grayish-brown to olive on top and, like most sharks, are lighter colored on their bellies.
The hammerhead's mouth is smaller than most other large sharks and can't open as wide. Inside its small mouth are small teeth that are sharp and heavily serrated up front but larger and flatter in the back. These hard back teeth are used for grinding up tougher prey, like shellfish. Scalloped, smooth and great hammerheads feed on bony fish, small sharks, shellfish and their favorite dinnertime treat, stingrays. Bonnetheads eat bony fish, shrimp and even seagrass, but mainly feed on crustaceans like blue crabs.
Hammerheads hunt alone during the daytime like most other sharks, but scalloped hammerheads have a fascinating characteristic that sets them apart from most other species of sharks -- they hang out together in schools. We’re not exactly sure why the scalloped hammerhead schools, but we have a few clues. Most small fish school to provide numbers against predators, but since the scalloped hammerhead is a large shark, this probably isn’t the reason. Another reason fish school is to surround their prey, but the hammerhead is a solitary hunter, so you can toss this one out too. Most researchers seem to think that the scalloped hammerhead schools because it enjoys the company.
Male scalloped hammerheads are outnumbered 6-to-1 by females, making them fairly choosey when selecting a mate. Their preference? Big girls. The larger the female, the more shark pups she can carry and the more sought after she'll become. When schooling, the largest females are typically located in the center. The male then heads to the center, picks a female from the school and swims off to mate, right there in broad daylight. An older large female can have up to 40 pups, compared to a mere 12 for a younger and smaller shark. They carry the pups inside them for about eight to 10 months before giving birth in shallow waters. The pups are about 18 inches long at birth and have soft "hammers" that are bent back toward the tail to make it easier on mom. The hammer then firms up as the shark grows larger.
In the next section, we'll learn about some of the different theories on why the hammerhead has a hammer-shaped head.
Hammerhead Shark Unique Characteristics
The distinctive shaped hammer head of these ocean predators is called a cephalofoil, and it has some small variations among the different species of hammerheads. The great hammerhead has a cephalofoil that's broad and nearly flat across the front, with a single shallow notch in the center. The scalloped hammerhead is arched more and has a pronounced center notch with two matching notches on either side, giving it a scalloped appearance. The smooth hammerhead is, you guessed it, smooth -- no notches and just a slight, broad arch. The bonnethead's cephalofoil is distinct from the others. It's rounded at the front and resembles a shovel more than a hammer.
But why do these sharks have such on oddly shaped head? Researchers aren't 100 percent sure why they evolved this way, but they have a few different theories, some of which hold up well under research. The first is that it acts as a lift when swimming, much like an airplane wing. Research indicates that while hammerheads have better maneuvering capabilities than other sharks, it's not likely due to the cephalofoil. They're more flexible and can therefore turn and pivot more easily with greater speed.
Another theory is that the hammerhead uses its cephalofoil to aid in trapping prey. One of the hammerhead's favorite foods is the stingray. Once a ray is located, the hammerhead pins the ray to the ground with its cephalofoil and starts eating. This theory has been observed in the wild, but it's probably a learned technique and not the central reason for the wide cephalofoil.
The most likely explanation is that the cephalofoil increases the hammerhead's ability to sense prey. All sharks have electrical sensors in their nose and heads called ampullae of Lorenzini, named for researcher Stephan Lorenzini. These sensors can detect weak electric emissions from other sea life. Because hammerheads have broad, flat heads, the ampullae are spread out over a greater surface area, giving the shark the ability to cover more ground and sense its prey easier. This theory is bolstered by the hammerhead's tendency to troll the bottom of the floor and its ability to find camouflaged stingrays buried beneath the sand.
For more articles on sharks and hammers, please click on ahead to the links on the following page.
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More Great Links
- "Bonnethead Hammerhead." flmnh.ufl.edu, 2008. http://www.flmnh.ufl.edu/fish/Gallery/Descript/Bonnethead/Bonnethead.html
- "Diving with Scalloped Hammerhead Sharks." dive-the-world.com, 2008. http://www.dive-the-world.com/creatures-scalloped-hammerhead-sharks.htm
- "Electroreception in Hammerhead Sharks." hawaii.edu, 2008. http://www.hawaii.edu/HIMB/sharklab/research/kajiura.html
- "From the Shore to the Deep Blue Sea." sdnhm.org, 2008. http://www.sdnhm.org/kids/sharks/shore-to-sea/hammerhead.html
- "Great Hammerhead." flmnh.ufl.edu, 2008. http://www.flmnh.ufl.edu/fish/Gallery/Descript/GreatHammerhead/GHammerhead.html
- "Hammerhead Shark." nationalgeographic.com, 2008. http://animals.nationalgeographic.com/animals/fish/hammerhead-shark.html
- "Hammerhead Shark." sharkinformation.org, 2008. http://www.sharkinformation.org/hammerhead-shark/
- "Hammerhead Sharks." divingwithsharks.com, 2008. http://www.divingwithsharks.com/shark-info/facts-hammers.html
- "ISAF Statistics on Attacking Species of Shark." flmnh.ufl.edu, 2008. http://www.flmnh.ufl.edu/fish/sharks/statistics/species2.htm
- "Scalloped Hammerhead." flmnh.ufl.edu, 2008. http://www.flmnh.ufl.edu/fish/Gallery/Descript/Schammer/ScallopedHammerhead.html
- "Smooth Hammerhead." flmnh.ufl.edu, 2008. http://www.flmnh.ufl.edu/fish/Gallery/Descript/SmHammer/SmoothHammerhead.html
- Kruppa, Donna. "Why Do Hammerhead Sharks Have Hammer Heads?" cdnn.info, 2008. http://www.cdnn.info/news/article/a020831.html
- Lovgren, Stefan. "Hammerhead Shark Gave "Virgin Birth" in Omaha Zoo." nationalgeographic.com, May 24, 2007. http://news.nationalgeographic.com/news/2007/05/070524-shark-virgin.html
- Rogerson, Simon. "Hammerhead: the other great white shark." bbc.co.uk, 2008. http://www.bbc.co.uk/nature/animals/features/311feature1.shtml