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.