12 Facts About Hammerhead Sharks

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Is there any fish in the world that casts a more distinctive shadow? Divers have little trouble recognizing a hammerhead when they see one. And yet not all hammerheads look alike. These fish are diverse, they’re weird, and someday they might change the way we fight skin cancer.

1. THERE ARE AT LEAST 10 KNOWN SPECIES ...

Experts have identified 10 living shark species in the hammerhead family (although it’s possible that even more exist). Nine belong to the genus Sphyrna (Greek for "hammer"), while the other—an oddball called the winghead shark—is the sole member of its own genus, Eusphyra. Keen-eyed observers can tell most of these guys apart by the slight differences in their skull shapes. Hammerheads also vary in terms of overall size: The smaller species max out at 3 to 5 feet in length, while the biggest is the great hammerhead (Sphyrna mokarran), which can be up to 18 feet long and weigh over 1000 pounds (with 10 to 13 feet and 500 pounds being closer to average).

2. ... AND SOME ARE ENDANGERED.

Three hammerhead species have a high risk of extinction: the great hammerhead, which is threatened by the shark fin trade and bycatch (unwanted fish captured as a byproduct of commercial fishing); the winghead (Eusphyra blochii), whose population is believed to have declined 50 percent in 42 years from overfishing and net entanglement; and the scalloped hammerhead (Sphyrna lewini), which, in 2014, became the first shark to ever receive protection from the U.S. Endangered Species Act.

3. IT LOOKS LIKE THEY EVOLVED SOMEWHAT RECENTLY.

In 2010, geneticists at the University of Colorado, Boulder compared DNA samples from eight hammerhead species in an attempt to map out the family’s evolutionary history. The molecular evidence suggested that the hammerheads started to diversify around 20 million years ago. The fossil record tells us sharks have existed for at least 420 million years—so if the University of Colorado team is correct, hammerheads are relative newcomers on the world stage. What did the earliest hammerheads look like? According to the researchers, these were probably large-bodied animals. They also argued that today’s modestly-sized bonnethead and winghead sharks independently evolved from big ancestors.

4. THEIR HEADS MAY GIVE THEM A HUNTING EDGE.

These sharks' broad, flat, hammer-shaped heads are called cephalofoils, and no other creature in the world has a head quite like it. Hammerheads, like all other sharks, have sensory organs that can detect the electric fields of prey in the water; some scientists hypothesize that the broad cephalofoils allow hammerheads to have more of these organs—therefore allowing them to better sense prey. A 2002 experiment seemingly lent credence to this notion. Although the researchers didn’t find a difference in sensitivity to the electric fields between a hammerhead and a cone-nosed sandbar shark, the hammerhead was able to search a larger area, which the researchers say would “increase the probability of prey-encounter.” The researchers also noted that the hammerhead was more maneuverable than the sandbar shark.

5. GREAT HAMMERHEADS LIKE TO SWIM SIDEWAYS.

A typical shark has eight fins on its body. Probably the most recognizable is the first dorsal fin; it typically acts like a sailboat keel, helping the shark stay balanced while it swims. Sharks also have a pair of pectoral fins, located on either side of the body just behind the head, which most species use to steer and generate lift. In the majority of sharks, the pectoral fins are longer than the first dorsal—but for great hammerheads, the opposite is true. And that has a big effect on how these animals move. A 2016 tagging study attached GoPro cameras to five great hammerheads that were living out in the wild. While being monitored, the sharks spent 90 percent of their swimming time tilted to one side—usually at an angle of 50 to 75 degrees. Why’d they do this? It’s thought that after a hammerhead rolls sideways, the creature’s first dorsal fin acts like one of the pectoral fins. This reduces drag while also increasing the animal’s “wingspan.” Both factors enable the shark to swim more efficiently.

6. ONE SPECIES EATS SEAGRASS.

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The bonnethead (Sphyrna tiburo) is a small hammerhead that frequents warm, shallow waters. It hunts crabs and shrimp—and sometimes it also ingests seagrass. One survey compared the gut contents of numerous wild bonnetheads and found that up to 62 percent of all the organic matter discovered in their stomachs was seagrass. And in a 2017 experiment, captive bonnetheads were fed a 90 percent seagrass diet. Rather than waste away, the sharks gained weight. A feces analysis showed that the sharks were digesting half of the grass they’d been eating; enzymes designed to break down plant matter are present in the bonnethead’s digestive tract. Experts aren’t sure if the sharks go out of their way to eat seagrass or just swallow it accidentally while hunting small animals. Either way, the bonnethead now qualifies as the only omnivorous shark known to science.

7. THE LARGER ONES USE THEIR HEADS TO PIN DOWN STINGRAYS.

If a stingray is found just above the ocean floor, a hungry great hammerhead will use its cephalofoil to pin the creature against the sand. Then, with a bite to the pectoral fin (or “wing”), the prey is immobilized. But the shark doesn't always escape unscathed: Great hammerheads are often found with stingray barbs on their faces.

8. THEY’VE GOT BETTER DEPTH PERCEPTION THAN OTHER SHARKS.

In 2009, biologist Michelle McComb and her team captured live bonnetheads, winghead sharks, and scalloped hammerheads to test their vision. They attached recording devices right below the sharks’ corneas and monitored the fishes' eye movements while sweeping light beams across their faces. The researchers found that the binocular overlap in the hammerheads' field of vision is up to three times higher than it is in lemon and blacknose sharks—both of which have cone-shaped snouts. That means hammerheads have superior depth perception when compared to other sharks.

Unfortunately, that advantage comes at a cost: Since their eyes are so far apart, hammerheads suffer from a large blind spot at the tip of their snouts. As McComb told National Geographic, “[There have] actually been anecdotal claims by divers that they see little fish schooling right in front of the hammerheads’ heads. It’s like the fish are swimming by and saying, Ha, ha, ha, you can’t see me!”

9. ONE BONNETHEAD HAD A VIRGIN BIRTH AT A NEBRASKA ZOO.

In 2001, a bonnethead was born inside one of the aquariums at Omaha’s Henry Doorly Zoo. The birth came as a complete surprise to the staff because all the bonnetheads in that tank were females—and none had seen a male of their species in three years. At first, it seemed likely that the mother must have been storing sperm; females of many animal species can keep semen alive for years before using it to fertilize eggs. But testing confirmed that Omaha's baby bonnethead had no paternal DNA; the mother had reproduced by fertilizing her own egg cells, a phenomenon known as parthenogenesis. It had never been documented in sharks before.

10. SOME HAMMERHEADS TRAVEL IN SCHOOLS.

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Though many sharks are solitary creatures, scalloped hammerheads—which can reach lengths of 10 to 12 feet and can weigh in at 300 pounds or more—form schools. The young sharks probably travel in schools for mutual protection, but nobody knows why full-grown scalloped hammerheads, which have few natural predators, congregate like this. The behavior may have something to do with their migration patterns or mating habits [PDF]. Some schools are exclusively made up of females while others contain sharks of both sexes and different ages. In adults-only groups, the fish tend to disperse at night before meeting back up during the day.

The scalloped hammerhead isn’t the only species which creates schools: The smooth hammerhead (Sphyrna zygaena) also travels in groups.

11. THE WINGHEAD SHARK HAS SOME CRAZY PROPORTIONS.

Biodiversity Heritage Library, Flickr // CC BY 2.0 (cropped from original)

Relative to its body size, the winghead shark has the widest head of any hammerhead—almost half as wide as its body is long. Wingheads live in the Indo-Pacific, where their oddly-shaped heads make them prone to getting tangled up in fishing nets.

12. SCALLOPED HAMMERHEADS CAN GET TANS.

It’s a myth that sharks don’t get cancer, but young scalloped hammerheads appear to develop cancer-free suntans. Researchers noticed that when young scalloped hammerheads are kept in shallow outdoor pools, their skin darkened, going from a light beige to a rich chocolate brown. To figure out what was happening, the scientists put opaque filters on their sharks’ pectoral fins. These partly blocked ultraviolet light, leaving the skin under the filter paler than the skin that had been exposed to the sun. “Our experiments demonstrated that the sharks were truly suntanning and that the response was, in fact, induced by the increase in solar radiation," the scientists said in a press release. "These sharks increased the melanin content in their skin by 14 percent over 21 days, and up to 28 percent over 215 days.” Despite all the tanning they’d done, there wasn’t a trace of skin cancer on any of the test sharks. If their secret is ever unlocked, it could revolutionize the way we treat melanoma in human beings.

Looking to Downsize? You Can Buy a 5-Room DIY Cabin on Amazon for Less Than $33,000

Five rooms of one's own.
Five rooms of one's own.
Allwood/Amazon

If you’ve already mastered DIY houses for birds and dogs, maybe it’s time you built one for yourself.

As Simplemost reports, there are a number of house kits that you can order on Amazon, and the Allwood Avalon Cabin Kit is one of the quaintest—and, at $32,990, most affordable—options. The 540-square-foot structure has enough space for a kitchen, a bathroom, a bedroom, and a sitting room—and there’s an additional 218-square-foot loft with the potential to be the coziest reading nook of all time.

You can opt for three larger rooms if you're willing to skip the kitchen and bathroom.Allwood/Amazon

The construction process might not be a great idea for someone who’s never picked up a hammer, but you don’t need an architectural degree to tackle it. Step-by-step instructions and all materials are included, so it’s a little like a high-level IKEA project. According to the Amazon listing, it takes two adults about a week to complete. Since the Nordic wood walls are reinforced with steel rods, the house can withstand winds up to 120 mph, and you can pay an extra $1000 to upgrade from double-glass windows and doors to triple-glass for added fortification.

Sadly, the cool ceiling lamp is not included.Allwood/Amazon

Though everything you need for the shell of the house comes in the kit, you will need to purchase whatever goes inside it: toilet, shower, sink, stove, insulation, and all other furnishings. You can also customize the blueprint to fit your own plans for the space; maybe, for example, you’re going to use the house as a small event venue, and you’d rather have two or three large, airy rooms and no kitchen or bedroom.

Intrigued? Find out more here.

[h/t Simplemost]

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The Reason Your Dog Stares at You

Dogs stare for a number of different reasons.
Dogs stare for a number of different reasons.
sankai/iStock via Getty Images

Sooner or later, every dog owner will find their pet expressing an innate curiosity over even the most mundane of actions. Watching television? The dog will observe you closely. Folding laundry? The dog will stare at you like you’re a Magic Eye poster.

You can tell the dog it’s rude, but they’ll continue doing it. So why do dogs stare at us?

It often has little to do with what we’re doing and is more about what we might do. Dogs are big on visual cues. They know a walk is preceded by you picking up their leash; dinnertime might involve going to the pantry; a car ride means grabbing the keys. If they get a treat by obeying a command, then they know you’re probably going to start pointing at them and want to make sure they don’t miss it. In keeping an eye on you, a dog is looking for hints that you’re going to do something they want.

Dogs may also use staring as a method to train their owner. Most people are more likely to slip a dog something off their dinner plate if the dog is looking up at them wistfully. If that behavior is rewarded, then the dog knows giving you a pleading look may result in some pork chops landing at their feet.

But not all dogs stare out of greed. For dogs, just like humans, making eye contact releases oxytocin, otherwise known as the “love hormone.” It’s a bonding experience for humans and their animal companions.

Of course, staring can have other connotations, particularly if it’s not a dog you know very well. An unblinking, focused stare with a rigid body posture can mean the dog is feeling territorial or might be considering taking a bite out of you. It’s best to back away. It’s also not advisable to hold a dog still and stare at them, as this might be considered an act of aggression.

The next time you catch your dog eyeing you, it’s likely they’re hoping for a walk, a treat, or just want to bond. Absent other methods of communication, staring is an effective way for getting their humans to behave.

[h/t American Kennel Club]