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.

A bonnethead shark swimming.
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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.

A school of scalloped hammerheads from below.
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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.

An illustration of the winghead shark and its head from the 1876 book 'The Fishes of India.'
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.

5 Dog Breeds That Get Along With Cats

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iStock/chendongshan

Dogs and cats may be seen as common enemies, but it does not have to be that way. If you're a strategic pet parent, you can add a cat into your dog family, or vice versa, seamlessly. here are plenty of dogs out there that would make a wonderful—and friendly—companion for your cat. Here are five of the best breeds for that.

1. JAPANESE CHINS

Japanese Chin against a burgundy background
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​Japanese Chins are the perfect dogs for your pet. The breed has even been described as a ​"cat in a dog suit" because of their acrobatic abilities and cat-like agility. ​Animal Planet describes these dogs as even-tempered and adaptable to new members in the family. They're also playful, mischievous, smart, determined, and affectionate.

2. GOLDEN RETRIEVERS

Friendly Golden Retriever looks at the camera
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These ​furry friends are as playful as it gets, but more importantly they're a versatile breed, so they know how to adapt to different friends they meet, including cats of all sizes and ages. Golden retrievers are very unlikely to be rough with your cat, even when playing, and will just consider the cat a part of its family. There's no jealousy on the part o these dogs.

3. PAPILLONS

Papillon and a cat snuggle under a blanket
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​Papillons are the dog versions of your friend who invites you to every social gathering. They have a ​"more the merrier" attitude, meaning they welcome any and all friends. These social butterflies are quick to befriend any and all creatures, cats included.

4. LABRADOR RETRIEVERS

Black Labrador Retriever and a cat share a couch
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A Labrador retriever grows to be pretty huge, there's no denying that, but that does not spell disaster for a cat-dog relationship. Labrador retrievers are very outgoing, but at the same time very gentle, and get along well with cats.

5. BEAGLES

A Beagle and a cat cuddle together
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Beagles are an easy choice to add to the family. According to the American Kennel Club, beagles are very friendly around all animals, and have an easygoing demeanor. Despite the fact you may see a beagle chase a cat outside from time to time, indoors, they won't keep that same energy. Inside, they play well with others, and will live peacefully—and possibly even snuggle—with cats.

12 Fun Facts About Pugs

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iStock/ClarkandCompany

These wrinkly-faced dogs have an adorably rich history. In honor of National Pug Day, read up on essential facts for any pug fanatic.    

1. THEY'RE AN ANCIENT BREED.


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Because the pug lineage stretches so far back, their early history is a little murky. Most believe that the breed originated in China and existed before 400 BCE and were called (or at least closely related to a breed called) “lo-sze.” Buddhist monks kept the dogs as pets in Tibetan monasteries.

2. THEY WERE TREATED LIKE ROYALTY.

Emperors of China kept pugs as lapdogs and treated them to all the luxuries of royal life. Sometimes the pampered pooches were given their own mini palaces and guards.  

3. A GROUP OF PUGS IS CALLED A GRUMBLE


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In Holland, the pug is called a mopshond, which comes from the Dutch for “to grumble.” 

4. THE BREED PROBABLY GETS ITS NAME FROM A MONKEY.

Marmosets were kept as pets in the early 18th century and were called pugs. The name made the jump to the dog because the two animals shared similar facial features. 

5. THE PUG IS THE OFFICIAL BREED OF THE HOUSE OF ORANGE.


iStock/Irishka1

In 1572, the Dutch were in the midst of the Eighty Years' War, a protracted struggle against the Spanish. The Prince of Orange, William the Silent, led the Dutch forces into battle. According to Dutch legend, while the prince was sleeping in his tent one night, Spanish assassins lurked just outside. Luckily, William’s pug, Pompey, was there to bark wildly and jump on his owner’s face. The Prince woke up and had his would-be assassins apprehended. Because of this, the pug was considered the official dog of the House of Orange. The effigy of Prince William I above his tomb also features Pompey at his feet (although weirdly, that dog doesn’t have a flat face, leading some to believe that it was a different breed).

Later, when Prince William III came to England to rule with his wife Mary II, he brought his pugs, who wore little orange ribbons to their master’s 1689 coronation. 

6. THE PERFECT PUG TAIL HAS TWO CURLS.

Pugs are known for their curly tails that curve up towards their bodies. According to the AKC, “the double curl is perfection.” 

7. THERE'S A PUG WITH AN MBA.

In 2009, Chester Ludlow the pug received an online graduate degree from Rochville University. He submitted his resume to the website and paid around $500 for entry. A week later, he received his grades, degree, and a school window decal in the mail. Although he never attended a class, he received a 3.19 and he got an A in Finance. Chester may have been the first pug to get his degree.

It’s too bad Rochville University isn't accredited. The whole thing was a stunt pulled by a website called GetEducated.com. The website reviews online colleges to protect students from being duped by diploma mill fraud. So while Chester the pug has a diploma, you won’t see him getting a job very soon (unless that job is acting in cute commercials). 

8. THERE WAS A SECRET ORGANIZATION NAMED AFTER THE DOG.

Around 1740, Roman Catholics formed a secret fraternal group called the Order of the Pug. The Pope forbad Catholics from joining the Freemasons, so this group formed as a replacement. They chose the pug as their symbol because the dogs were loyal and trustworthy. The Grand Master was a man, but each division of the group had two “Big Pugs” that were always one male and one female. 

To join, members were expected to prove their devotion by kissing the rear of the Grand Pug under his tail (luckily, the Grand Pug was porcelain). Other wacky habits included wearing dog collars, scratching at the lodge door for entry, and barking loudly. 

This outcome probably wasn’t the result anyone was expecting from the freemason ban, so this new, stranger group got banned in several regions, until ultimately fizzling out. Probably due to a lack of people willing to kiss a pug’s posterior.  

9. JOSEPHINE BONAPARTE'S PUG DIDN'T MESS AROUND.


iStock/Camrocker

Napoleon’s wife Josephine had a pet pug named Fortuné that she loved so much that she refused to let the dog sleep anywhere but in her bed. It’s rumored that when Napoleon entered the bed with his new wife for the first time, her pug bit him on the leg. 

10. THEY'VE GOT ROYAL CONNECTIONS IN THE UK, TOO.

Long before Queen Elizabeth II met her first corgi, Queen Victoria was the top British dog fancier, and she loved pugs. Victoria was such a dog lover that she also banned the practice of cropping ears, enabling pug owners to enjoy their pups’ velvety ears in all their glory.

11. THEIR SHORT NOSES CAUSE SOME TROUBLE.


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Pugs are brachycephalic, meaning their noses are pushed in more than other dogs. While cute, these smushed faces can lead to some breathing problems. Their facial structure makes it difficult to take long and deep breaths, which is why you might hear a pug snuffling while running around. The dogs are still very energetic, but they might not be the best swimmers and may have trouble on airplanes

12. PUGS ARE MADE TO BE COMPANIONS.

Pugs are excellent pets because of their adaptable personality. Whether you like to stay at home or enjoy the outdoors, the little dogs will be up for anything. Bred to be companions, their favorite place is right by your side.

An earlier version of this article ran in 2015.

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