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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.

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Whale Sharks Can Live for More Than a Century, Study Finds
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Some whale sharks alive today have been swimming around since the Gilded Age. The animals—the largest fish in the ocean—can live as long as 130 years, according to a new study in the journal Marine and Freshwater Research. To give you an idea of how long that is, in 1888, Grover Cleveland was finishing up his first presidential term, Thomas Edison had just started selling his first light bulbs, and the U.S. only had 38 states.

To determine whale sharks' longevity, researchers from the Nova Southeastern University in Florida and the Maldives Whale Shark Research Program tracked male sharks around South Ari Atoll in the Maldives over the course of 10 years, calculating their sizes as they came back to the area over and over again. The scientists identified sharks that returned to the atoll every few years by their distinctive spot patterns, estimating their body lengths with lasers, tape, and visually to try to get the most accurate idea of their sizes.

Using these measurements and data on whale shark growth patterns, the researchers were able to determine that male whale sharks tend to reach maturity around 25 years old and live until they’re about 130 years old. During those decades, they reach an average length of 61.7 feet—about as long as a bowling lane.

While whale sharks are known as gentle giants, they’re difficult to study, and scientists still don’t know a ton about them. They’re considered endangered, making any information we can gather about them important. And this is the first time scientists have been able to accurately measure live, swimming whale sharks.

“Up to now, such aging and growth research has required obtaining vertebrae from dead whale sharks and counting growth rings, analogous to counting tree rings, to determine age,” first author Cameron Perry said in a press statement. ”Our work shows that we can obtain age and growth information without relying on dead sharks captured in fisheries. That is a big deal.”

Though whale sharks appear to be quite long-lived, their lifespan is short compared to the Greenland shark's—in 2016, researchers reported they may live for 400 years. 

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Animal Welfare Groups Are Building a Database of Every Cat in Washington, D.C.
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There are a lot of cats in Washington, D.C. They live in parks, backyards, side streets, and people's homes. Exactly how many there are is the question a new conservation project wants to answer. DC Cat Count, a collaboration between Humane Rescue Alliance, the Humane Society, PetSmart Charities, and the Smithsonian Conservation Biology Institute, aims to tally every cat in the city—even house pets, The New York Times reports.

Cities tend to support thriving feral cat populations, and that's a problem for animal conservationists. If a feline is born and grows up without human contact, it will never be a suitable house cat. The only options animal control officials have are to euthanize strays or trap and sterilize them, and release them back where they were found. If neither action is taken, it's the smaller animals that belong in the wild who suffer. Cats are invasive predators, and each year they kill billions of birds in the U.S. alone.

Before animal welfare experts and wildlife scientists can tackle this problem, they need to understand how big it is. Over the next three years, DC Cat Count will use various methods to track D.C.'s cats and build a feline database for the city. Sixty outdoor camera traps will capture images of passing cats, relying on infrared technology to sense them most of the time.

Citizens are being asked to help as well. An app is currently being developed that will allow users to snap photos of any cats they see, including their own pets. The team also plans to study the different ways these cats interact with their environments, like how much time pets spend indoors versus outdoors, for example. The initiative has a $1.5 million budget to spend on collecting data.

By the end of the project, the team hopes to have the tools both conservationists and animal welfare groups need to better control the local cat population.

Lisa LaFontaine, president and CEO of the Humane Rescue Alliance, said in a statement, “The reality is that those in the fields of welfare, ecology, conservation, and sheltering have a common long-term goal of fewer free-roaming cats on the landscape. This joint effort will provide scientific management programs to help achieve that goal, locally and nationally."

[h/t The New York Times]

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