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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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Scientific Reports, Fernando Ramirez Rozzi
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Stones, Bones, and Wrecks
Humans Might Have Practiced Brain Surgery on Cows 5000 Years Ago
Scientific Reports, Fernando Ramirez Rozzi
Scientific Reports, Fernando Ramirez Rozzi

In the 1970s, archaeologists discovered a site in France containing hundreds of cow skeletons dating back 5000 to 5400 years. The sheer number wasn't surprising—human agriculture in that part of the world was booming by 3000 BCE. What perplexed scientists was something uncovered there a few decades later: a cow skull bearing a thoughtfully drilled hole. Now, a team of researchers has released evidence that suggests the hole is an early example of animal brain surgery.

Fernando Ramírez Rozzi, a paleontologist with the French National Center for Scientific Research, and Alain Froment, an anthropologist at the Museum of Mankind in Paris, published their findings in the journal Nature Scientific Reports. After comparing the opening to the holes chiseled into the skulls of humans from the same era, they found the bones bore some striking similarities. They didn't show any signs of fracturing from blunt force trauma; rather, the hole in the cow skull, like those in the human skulls, seemed to have been carved out carefully using a tool made for exactly that purpose. That suggests that the hole is evidence of the earliest known veterinary surgery performed by humans.

Trepanation, or the practice of boring holes into human skulls, is one of the oldest forms of surgery. Experts are still unsure why ancient humans did this, but the level of care that went into the procedures suggests that the surgery was likely used to treat sick patients while they were still alive. Why a person would perform this same surgery on a cow, however, is harder to explain.

The authors present a few theories, the first being that these ancient brain surgeons were treating a sick cow the same way they might treat a sick human. If a cow was suffering from a neural disease like epilepsy, perhaps they though that cutting a hole in its head would relieve whatever was agitating the brain. The cow would have needed to be pretty special to warrant such an effort when there were hundreds of healthy cows living on the same plot of land, as evidenced by the skeletons it was found with.

Another possible explanation was that whoever operated on the cow did so as practice to prepare them for drilling into the heads of live humans one day. "Cranial surgery requires great manual dexterity and a complete knowledge of the anatomy of the brain and vessel distribution," the authors write in the study. "It is possible that the mastery of techniques in cranial surgery shown in the Mesolithic and Neolithic periods was acquired through experimentation on animals."

Either way, the bovine patient didn't live to see the results of the procedure: The bone around the hole hadn't healed at all, which suggests the cow either died during surgery or wasn't alive to begin with.

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Animals
15 Incredible Facts About Pigeons
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Though they're often described as "rats with wings" (a phrase popularized by the movie Stardust Memories), pigeons are actually pretty cool. From homing instincts to misleading rump feathers, here are 15 things you might not know about these avian adventurers.

1. THEY MIGHT BE THE FIRST DOMESTICATED BIRD.

The common city pigeon (Columba livia), also known as the rock pigeon, might be the first bird humankind ever domesticated. You can see them in art dating back as far as 4500 BCE in modern Iraq, and they've been a valuable source of food for thousands of years.

2. THEY WON OVER CHARLES DARWIN—AND NIKOLA TESLA.

Pigeon-breeding was a common hobby in Victorian England for everyone from well-off businessmen to average Joes, leading to some fantastically weird birds. Few hobbyists had more enthusiasm for the breeding process than Charles Darwin, who owned a diverse flock, joined London pigeon clubs, and hobnobbed with famous breeders. Darwin's passion for the birds influenced his 1868 book The Variation of Animals and Plants Under Domestication, which has not one but two chapters about pigeons (dogs and cats share a single chapter).

Nikola Tesla was another great mind who enjoyed pigeons. He used to care for injured wild pigeons in his New York City hotel room. Hands down, Tesla's favorite was a white female—about whom he once said, "I loved that pigeon, I loved her as a man loves a woman and she loved me. When she was ill, I knew and understood; she came to my room and I stayed beside her for days. I nursed her back to health. That pigeon was the joy of my life. If she needed me, nothing else mattered. As long as I had her, there was a purpose in my life." Reportedly, he was inconsolable after she died.

3. THEY UNDERSTAND SPACE AND TIME.

In a 2017 Current Biology study, researchers showed captive pigeons a series of digital lines on a computer screen for either two or eight seconds. Some lines were short, measuring about 2.3 inches across; others were four times longer. The pigeons were trained to evaluate either the length of the line or how long it was displayed. They found that the more time a line was displayed, the longer in length the pigeon judged it to be. The reverse was true too: If the pigeons encountered a longer line, they thought it existed in time for a greater duration. Pigeons, the scientists concluded, understand the concepts of both time and space; the researchers noted "similar results have been found with humans and other primates."

It's thought that humans process those concepts with a brain region called the parietal cortex; pigeon brains lack that cortex, so they must have a different way of judging space and time.

4. THEY CAN FIND THEIR WAY BACK TO THE NEST FROM 1300 MILES AWAY.

A pigeon flying in front of trees.
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The birds can do this even if they've been transported in isolation—with no visual, olfactory, or magnetic clues—while scientists rotate their cages so they don't know what direction they're traveling in. How they do this is a mystery, but people have been exploiting the pigeon's navigational skills since at least 3000 BCE, when ancient peoples would set caged pigeons free and follow them to nearby land.

Their navigational skills also make pigeons great long-distance messengers. Sports fans in ancient Greece are said to have used trained pigeons to carry the results of the Ancient Olympics. Further east, Genghis Khan stayed in touch with his allies and enemies alike through a pigeon-based postal network.

5. THEY SAVED THOUSANDS OF HUMAN LIVES DURING WORLD WARS I AND II.

Pigeons' homing talents continued to shape history during the 20th century. In both World Wars, rival nations had huge flocks of pigeon messengers. (America alone had 200,000 at its disposal in WWII.) By delivering critical updates, the avians saved thousands of human lives. One racing bird named Cher Ami completed a mission that led to the rescue of 194 stranded U.S. soldiers on October 4, 1918.

6. TWO PIGEONS ALMOST DISTRACTED FROM THE DISCOVERY OF EVIDENCE OF THE BIG BANG.

In 1964, scientists in Holmdel, New Jersey, heard hissing noises from their antenna that would later prove to be signals from the Big Bang. But when they first heard the sound, they thought it might be, among other things, the poop of two pigeons that were living in the antenna. "We took the pigeons, put them in a box, and mailed them as far away as we could in the company mail to a guy who fancied pigeons," one of the scientists later recalled. "He looked at them and said these are junk pigeons and let them go and before long they were right back." But the scientists were able to clean out the antenna and determine that they had not been the cause of the noise. The trap used to catch the birds (before they had to later be, uh, permanently removed) is on view at the Smithsonian Air & Space Museum.

7. YOU CAN TRAIN THEM TO BE ART SNOBS …

Japanese psychologist Shigeru Watanabe and two colleagues earned an Ig Nobel Prize in 1995 for training pigeons, in a lab setting, to recognize the paintings of Claude Monet and Pablo Picasso and to distinguish between the painters. The pigeons were even able to use their knowledge of impressionism and cubism to identify paintings of other artists in those movements. Later, Watanabe taught other pigeons to distinguish watercolor images from pastels. And in a 2009 experiment, captive pigeons he'd borrowed were shown almost two dozen paintings made by students at a Tokyo elementary school, and were taught which ones were considered "good" and which ones were considered "bad." He then presented them with 10 new paintings and the avian critics managed to correctly guess which ones had earned bad grades from the school's teacher and a panel of adults. Watanabe's findings indicate that wild pigeons naturally categorize things on the basis of color, texture, and general appearance.

8. … AND TO DISTINGUISH WRITTEN WORDS.

In a 2016 study, scientists showed that pigeons can differentiate between strings of letters and actual words. Four of the birds built up a vocabulary of between 26 and 58 written English words, and though the birds couldn't actually read them, they could identify visual patterns and therefore tell them apart. The birds could even identify words they hadn't seen before.

9. FLUFFY PIGEON FEET MIGHT ACTUALLY BE PARTIAL WINGS.

A white pigeon with curly feathers and fluffy feet.
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A few pigeon breeds have fuzzy legs—which hobbyists call "muffs"—rather than scaly ones. According to a 2016 study, the DNA of these fluffy-footed pigeons leads their hind legs to take on some forelimb characteristics, making muffed pigeon legs look distinctly wing-like; they're also big-boned. Not only do they have feathers, but the hindlimbs are somewhat big-boned, too. According to biologist Mike Shapiro, who led the study, "pigeons' fancy feathered feet are partially wings."

10. SOME PIGEONS DISTRACT FALCONS WITH WHITE RUMP FEATHERS.

In a life-or-death situation, a pigeon's survival could depend upon its color pattern: Research has shown that wild falcons rarely go after pigeons that have a white patch of feathers just above the tail, and when the predators do target these birds, the attacks are rarely successful.

To figure out why this is, Ph.D. student Alberto Palleroni and a team tagged 5235 pigeons in the vicinity of Davis, California. Then, they monitored 1485 falcon-on-pigeon attacks over a seven-year span. The researchers found that although white-rumped pigeons comprised 20 to 25 percent of the area's pigeon population, they represented less than 2 percent of all the observed pigeons that were killed by falcons; the vast majority of the victims had blue rumps. Palleroni and his team rounded up 756 white- and blue-rumped pigeons and swapped their rump feathers by clipping and pasting white feathers on blue rumps, and vice versa. The falcons had a much easier time spotting and catching the newly blue-rumped pigeons, while the pigeons that received the white feathers saw predation rates plummet.

Close observation revealed that the white patches distract birds of prey. In the wild, falcons dive-bomb other winged animals from above at high speeds. Some pigeons respond by rolling away in midair, and on a spiraling bird, white rump feathers can be eye-catching, which means that a patch of them may divert a hungry raptor's focus long enough to make the carnivore miscalculate and zip right past its intended victim.

11. DODOS WERE RELATED TO TODAY'S PIGEONS.

Two blue and green Nicobar pigeons.
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Though most of this list focuses on the rock pigeon, there are 308 living species of pigeons and doves. Together, they make up an order of birds known as the columbiformes. The extinct dodo belonged to this group as well.

Flightless and (somewhat) docile, dodos once inhabited Mauritius, an island near Madagascar. The species had no natural predators, but when human sailors arrived with rats, dogs, cats, and pigs, it began to die out, and before the 17th century came to a close, the dodo had vanished altogether. DNA testing has confirmed that pigeons are closely related to the dodo, and the vibrant Nicobar pigeon (above) is its nearest genetic relative. A multi-colored bird with iridescent feathers, this near-threatened creature is found on small islands in the South Pacific and off Asia. Unlike the dodo, it can fly.

12. AT ONE POINT, MORE THAN ONE-QUARTER OF ALL THE BIRDS LIVING IN THE U.S. MAY HAVE BEEN PASSENGER PIGEONS.

Wild/feral rock pigeons reside in all 50 states, which makes it easy to forget that they're invasive birds. Originally native to Eurasia and northern Africa, the species was (most likely) introduced to North America by French settlers in 1606. At the time, a different kind of columbiform—this one indigenous—was already thriving there: the passenger pigeon (Ectopistes migratorius). As many as 5 billion of them were living in America when England, Spain, and France first started colonizing, and they may have once represented anywhere from 25 to 40 percent of the total U.S. bird population. But by the early 20th century, they had become a rare sight, thanks to overhunting, habitat loss, and a possible genetic diversity issue. The last known passenger pigeon—a captive female named Martha—died on September 1, 1914.

13. THEY'RE REALLY GOOD AT MULTITASKING.

According to one study, they're more efficient multitaskers than people are. Scientists at Ruhr-Universitat Bochum put together a test group of 15 humans and 12 pigeons and trained all of them to complete two simple jobs (like pressing a keyboard once a light bulb came on). They were also put in situations wherein they'd need to stop working on one job and switch over to another. In some trials, the participants had to make the change immediately. During these test runs, humans and pigeons switched between jobs at the same speed.

But in other trials, the test subjects were allowed to complete one assignment and then had to wait 300 milliseconds before moving on to the next job. Interestingly, in these runs, the pigeons were quicker to get started on that second task after the period ended. In the avian brain, nerve cells are more densely packed, which might enable our feathered friends to process information faster than we can under the right circumstances.

14. PIGEONS PRODUCE FAKE "MILK."

Only mammals produce genuine milk, but pigeons and doves (along with some other species of birds) feed their young with something similar—a whitish liquid filled with nutrients, fats, antioxidants, and healthy proteins called "crop milk." Both male and female pigeons create the milk in the crop, a section of the esophagus designed to store food temporarily. As is the case with mammal milk, the creation of crop milk is regulated by the hormone prolactin. Newly-hatched pigeons drink crop milk until they're weaned off it after four weeks or so. (And if you've ever asked yourself, "Where are all the baby pigeons?" we have the answer for you right here.)

15. ONE STUDY SUGGESTS THAT, GIVEN THE RIGHT CONDITIONS, THEY'RE AS GOOD AT IDENTIFYING CANCER AS DOCTORS.

We've already established that pigeons are excellent at differentiating between artists and words, but a 2015 study revealed they can also distinguish between malignant and benign growths in the right conditions. Researchers at University of California Davis Medical Center put 16 pigeons in a room with magnified biopsies of potential breast cancers. If the pigeons correctly identified them as either benign or malignant, they got a treat, According to Scientific American.

"Once trained, the pigeons' average diagnostic accuracy reached an impressive 85 percent. But when a "flock sourcing" approach was taken, in which the most common answer among all subjects was used, group accuracy climbed to a staggering 99 percent, or what would be expected from a pathologist. The pigeons were also able to apply their knowledge to novel images, showing the findings weren't simply a result of rote memorization."

Mammograms proved to be more of a challenge, however; the birds could memorize signs of cancer in the images they were trained on but could not identify the signs in new images.

No matter how impressive their results, "I don't anticipate that pigeons, no matter how good they become at pathology or radiology, will be playing a role in actual patient care—certainly for the foreseeable future," study co-author Richard M. Levenson told Scientific American. "There are just too many regulatory barriers—at least in the West."

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