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9 Surprising Facts About Sharks

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Far from mindless killing machines, as they’re so often portrayed, these members of the class Chondrichthyes, or cartilaginous fish, are fascinating age-old survivors with a critical role in the ocean ecosystem.

1. The Shark Immune System Could Help Humans.

Sharks are among the oldest animals with a modern immune system, one similar to ours but with a sophisticated twist that may prove beneficial to humans. Shark blood contains large quantities of urea, which protects them from dehydrating in their salt water habitat. Urea can also destabilize sensitive protein molecules such as antibodies, though, and similar levels would destroy those in humans. Sharks have an additional salt bridge between structurally important amino acid chains and a particularly large non-polar nucleus of the Immunoglobulin fold in their antibodies—a complicated way of saying they have special adaptations to handle all that urea. Researchers now have integrated these adaptations into human antibodies, resulting in increased stability that could lead to improved therapy and diagnosis for human diseases.

2. Great Whites Live Much Longer than We Previously Thought.

Estimating the age of a great white shark presents a challenge—and no, getting close enough to one to ask isn’t the problem. Scientists determine the age of bony fish by analyzing mineralized tissues—ear bones, vertebrae, and fin rays—that have annual rings much the same as trees. Sharks have skeletons made of cartilage, not bone, except for their vertebrae. And while vertebrae do contain layers of tissue laid down sequentially over time, the bands can be less distinct and don't necessarily equate to annual growth. Using this method had previously yielded top ages for great whites of 23 years. When researchers used radiocarbon to analyze collagen in the vertebrae, they estimated the largest male was 73 years old, making great whites among the longest-lived cartilaginous fishes. Guess they need that unlimited supply of teeth.

3. Some sharks return to their birthplace to reproduce.

Sea turtles are famous for returning to the beaches where they hatched to lay their own eggs, many miles and decades later. Scientists call that natal philopatry and long-term fidelity to parturition sites. Turns out, some sharks have that, too.

A 19-year study that began in 1995 and involved the capture, tagging, and release of more than 2000 baby sharks revealed that female lemon sharks returned to where they were born to give birth, up to 15 years later. The discovery means that preserving local nursery habitats could protect future generations of sharks.

4. Oils stored in the liver fuel a great white’s long migration.

Great white sharks make non-stop trips of more than 2500 miles across the Pacific Ocean, crossing large swaths of open water with little if anything for them to eat. A study reveals that fuel for the journey comes from fat stored in the sharks’ livers, which account for up to a quarter of their body weight. It’s an approach similar to how hibernating bears bulk up and migrating whales pack on the blubber. Oils stored in the liver also provide the sharks with increased buoyancy.

Scientists used data records from white sharks in the eastern Pacific, equipped with electronic tags that record location, depth, and water temperature, to identify periods of drift diving—when marine animals descend passively and allow momentum to carry them forward like underwater hang gliders. The researchers estimated the amount of oil in an animal’s liver by measuring the rate at which it sank during drift dives; less oil to provide buoyancy meant a quicker descent while more oil equaled a slower one. Consistent decreases in buoyancy during migration indicated gradual but steady depletion of oil in the liver, meaning the sharks depended on that stored energy for their journey.

5. Shark embryos can detect danger.

Shark embryos inside an external egg case can detect the presence of predators and freeze, Bambi-style, to avoid being detected themselves. Adult sharks detect electric fields emitted by potential prey, and their embryos employ similar receptors to detect potential predators. When researchers created electric fields mimicking a predator, brown-banded bamboo shark embryos grew more still by reducing respiratory gill movements. Knowing about this behavior could help humans develop more effective ways of repelling grown-up sharks.

6. Humans and sharks share a common ancestor and similar genes.

Jawed vertebrates on earth—including sharks and humans—have a common ancestor, most likely Acanthodes bronni. Using more than 100 body characteristics, researchers compared resemblance among the earliest jawed fishes and found that Acanthodians as a whole clustered with ancient sharks. Perhaps it won’t surprise you that our relationship didn’t last long; the descendants of this shark-like fish from the Paleozoic era split more than 420 million years ago into early sharks and the first bony fishes, with humans eventually evolving from the latter. We remain connected, though, as recent large-scale analysis of the genes of great white sharks revealed that the proportion of its genes associated with metabolism and its heart RNA molecules were more similar to those of humans than of zebrafish, part of the bony fish line.

7. The shark family is large and diverse.

There are nearly 500 species of sharks. This large extended family includes the 6 inch dwarf lantern shark and the 40 foot whale shark, the round and flattened angel shark and the gape-mouthed basking shark. There are families of gulper sharks, lantern sharks, sleeper sharks and dogfish sharks; angel, bullhead, and carpet sharks. There are even zebra, crocodile, mackerel, hound, weasel, and cat sharks (something of a theme there). The family includes rays and skates as well. And as-yet undiscovered species likely lurk in the abyss, where as recently as 1976 we discovered Megamouth, a deep-water shark reaching lengths of 16 feet with a short snout and large mouth sporting 50 rows of teeth. And you thought your relatives were strange.

8. Shark skin reduces drag—and provides thrust.

Sharks are legendary for their efficiency at moving through the water, thanks to a streamlined body and tiny denticles, or tooth-like scales, on their skin that reduce drag. Sharkskin has inspired the design of suits worn by human swimmers and other marvels of engineering. It turns out that as a shark’s body flexes when it swims, the denticles alter the structure of water flow—technically they ‘promote enhanced leading-edge suction’—which may actually provide thrust in addition to reducing drag. Advantage: shark.

9. Sharks and human hunter-gatherers share the same foraging pattern.

Sharks, bees, and many other animals follow a pattern known as the Levy walk when they forage. This pattern of movement is similar to the mathematical ratio phi, which has been found to describe proportions in plants and animals throughout nature. A recent study confirmed that hunter-gatherer tribes of humans also follow the pattern, showing yet again that we have more in common with sharks than you might think.

BONUS RAY FACT: Devil Rays dive deeper than a mile.

Devil rays grow up to 13 feet across, travel across large areas of the ocean, and are often spotted in warm, shallow waters. But scientists recently discovered that these rays dive deeper than a mile. They tracked 15 animals in the central North Atlantic using pop-up satellite archival transmitting tags, which stay on the animals for up to 9 months, recording water temperature, depth, and light levels. The tags eventually pop off, float to the surface, and beam their data via satellite to waiting computers on shore. These 15 tags revealed that their bearers routinely descended almost 1.24 miles deep, traveling at speeds up to 13.4 miles per hour, and remaining there for two or three hours. That’s a hell of a dive.

All images courtesy of iStock.

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Big Questions
Why Can't Dogs Eat Chocolate?
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Even if you don’t have a dog, you probably know that they can’t eat chocolate; it’s one of the most well-known toxic substances for canines (and felines, for that matter). But just what is it about chocolate that is so toxic to dogs? Why can't dogs eat chocolate when we eat it all the time without incident?

It comes down to theobromine, a chemical in chocolate that humans can metabolize easily, but dogs cannot. “They just can’t break it down as fast as humans and so therefore, when they consume it, it can cause illness,” Mike Topper, president of the American Veterinary Medical Association, tells Mental Floss.

The toxic effects of this slow metabolization can range from a mild upset stomach to seizures, heart failure, and even death. If your dog does eat chocolate, they may get thirsty, have diarrhea, and become hyperactive and shaky. If things get really bad, that hyperactivity could turn into seizures, and they could develop an arrhythmia and have a heart attack.

While cats are even more sensitive to theobromine, they’re less likely to eat chocolate in the first place. They’re much more picky eaters, and some research has found that they can’t taste sweetness. Dogs, on the other hand, are much more likely to sit at your feet with those big, mournful eyes begging for a taste of whatever you're eating, including chocolate. (They've also been known to just swipe it off the counter when you’re not looking.)

If your dog gets a hold of your favorite candy bar, it’s best to get them to the vet within two hours. The theobromine is metabolized slowly, “therefore, if we can get it out of the stomach there will be less there to metabolize,” Topper says. Your vet might be able to induce vomiting and give your dog activated charcoal to block the absorption of the theobromine. Intravenous fluids can also help flush it out of your dog’s system before it becomes lethal.

The toxicity varies based on what kind of chocolate it is (milk chocolate has a lower dose of theobromine than dark chocolate, and baking chocolate has an especially concentrated dose), the size of your dog, and whether or not the dog has preexisting health problems, like kidney or heart issues. While any dog is going to get sick, a small, old, or unhealthy dog won't be able to handle the toxic effects as well as a large, young, healthy dog could. “A Great Dane who eats two Hershey’s kisses may not have the same [reaction] that a miniature Chihuahua that eats four Hershey’s kisses has,” Topper explains. The former might only get diarrhea, while the latter probably needs veterinary attention.

Even if you have a big dog, you shouldn’t just play it by ear, though. PetMD has a handy calculator to see just what risk levels your dog faces if he or she eats chocolate, based on the dog’s size and the amount eaten. But if your dog has already ingested chocolate, petMD shouldn’t be your go-to source. Call your vet's office, where they are already familiar with your dog’s size, age, and condition. They can give you the best advice on how toxic the dose might be and how urgent the situation is.

So if your dog eats chocolate, you’re better off paying a few hundred dollars at the vet to make your dog puke than waiting until it’s too late.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Animals
Elusive Butterfly Sighted in Scotland for the First Time in 133 Years

Conditions weren’t looking too promising for the white-letter hairstreak, an elusive butterfly that’s native to the UK. Threatened by habitat loss, the butterfly's numbers have dwindled by 96 percent since the 1970s, and the insect hasn’t even been spotted in Scotland since 1884. So you can imagine the surprise lepidopterists felt when a white-letter hairstreak was seen feeding in a field in Berwickshire, Scotland earlier in August, according to The Guardian.

A man named Iain Cowe noticed the butterfly and managed to capture it on camera. “It is not every day that something as special as this is found when out and about on a regular butterfly foray,” Cowe said in a statement provided by the UK's Butterfly Conservation. “It was a very ragged and worn individual found feeding on ragwort in the grassy edge of an arable field.”

The white-letter hairstreak is a small brown butterfly with a white “W”-shaped streak on the underside of its wings and a small orange spot on its hindwings. It’s not easily sighted, as it tends to spend most of its life feeding and breeding in treetops.

The butterfly’s preferred habitat is the elm tree, but an outbreak of Dutch elm disease—first noted the 1970s—forced the white-letter hairstreak to find new homes and food sources as millions of Britain's elm trees died. The threatened species has slowly spread north, and experts are now hopeful that Scotland could be a good home for the insect. (Dutch elm disease does exist in Scotland, but the nation also has a good amount of disease-resistant Wych elms.)

If a breeding colony is confirmed, the white-letter hairstreak will bump Scotland’s number of butterfly species that live and breed in the country up to 34. “We don’t have many butterfly species in Scotland so one more is very nice to have,” Paul Kirkland, director of Butterfly Conservation Scotland, said in a statement.

Prior to 1884, the only confirmed sighting of a white-letter hairstreak in Scotland was in 1859. However, the insect’s newfound presence in Scotland comes at a cost: The UK’s butterflies are moving north due to climate change, and the white-letter hairstreak’s arrival is “almost certainly due to the warming climate,” Kirkland said.

[h/t The Guardian]

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