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Clever Cuttlefish Muffle Electrical Signals to Hide from Sharks

At first glance, the cuttlefish looks like a pretty easy meal. It’s squishy, stubby, and seemingly defenseless. But the cuttlefish doesn't give up so easily.

Like its cousins the octopus and the squid, the cuttlefish has no external shell, and has therefore been forced to get creative in its defenses. Biologists and material scientists alike are captivated by the cuttlefish’s talent for subterfuge. By compressing, stretching, or relaxing pigment and light-reflecting cells, the cuttlefish can change color and even create patterns on its skin. As if that’s not enough, they’ve become shape-shifters, too. Any cuttlefish with a half-second’s head start can vanish into its surroundings.

But all this razzle-dazzle will only thwart predators who hunt by sight. Those that rely on scent or touch won’t be fooled, nor will those hunters who locate prey via electrosensing. 

It’s more common than you’d think. Every living thing gives off at least some electricity. Some, like the electric eel, carry a significant charge. But most of us just trundle along, emitting a faint electrical aura as we go about our daily business.

Scientists’ list of animals that can sense other animals’ electrical fields is growing all the time, and a lot of those animals are aquatic. At the top of the list are sharks, with electrosensing skills 10,000 times more powerful than those of any other animal.  

And what do sharks like to eat? A lot of things, including—you guessed it—the cuttlefish. Once again, it looks like the cuttlefish doesn’t stand a chance—and once again, looks are deceiving. Researchers recently discovered that cuttlefish can actually muffle their electrical fields, rendering themselves nearly invisible.

This insight comes courtesy of Duke University biologist Christine Bedore, who has made it her business to study electrosensing in sea critters. Bedore found that the electric field given off by the common cuttlefish (Sepia officinalis) is quite weak, about 75,000 times weaker than a single AAA battery. But even a weak field is still recognizable to a hungry shark.

To find out how a cuttlefish responds to the sight of a shark, Bedore set an iPad screen against the wall of a cuttlefish tank. She then played the cuttlefish videos of what looked like the silhouettes of approaching crabs, sharks, and groupers (another cuttlefish predator).

The crab silhouette, which presented no threat, didn’t inspire any changes in the cuttlefish’s behavior. But each time the silhouette of a shark or grouper approached, the cuttlefish in the tank froze. Its breathing slowed, and it seemed to be covering parts of its body with its little arms.

Throughout the experiment, Bedore was tracking the cuttlefish’s electrical output. Watch for yourself:

The cuttlefish’s tactic had a huge effect on the strength of its electrical field. By covering its siphons with its arms, the cuttlefish masked its electrical output by as much as 89 percent.

For the second phase of the study, Bedore and her colleagues offered real sharks access to a simulated cuttlefish in two positions: relaxed and frozen. The “cuttlefish” at rest was not only visible, but it was irresistible; the sharks started biting the equipment. The muted electrical field of a frozen cuttlefish, on the other hand, attracted only half as many shark bites. 

Bedore and her colleagues published their findings in the December 2 issue of the Proceedings of the Royal Society B. 

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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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Animals
10 Notable Gestation Periods in the Animal Kingdom
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The gestation periods of the animal kingdom are varied and fascinating. Some clock in at just a few weeks, making any human green with envy, while others can last more than a year. Here are 10 notable gestation times for animals around the globe. The lesson? Be thankful that you’re not a pregnant elephant.

1. ELEPHANTS: 640-660 DAYS

Elephants are pregnant for a long time. Like really, really long. At an average of 95 weeks, the gestation period is more than double the length of a human pregnancy, so it shouldn't come as a shock that female elephants don't often have more than four offspring during their lifetimes. Who has the time?

2. HIPPOS: 8 MONTHS

A photo of a mother hippo and her baby in Uganda

Yes, it takes less time to make a hippopotamus than it takes to make a human.

3. GIRAFFE: 14-15 MONTHS

Baby giraffes can weigh more than 150 pounds and can be around 6 feet tall. Another fascinating tidbit: giraffes give birth standing up, so it's pretty normal for a baby to fall 6 feet to the ground.

4. KILLER WHALE: 17 MONTHS

There’s a reason for the long wait: after that 17 months, Baby Shamu emerges weighing anywhere from 265 to 353 pounds and measuring about 8.5 feet long. Yikes.

5. OPOSSUM: 12-13 DAYS

A baby opossum wrapped up in a blanket

Blink and you'll miss it: This is the shortest gestation period of any mammal in North America. But since the lifespan of an opossum is only two to four years, it makes sense.

6. GERBILS: 25 DAYS

Hey, they get off pretty easy.

7. GORILLAS: 8.5 MONTHS

It's not a huge surprise that their gestational periods are pretty similar to ours, right?

8. BLACK BEAR: 220 DAYS

A pair of black bear cubs

Also less than a human. Interestingly, cubs might only be 6 to 8 inches in length at birth and are completely hairless. 

9. PORCUPINE: 112 DAYS

This is the longest gestation period of any rodent. Thankfully for the mother, porcupine babies (a.k.a. porcupettes) are actually born with soft quills, and it's not until after birth that they harden up.

10. WALRUS: 15 MONTHS

Baby walruses? Kind of adorable. They certainly take their sweet time coming out, though.

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