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Bats Listen for the Sounds of Fly Sex

“There are certain rules that one must abide by in order to successfully survive a horror movie,” Randy told us in Scream. “For instance, Number One: You can never have sex. Sex equals death, OK?”

He’s right. For some reason, sex often draws out slasher movie villains, and when supporting characters hook up, they often also get chopped up. The connection between sex and death isn’t just a horror movie trope, though. Biologists have often suggested that animals are at a higher risk for getting killed and eaten during mating, either because they’re more conspicuous, less vigilant, or can’t escape as fast.

For flies, it’s that first factor that’s the problem. Mating can mean certain doom because the sounds they make catch the attention of a killer lurking in the shadows.

In Europe, Natterer’s bats (Myotis nattereri) often roost in barns and cow sheds, drawn by flies that are attracted to animal manure. There’s plenty to eat, but catching the flies isn’t easy. Natterer’s bats hunt by echolocation, generating ultrasonic calls and using the echoes that bounce back to find prey and map their surroundings. Inside the farm buildings, finding flies this way is “almost impossible,” says biologist Stefan Greif, because the faint echoes made by the flies are masked by the more massive ones bouncing back from the walls and ceilings. What’s more, the flies also stop flying around at night, when the bats hunt, and either move around by walking or sit still on the ceiling, making them even harder find. 

Over four years, Greif and his team of researchers filmed thousands of flies walking along the ceiling of a cow shed in Weitershausen, Germany that was home to a colony of 45 bats. In all that time, not one of them was attacked by a bat. Yet, the bats stayed, and Greif knew that flies made up the majority of the species’ diet. So how were they catching bugs that were all but invisible to them?

It turns out that the bats are a little bit like B-movie killers, and make their attacks when the flies are having sex. While flies just walking around were inconspicuous, Greif found that ones that were getting it on were a clear sonic target. This is because when flies mate, the males flutter their wings, producing a burst of click-like noises. Humans can hear the lower frequency noises as a soft buzz, while the higher ones ring as loud and clear to the bats as a dinner bell. More than a quarter of the mating flies that Greif caught on camera during the study were attacked by the bats, and more than half of those attacks were successful, with the bat devouring both flies in all but two cases. 

To make sure that it was the sounds of sex that got the bats’ attention, the researchers tried two experiments. First, they stuck dead flies together in mating positions and mounted them on the ceiling. These were ignored by the bats, so it isn’t the bigger target and larger echo that mating flies create that gives them away. When the scientists recorded the flies’ mating sounds in the second experiment and played them back over a sound system, though, the bats went on the hunt. The researchers watched as “the bats approached, hovered and inspected the speaker, and in several cases tried to glean the acoustically simulated flies from the speaker with their tail membrane—just as they did with real pairs of copulating flies on the ceiling.” 

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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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