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Smithsonian Tropical Research Institute

This Snail's Sex Change Is Triggered by Touch

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Smithsonian Tropical Research Institute

Tropical slipper limpets are pretty unremarkable. They spend most of their lives sitting in one spot under a rock filtering their food out of the water. When it comes time for the limpets to mate, though, things get interesting. 

Compared to their body size, male limpets have some of the most impressive penises in the animal kingdom—almost as long as the snail’s entire body. They don’t hang on to these massive members forever, though. Slipper limpets begin their lives as males and turn into females as they age and grow—a process called sequential hermaphroditism. Their penises slowly shrink and then disappear while their female organs develop. 

A recent study in The Biological Bulletin shows that this dramatic change is set off by a simple touch.

Plenty of animals—including some starfish, crustaceans, and fish—change sex at some point in their lives. The timing of a sex change has to do both with an animal’s size and the company it keeps. Bigger animals can produce and carry more eggs than smaller ones, while sperm production is less size-dependent, so the switch usually happens when an animal reaches a certain size. For many sex-changing animals, there’s also a social factor, and the animals in a mating group or population can “tell” each other when it’s the best time to switch. 

Chemical cues are used by sea snails and other mollusks to warn one another about predators, recognize each other, and choose sites to live, among other things. Rachel Collin, a biologist at the Smithsonian Tropical Research Institute, figured that limpets used similar chemical signals to prompt each other's sex changes, too.

To test that idea, she and intern Allan Carrillo-Baltodano collected small male slipper limpets at Playa Chumical on Panama’s Pacific coast. They paired the snails off and placed them in different cups of seawater that allowed them different degrees of access to any chemical cues their cup mates might release. In some cups, the snails were free to roam around and come in contact with each other. In others, a mesh barrier kept them on separate sides of the cup. Some of the barriers had a finer mesh that only allowed water through, while others were a little wider and let the snails touch each other through the screen. Still other pairs were separated by fine mesh, but the snails were switched from one side of the cup to the other every week, letting them come in contact with their cup mate’s pedal mucus, the gluey substance snails and slugs use to help them move around.

When the limpets were allowed total access to each other, the larger one in each pair grew faster and changed sex sooner than their counterparts that were separated from their partners, while smaller, free-roaming partners delayed their own sex changes longer than the smaller snails in the separated pairs. The snails that were separated by the fine mesh and those that switched sides in their cups changed sex on the same, slower schedules, while the ones that were separated by the wider mesh fell in the middle.

Collin and Carrillo-Baltodano said they expected the sex change to be triggered by chemical cues since limpets are sedentary, but their results suggest that the change “requires some kind of physical interaction that is lost when the snails are separated.” For now, they aren’t sure what it is about physical contact that sets the change in motion. It might have to do with the way one snail positions itself relative to another, or a certain type of physical stimulation. Chemical signals could also still be at work, but ones that are contact-based instead of waterborne.

Watch a School of Humpback Whales 'Fish' Using Nets Made of Bubbles 

Just like humans, humpback whales catch many fish at once by using nets—but instead of being woven from fibers, their nets are made of bubbles.

Unique to humpbacks, the behavior known as bubble-net feeding was recently captured in a dramatic drone video that was created by GoPro and spotted by Smithsonian. The footage features a school of whales swimming off Maskelyne Island in British Columbia, Canada, in pursuit of food. The whales dive down, and a large circle of bubbles forms on the water's surface. Then, the marine mammals burst into the air, like circus animals jumping through a ring, and appear to swallow their meal.

The video offers a phenomenal aerial view of the feeding whales, but it only captures part of the underwater ritual. It begins with the group's leader, who locates schools of fish and krill and homes in on them. Then, it spirals to the water's surface while expelling air from its blowhole. This action creates the bubble ring, which works like a net to contain the prey.

Another whale emits a loud "trumpeting feeding call," which may stun and frighten the fish into forming tighter schools. Then, the rest of the whales herd the fish upwards and burst forth from the water, their mouths open wide to receive the fruits of their labor.

Watch the intricate—and beautiful—feeding process below:

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Big Questions
Why Do Dogs Love to Dig?
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Dog owners with green thumbs beware: It's likely just a matter of time before Fido turns your azalea bed into a graveyard of forgotten chew toys. When dogs aren't digging up your prized garden, they can be found digging elsewhere in your yard, at the beach, and even between your couch cushions at home. But what exactly is behind your dog's drive to turn every soft surface he or she sees into an excavation site?

According to Dr. Emma Grigg, an animal behaviorist and co-author of The Science Behind a Happy Dog, this behavior is completely normal. "When people say 'why do dogs dig,' the first thing that always comes to mind is 'well, because they're dogs,'" she tells Mental Floss. The instinct first appeared in dogs' wolf ancestors, then it was amplified in certain breeds through artificial selection. That's why dogs that were bred to hunt rodents, like beagles and terriers, are especially compelled to dig in places where such animals might make their homes.

But this tendency isn't limited to just a few specific breeds. No matter their original roles, dogs of all breeds have been known to kick up some dirt on occasion. Beyond predatory urges, Dr. Grigg says there are two main reasons a dog may want to dig. The first is to cool off on a hot day. When stuck on an open lawn with little to no shade, unearthing a fresh layer of dirt untouched by the sun is a quick way to beat the heat.

The second reason is to stash away goodies. Imagine your dog gets bored with chewing his favorite bone but knows he wants to come back for it later. Instead of leaving it out in the open where anyone can snatch it up, he decides to bury it in a secret place where only he'll be able to find it. Whether or not he'll actually go back for it is a different story. "There's a disconnect with modern dogs: They know the burying part but they don't always know to dig it up," Dr. Grigg says.

Because digging is part of a dog's DNA, punishing your pet for doing so isn't super effective. But that doesn't mean you should stand idly by as your yard gets turned inside-out. When faced with this behavior in your own dog, one option is to redirect it. This can mean allowing him to dig in a designated corner of the yard while keeping other parts off-limits, or setting up a raised flowerbed or sandbox especially to satisfy that urge. "You can get him interested in the area by burying a couple bones or some interesting things in there for him to dig," Dr. Grigg says. "I like the idea of buried treasure."

If your dog's motive for digging is more destructive than practical, he may have an energy problem. Dogs require a certain amount of stimulation each day, and when their humans don't provide it for them they find their own ways to occupy themselves. Sometimes it's by chewing up shoes, toppling trash cans, or digging ditches the perfect size for twisting ankles. Fortunately, this is nothing more walks and playtime can't improve.

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