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Some Sloths Are More Slothful Than Others

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As we wrote about in detail a few months ago, sloths have a pretty low-key lifestyle. They can hang upside down much of the day, moving slowly when they move at all, while only going to the bathroom once a week or so. That’s a consequence of having to live on very little nutritional energy from a diet mostly composed of tree leaves. New research published in the journal The American Naturalist suggests that in this low-energy group, some sloths are more slothful than others—and one species has the lowest field metabolic rate ever recorded in a mammal. 

An animal’s field metabolic rate, or FMR, is its daily energy expenditure in the wild. Scientists had calculated it for three-toed sloths in the 1980s, but no one had ever gotten around to doing the same for two-toed sloths (which split from their distant cousins more than 40 million years ago). Ecologist Jonathan Pauli and his team from the University of Wisconsin decided they should, and while they were at it, they wanted to see how the two groups of sloths stacked up against each other.

To measure how much energy the animals used over the course of a day, the researchers captured several brown-throated three-toed sloths and Hoffmann’s two-toed sloths in Costa Rica. They took blood samples, and then injected the sloths with water “labeled” with specific isotopes of oxygen and hydrogen. After tracking the sloths for a week and a half, the scientists drew their blood again and looked at how much of the isotopes remained to calculate the sloths’ FMRs. During that week and a half, the researchers also monitored the sloths’ body temperatures, how much they moved each day, and where they went.

They found that the brown-throated three-toed sloths' FMRs were 31 percent lower than the two-toed sloths'. That’s impressive conservation in a group of animals that is already pretty judicious about expending energy. What’s more amazing is that the three-toed sloth’s FMR is lower than what’s been measured in any other mammal outside of hibernation.

How do the sloths get away with using that little energy day to day? The researchers say it's a combination of behavioral and physiological characteristics. First, the three-toed sloths aren’t very mobile, even by the low standards of sloths, and only traveled around 160 feet per day (the two-toed sloths, meanwhile covered about 480 feet daily). Second, their body temperatures fluctuated much more than the other sloths and rose and fell with the ambient temperature. The sloths still need to keep their temperature within a certain range, but instead of regulating their temps with metabolic processes, they adjusted their thermostats behaviorally. They inched their way higher into the trees during the cool mornings to warm up in the sunlight, and then made their way back down into the shade as the day went on and the ambient temperature increased.

The researchers decided to take their comparisons a step further and compared the sloths’ FMRs to those available for other arboreal folivores (that is, animals that live in trees and eat their leaves). They found that the more specialized in this lifestyle an animal was, the lower its daily energy use.

The adaptations sloths need to live on such little energy—slow movements, low metabolic and digestive rates, and thermoregulating behaviors—are a unique combination (they also need another odd adaptation to live hanging upside down from branches).

These “unexpected, and even bizarre, characteristics,” Pauli’s team concludes, may help explain why arboreal folivory is such a rare lifestyle (it occurs in less than 0.2 percent of mammals), and why sloths and other arboreal folivores haven’t diversified as much as groups with other lifestyles. “A suite of adaptations, rather than a single key innovation, is necessary to support an organism exploiting a lifestyle in the trees,” the team writes. The constraints of the diet available up there and traits required to exploit it don’t make it a very attractive place for the arboreal folivores to branch out—so they haven’t.

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Animals
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.

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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