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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
10 Biting Facts About Snapping Turtles
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Here in the Americas, lake monster legends are a dime a dozen. More than a few of them were probably inspired by these ancient-looking creatures. In honor of World Turtle Day, here are 10 things you might not have known about snapping turtles.

1. THE COMMON SNAPPING TURTLE IS NEW YORK'S OFFICIAL STATE REPTILE.

Elementary school students voted to appoint Chelydra serpentina in a 2006 statewide election. Weighing as much as 75 pounds in the wild (and 86 in captivity), this hefty omnivore’s natural range stretches from Saskatchewan to Florida.

2. ALLIGATOR SNAPPING TURTLES CAN BE LARGE. (VERY LARGE.)

An alligator snapping turtle
NorbertNagel, Wikimedia Commons // CC BY-SA 3.0

Utterly dwarfing their more abundant cousin, alligator snappers (genus: Macrochelys) are the western hemisphere’s biggest freshwater turtles. The largest one on record, a longtime occupant of Chicago’s Shedd Aquarium, weighed 249 pounds.  

A monstrous 403-pounder was reported in Kansas during the Great Depression, though this claim was never confirmed.  

3. COMMON SNAPPERS HAVE LONGER NECKS AND SPIKIER TAILS.

Alligator snappers also display proportionately bigger heads and noses plus a trio of tall ridges atop their shells. Geographically, alligator snapping turtles are somewhat restricted compared to their common relatives, and are limited mainly to the southeast and Great Plains.

4. BOTH VARIETIES AVOID CONTACT WITH PEOPLE.

If given the choice between fight and flight, snapping turtles almost always distance themselves from humans. The animals spend the bulk of their lives underwater, steering clear of nearby Homo sapiens. However, problems can arise on dry land, where the reptiles are especially vulnerable. Females haul themselves ashore during nesting season (late spring to early summer). In these delicate months, people tend to prod and handle them, making bites inevitable.

5. YOU REALLY DON'T WANT TO GET BITTEN BY ONE. 

Snapping turtle jaw strength—while nothing to sneeze at—is somewhat overrated. Common snapping turtles can clamp down with up to 656.81 newtons (N) of force, though typical bites register an average of 209 N. Their alligator-like cousins usually exert 158 N. You, on the other hand, can apply 1300 N between your second molars.

Still, power isn’t everything, and neither type of snapper could latch onto something with the crushing force of a crocodile’s mighty jaws. Yet their sharp beaks are well-designed for major-league shearing. An alligator snapping turtle’s beak is capable of slicing fingers clean off and (as the above video proves) obliterating pineapples.

Not impressed yet? Consider the following. It’s often said that an adult Macrochelys can bite a wooden broom handle in half. Intrigued by this claim, biologist Peter Pritchard decided to play MythBuster. In 1989, he prodded a 165-pound individual with a brand new broomstick. Chomp number one went deep, but didn’t quite break through the wood. The second bite, though, finished the job.

6. SCIENTISTS RECENTLY DISCOVERED THAT THERE ARE THREE SPECIES OF ALLIGATOR SNAPPING TURTLES.

A 2014 study trisected the Macrochelys genus. For over a century, naturalists thought that there was just a single species, Macrochelys temminckii. Closer analysis proved otherwise, as strong physical and genetic differences exist between various populations. The newly-christened M. suwanniensis and M. apalachicolae are named after their respective homes—namely, the Suwannee and Apalachicola rivers. Further west, good old M. temminckii swims through the Mobile and the Mississippi.

7. THANKS TO A 19TH CENTURY POLITICAL CARTOON, COMMON SNAPPING TURTLES ARE ALSO KNOWN AS "OGRABMES." 

Snapping turtle cartoon
Urban~commonswiki via Wiki Commons // CC BY PD-US

Drawn by Alexander Anderson, this piece skewers Thomas Jefferson’s signing of the unpopular Embargo Act. At the president’s command, we see a snapping turtle bite some poor merchant’s hind end. Agitated, the victim calls his attacker “ograbme”—“embargo” spelled backwards.

8. ALLIGATOR SNAPPERS ATTRACT FISH WITH AN ORAL LURE …

You can’t beat live bait. Anchored to the Macrochelys tongue is a pinkish, worm-like appendage that fish find irresistible. Preferring to let food come to them, alligator snappers open their mouths and lie in wait at the bottoms of rivers and lakes. Cue the lure. When this protrusion wriggles, hungry fish swim right into the gaping maw and themselves become meals.

9.  … AND THEY FREQUENTLY EAT OTHER TURTLES. 


Complex01, WikimediaCommons

Alligator snappers are anything but picky. Between fishy meals, aquatic plants also factor into their diet, as do frogs, snakes, snails, crayfish, and even relatively large mammals like raccoons and armadillos. Other shelled reptiles are fair game, too: In one Louisiana study, 79.82% of surveyed alligator snappers had turtle remains in their stomachs.

10. YOU SHOULD NEVER PICK A SNAPPER UP BY THE TAIL.

Ideally, you should leave the handling of these guys to trained professionals. But what if you see a big one crossing a busy road and feel like helping it out? Before doing anything else, take a few moments to identify the turtle. If it’s an alligator snapper, you’ll want to grasp the lip of the upper shell (or “carapace”) in two places: right behind the head and right above the tail.

Common snappers demand a bit more finesse (we wouldn’t want one to reach back and nip you with that long, serpentine neck). Slide both hands under the hind end of the shell, letting your turtle’s tail dangle between them. Afterwards, clamp down on the carapace with both thumbs.

Please note that lifting any turtle by the tail can permanently dislocate its vertebrae. Additionally, remember to move the reptile in the same direction that it’s already facing. Otherwise, your rescue will probably turn right back around and try to cross the road again later. 

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Big Questions
Why Do Cats 'Blep'?
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As pet owners are well aware, cats are inscrutable creatures. They hiss at bare walls. They invite petting and then answer with scratching ingratitude. Their eyes are wandering globes of murky motivations.

Sometimes, you may catch your cat staring off into the abyss with his or her tongue lolling out of their mouth. This cartoonish expression, which is atypical of a cat’s normally regal air, has been identified as a “blep” by internet cat photo connoisseurs. An example:

Cunning as they are, cats probably don’t have the self-awareness to realize how charming this is. So why do cats really blep?

In a piece for Inverse, cat consultant Amy Shojai expressed the belief that a blep could be associated with the Flehmen response, which describes the act of a cat “smelling” their environment with their tongue. As a cat pants with his or her mouth open, pheromones are collected and passed along to the vomeronasal organ on the roof of their mouth. This typically happens when cats want to learn more about other cats or intriguing scents, like your dirty socks.

While the Flehmen response might precede a blep, it is not precisely a blep. That involves the cat’s mouth being closed while the tongue hangs out listlessly.

Ingrid Johnson, a certified cat behavior consultant through the International Association of Animal Behavior Consultants and the owner of Fundamentally Feline, tells Mental Floss that cat bleps may have several other plausible explanations. “It’s likely they don’t feel it or even realize they’re doing it,” she says. “One reason for that might be that they’re on medication that causes relaxation. Something for anxiety or stress or a muscle relaxer would do it.”

A photo of a cat sticking its tongue out
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If the cat isn’t sedated and unfurling their tongue because they’re high, then it’s possible that an anatomic cause is behind a blep: Johnson says she’s seen several cats display their tongues after having teeth extracted for health reasons. “Canine teeth help keep the tongue in place, so this would be a more common behavior for cats missing teeth, particularly on the bottom.”

A blep might even be breed-specific. Persians, which have been bred to have flat faces, might dangle their tongues because they lack the real estate to store it. “I see it a lot with Persians because there’s just no room to tuck it back in,” Johnson says. A cat may also simply have a Gene Simmons-sized tongue that gets caught on their incisors during a grooming session, leading to repeated bleps.

Whatever the origin, bleps are generally no cause for concern unless they’re doing it on a regular basis. That could be sign of an oral problem with their gums or teeth, prompting an evaluation by a veterinarian. Otherwise, a blep can either be admired—or retracted with a gentle prod of the tongue (provided your cat puts up with that kind of nonsense). “They might put up with touching their tongue, or they may bite or swipe at you,” Johnson says. “It depends on the temperament of the cat.” Considering the possible wrath involved, it may be best to let them blep in peace.

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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