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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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Pigeons Are Secretly Brilliant Birds That Understand Space and Time, Study Finds
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Of all the birds in the world, the pigeon draws the most ire. Despite their reputation as brainless “rats with wings,” though, they’re actually pretty brilliant (and beautiful) animals. A new study adds more evidence that the family of birds known as pigeons are some of the smartest birds around, as Quartz alerts us.

In addition to being able to distinguish English vocabulary from nonsense words, spot cancer, and tell a Monet from a Picasso, pigeons can understand abstract concepts like space and time, according to the new study published in Current Biology. Their brains just do it in a slightly different way than humans’ do.

Researchers at the University of Iowa set up an experiment where they showed pigeons a computer screen featuring a static horizontal line. The birds were supposed to evaluate the length of the line (either 6 centimeters or 24 centimeters) or the amount of time they saw it (either 2 or 8 seconds). The birds perceived "the longer lines to have longer duration, and lines longer in duration to also be longer in length," according to a press release. This suggests that the concepts are processed in the same region of the brain—as they are in the brains of humans and other primates.

But that abstract thinking doesn’t occur in the same way in bird brains as it does in ours. In humans, perceiving space and time is linked to a region of the brain called the parietal cortex, which the pigeon brains lack entirely. So their brains have to have some other way of processing the concepts.

The study didn’t determine how, exactly, pigeons achieve this cognitive feat, but it’s clear that some other aspect of the central nervous system must be controlling it. That also opens up the possibility that other non-mammal animals can perceive space and time, too, expanding how we think of other animals’ cognitive capabilities.

[h/t Quartz]

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The Queen's Racing Pigeons Are in Danger, Due to an Increase in Peregrine Falcons
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Queen Elizabeth is famous for her love of corgis and horses, but her pet pigeons don't get as much press. The monarch owns nearly 200 racing pigeons, which she houses in a luxury loft at her country estate, Sandringham House, in Norfolk, England. But thanks to a recent boom in the region’s peregrine falcon population, the Queen’s swift birds may no longer be able to safely soar around the countryside, according to The Telegraph.

Once endangered, recent conservation efforts have boosted the peregrine falcon’s numbers. In certain parts of England, like Norfolk and the city of Salisbury in Wiltshire, the creatures can even find shelter inside boxes installed at local churches and cathedrals, which are designed to protect potential eggs.

There’s just one problem: Peregrine falcons are birds of prey, and local pigeon racers claim these nesting nooks are located along racing routes. Due to this unfortunate coincidence, some pigeons are failing to return to their owners.

Pigeon racing enthusiasts are upset, but Richard Salt of Salisbury Cathedral says it's simply a case of nature taking its course. "It's all just part of the natural process,” Salt told The Telegraph. "The peregrines came here on their own account—we didn't put a sign out saying 'room for peregrines to let.' Obviously we feel quite sorry for the pigeons, but the peregrines would be there anyway."

In the meantime, the Queen might want to keep a close eye on her birds (or hire someone who will), or consider taking advantage of Sandringham House's vast open spaces for a little indoor fly-time.

[h/t The Telegraph]

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