Tag: mammals

Why Sloths Live Life In the Slow Lane

Forests cover more than one-third of the land on Earth, yet few vertebrates make the canopy their home, and even fewer subsist solely on a diet of tree leaves.

In a new study in American Naturalist, researchers from the University of Wisconsin – Madison explain why this lifestyle is so rare and why animals that live in trees and eat leaves tend to live life at a slower pace.

Those species that do take advantage of this niche do not often radiate afterwards; that is, they don’t diversify and take on a variety of specialized forms.

The energetic constraints of a leafy diet are thought to prevent such adaptive radiation.

Leaves are an energetically and nutritionally poor food source. Most animals that live off plant leaves tend to be large, such as moose, elk, and deer.

Leaves are everywhere, but you need pretty complex gut machinery to be able to extract energy and nutrients from them,” says Jonathan Pauli, one of the study’s authors.

Most herbivores are big-bodied and they carry around big guts to break down and detoxify plant leaves.”

But animals that live in the treetops cannot be too big, or else the branches won’t support their body weight. So how do they make it on a nutritionally challenging diet?

Pauli and Zachariah Peery, along with co-authors Emily Fountain and William Karasov, set out to answer this question by measuring the daily energy expenditure of both two-toed and three-toed sloths in Costa Rica.

Both species of sloth are at the extreme end of specialization for a tree-dwelling, leaf-eating, lifestyle.

Pauli, Peery, and colleagues found that both sloth species expended very little energy, but three-toed sloths were especially slothful.

Three-toed sloths expended as little as 460 kilojoules of energy a day, the equivalent of burning only 110 calories. It is the lowest measured energetic output for any mammal.

Three-toed sloths use both behavioral and thermal strategies to limit their energy output. “They really are a slothful bunch,” says Pauli.

While two-toed sloths have bigger home ranges and move around quite a bit, three-toed sloths have very small home ranges and spend most of their time in just one or a few individual trees. “

“To limit energy costs, three-toed sloths find a good tree and camp out for a while and eat from it.”

The researchers then compared their sloth data to similar studies of other tree-dwelling, leaf-eating species from around the world.

Overall, the more specialized for the niche an animal was, the lower its daily energy expenditure.

While these species had lower metabolic rates than most mammals in general, they also relied heavily on thermoregulation and behavioral strategies to reduce their energetic expenditure.

The findings support the idea that tree-dwelling, leaf-eating mammals are tightly constrained by the poor nutritional quality of their diet, and thus, exhibit extremely low energetic output.

The researchers believe this impedes the opportunity for organisms to rapidly radiate into this niche.

For tree-dwelling, leaf-eating animals, there is a whole series of key innovations that are needed before they can crack into that open niche.

Sloths are the poster children for making a living in the treetops by saving energy. For them, slothfulness is a necessary virtue, not a deadly sin.

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Pass it on: New Scientist

Narwhals Wearing Heart Monitors Reveal Danger Of Human Encounters

Normally when an animal is scared, it either remains very still and slows its heart rate and metabolism in hope that danger will pass like a possum playing dead, in an extreme case or the body revs up to power a “fight or flight” response.

But when narwhals get caught in fishing nets, surprisingly, they do both. Even as the narwhals pump their fins and tails as fast as they can to escape, their heart rates plummet to just three to four beats per minute.

Scientists report Thursday in the journal Science. For reference, that’s about as many beats per minute as a ground squirrel while it’s hibernating.

It’s the first time anyone has measured heart rate and performance at the same time for a diving cetacean, the group that includes whales, dolphins, and narwhals, says Terrie Williams, a wildlife ecophysiologist at the University of California, Santa Cruz and lead author of the new study.

The narwhals’ super-slow heartbeats were a surprise because animals need to pump enough blood around the body to supply oxygen to the brain and stay warm, Williams says.

What’s more, the lack of blood flow could prevent the narwhals’ bodies from removing nitrogen, which leads to decompression sickness, also known as the bends.

Narwhals have always been a bit of a mystery. It’s thought that there may be as many as 173,000 narwhals on Earth, but because of the animal’s remote habitat and cryptic nature, no one is really sure.

For starters, the animals live entirely above the Arctic Circle and tend to hang out in places where sea ice is so thick that it’s nearly impenetrable to ships.

They spend much of the day diving to depths of 4,500 feet or more where they hunt for halibut, cod, shrimp, and squid.

No one has ever seen a narwhal eat, by the way. Scientists know about their menu only from studying the gut contents of narwhals killed by local hunters.

In fact, the current study probably wouldn’t exist without indigenous hunters, since Williams and her coauthors relied on cooperation from the people of Ittoqqortoormiit, Greenland, to get access to the narwhals caught in their nets.

The local people would have normally harvested these animals, but instead agreed to let the narwhals go so they could be studied.

Before each animal was released, the scientists used suction cups to attach heart rate monitors and motion sensors called accelerometers to the entangled narwhals.

Williams suspects that other deep-diving cetacean species may also use this bizarre escape response when they become stranded.

If these animals also experience a cardiac freeze either as the result of noise pollution, entanglements, or other human impacts then perhaps the animals’ brains aren’t getting enough oxygen.

This might explain why some whales strand themselves again even after rescuers help them back into the waves.

I don’t have the proof,” says Williams, who published a paper earlier this year that showed noise pollution increases the amount of energy beaked whales expend on their dives.

But we are certainly seeking the proof.”

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Pass it on: Popular Science

Rodents Of Unusual Size? Researchers Find Giant, Tree-Dwelling Rat In Solomon Islands

Deep in the forests of Vangunu in the Solomon Islands lives a rat like no other you’ve likely ever seen. It’s more than four times the size of an average rat and weighs more than a kilogram.

Meet Uromys vika, a new giant rat species.

The Solomon Islands in the South Pacific are home to some unique species, a result of the relative isolation of the islands. In particular, they are home to a number of giant rats species.

In 2010 while on a visit to the island of Vangunu, mammalogist Tyrone Lavery heard stories of a giant, coconut-cracking rat from locals.

He was convinced that this was a new kind of rat because while several giant rat species had been found in parts of the Solomon Islands, none had yet been discovered in the region known as the Western Province, which includes Vangunu and several other islands.

Those islands have also never been connected to the other Solomon islands, so I knew that if something had managed to arrive in the Western Province, it was a really good chance it would be a new species,” said Lavery, lead author of the findings, which were published in the Journal of Mammalogy Wednesday.

You’d think finding a giant rat would be easy, but Lavery spent five years searching for the elusive rodent.

While spending time on a tropical island may sound like paradise, Lavery said that it was a gruelling experience.

Long hikes through the forest, plenty of long rainy days setting up traps and cameras, and digging through layers of vegetation to try to find some clue as to the rat’s existence.

And then by accident he and his colleague Hikuna Judge found vika.

The rat was discovered near the village of Zaira during a hike in 2015.

Lavery and his colleague spotted the rat scurrying out from a tree that had been logged near the community trying to protect its rich forest from logging companies active throughout the islands where various species of these giant rats live.

The researchers captured the injured rat, which later died.

“Logging is quite a threat to a number of [mammal] species,” Lavery said.

That’s because many species — including many bats that Lavery studies in the islands — rely on old trees, those with hollows in them where the mammals can live.

And logging removes most of those trees,” he said.

The researchers compared the dead rat’s skull to existing giant rat skulls from other museums and collections. They found that this rat was like no other ever documented.

The new species, Uromys vika, became the first rat discovered in the Solomon Islands in 80 years.

It’s important to document these animals to know they’re there and conserve them,” Lavery said.

There are other giant rats living in the forests across the Solomon Islands. To date there have been eight species identified.

And while this vika rat may seem like a nightmare to some, Lavery looks at it quite fondly.

People … not having seen this rodent, [believe] it’s scary to think of a rodent that large,” he said.

I don’t think of it as scary. I think they’re quite unique animals. And I think this rat is quite cute for a rat.

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Pass it on: Popular Science