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Why Do We Yawn?

The short answer is that no one really knows.

The long answer is that no one really knows, but there are plenty of interesting theories:

1. The idea that we yawn to get rid of carbon dioxide and take in more oxygen has been disproved by research, but persists as the "common wisdom" answer. According to this theory, people breathe more slowly when they're bored or tired and less oxygen gets to the lungs. As CO2 builds up in the blood, the brain reflexively prompts a deep, oxygen-rich breath.

The problem with this theory is a 1987 study by Dr. Robert Provine, who is regarded as the world's foremost yawn expert. Provine set up an experiment in which volunteers breathed one of four gases that contained varying ratios of CO2 to O2 for 30 minutes. Normal air contains 20.95% oxygen and 0.03% carbon dioxide, but neither of the gases in the experiment with higher concentrations of CO2 (3% and 5%) caused the research subjects to yawn more.

2.

Last year, a team of researchers at the University of Albany proposed that the purpose of yawning is to cool the brain. They conducted an experiment similar to Provine's and again found that raising or lowering oxygen and carbon dioxide levels in the blood did not change the amount or length of yawns.

Subsequent experiments focused on two well-established brain cooling mechanisms: nasal breathing and forehead cooling. When you breathe through your nose, it cools the blood vessels in the nasal cavity and sends that cooler blood to the brain. Likewise, when you cool your forehead, the veins there, some of which are directly connected to the brain, deliver cooler blood. The researchers found that their test subjects with warm or room temperature towels pressed against their heads yawned more than those with cold towels. Subjects who breathed through their noses during the experiment did not yawn at all.

The researchers said their evidence suggests that taking in a big gulp of air with a yawn cools the brain and maintains mental efficiency.

3. Another theory says that yawning has more to do with sociology than physiology and also tackles the question of contagious yawning.

Almost all vertebrates yawn spontaneously, but only humans, chimps and macaques yawn as a result of watching another individual do it. Given that these are social creatures that live in groups, the contagious yawn may have evolved as a way to coordinate behavior and maintain group vigilance. When one individual yawned, the group took that as evidence that their brain temperature was up and their mental efficiency was down. If all members of the group then yawned, the overall level of vigilance in the group was enhanced. In humans, who have color-coded charts to signal how vigilant they should be, yawns may still be contagious as a vestigial response.

charlotte.jpgWhile yawns are still largely a mystery, here are some things we know for certain:

"¢ The average yawn lasts about six seconds.

"¢ In humans, the earliest occurrence of a yawn happens about 11 weeks after conception "“ while we're still in the womb.

"¢ Your heart rate can rise as much as 30% during a yawn.

"¢ 55% of people will yawn within five minutes of seeing someone else yawn.

"¢ Blind people yawn more after hearing an audio tape of people yawning.

"¢ Reading or even thinking about yawning can cause you to yawn.

"¢ While researching and writing this story, I yawned 37 times.

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Dodo: © Oxford University, Oxford University Museum of Natural History. Background: iStock
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science
Head Case: What the Only Soft Tissue Dodo Head in Existence Is Teaching Scientists About These Extinct Birds
Dodo: © Oxford University, Oxford University Museum of Natural History. Background: iStock
Dodo: © Oxford University, Oxford University Museum of Natural History. Background: iStock

Of all the recently extinct animals, none seems to excite the imagination quite like the dodo—a fact Mark Carnall has experienced firsthand. As one of two Life Collections Managers at the UK's Oxford University Museum of Natural History, he’s responsible for nearly 150,000 specimens, “basically all the dead animals excluding insects and fossils,” he tells Mental Floss via email. And that includes the only known soft tissue dodo head in existence.

“In the two and a bit years that I’ve been here, there’s been a steady flow of queries about the dodo from researchers, artists, the public, and the media,” he says. “This is the third interview about the dodo this week! It’s definitely one of the most popular specimens I look after.”

The dodo, or Raphus cucullatus, lived only on the island of Mauritius (and surrounding islets) in the Indian Ocean. First described by Vice Admiral Wybrand van Warwijck in 1598, it was extinct less than 100 years later (sailors' tales of the bird, coupled with its rapid extinction, made many doubt that the dodo was a real creature). Historians still debate the extent that humans ate them, but the flightless birds were easy prey for the predators, including rats and pigs, that sailors introduced to the isolated island of Mauritius. Because the dodo went extinct in the 1600s (the actual date is still widely debated), museum specimens are very, very rare. In fact, with the exception of subfossils—the dark skeletons on display at many museums—there are only three other known specimens, according to Carnall, “and one of those is missing.” (The fully feathered dodos you might have seen in museums? They're models, not actual zoological specimens.)

A man standing with a Dodo skeleton and a reconstructed model of the extinct bird
A subfossil (bone that has not been fully fossilized) Dodo skeleton and a reconstructed model of the extinct bird in a museum in Wales circa 1938.
Becker, Fox Photos/Getty Images

Since its extinction was confirmed in the 1800s, Raphus cucullatus has been an object of fascination: It’s been painted and drawn, written about and scientifically studied, and unfairly become synonymous with stupidity. Even now, more than 300 years since the last dodo walked the Earth, there’s still so much we don’t know about the bird—and Oxford’s specimen might be our greatest opportunity to unlock the mysteries surrounding how it behaved, how it lived, how it evolved, and how it died.

 
 

To put into context how old the dodo head is, consider this: From the rule of Oliver Cromwell to the reign of Queen Elizabeth II, it has been around—and it’s likely even older than that. Initially an entire bird (how exactly it was preserved is unclear), the specimen belonged to Elias Ashmole, who used his collections to found Oxford’s Ashmolean Museum in 1677. Before that, it belonged to John Tradescant the Elder and his son; a description of the collection from 1656 notes the specimen as “Dodar, from the Island Mauritius; it is not able to flie being so big.”

And that’s where the dodo’s provenance ends—beyond that, no one knows where or when the specimen came from. “Where the Tradescants got the dodo from has been the subject of some speculation,” Carnall says. “A number of live animals were brought back from Mauritius, but it’s not clear if this is one of [those animals].”

Initially, the specimen was just another one of many in the museum’s collections, and in 1755, most of the body was disposed of because of rot. But in the 19th century, when the extinction of the dodo was confirmed, there was suddenly renewed interest in what remained. Carnall writes on the museum’s blog that John Duncan, then the Keeper of the Ashmolean Museum, had a number of casts of the head made, which were sent to scientists and institutions like the British Museum and Royal College of Surgeons. Today, those casts—and casts of those casts—can be found around the world. (Carnall is actively trying to track them all down.)

The Oxford University Dodo head with scoleric bone and the skin on one side removed.
The Oxford University Dodo head with skin and sclerotic ring.
© Oxford University, Oxford University Museum of Natural History // Used with permission

In the 1840s, Sir Henry Acland, a doctor and teacher, dissected one side of the head to expose its skeleton, leaving the skin attached on the other side, for a book about the bird by Alexander Gordon Melville and H.E. Strickland called The dodo and its kindred; or, The history, affinities, and osteology of the dodo, solitaire, and other extinct birds of the islands Mauritius, Rodriguez and Bourbon. Published in 1848, “[It] brought together all the known accounts and depictions of the dodo,” Carnall says. The Dodo and its kindred further raised the dodo’s profile, and may have been what spurred schoolteacher George Clark to take a team to Mauritius, where they found the subfossil dodo remains that can be seen in many museums today.

Melville and Strickland described Oxford’s specimen—which they believed to be female—as being “in tolerable preservation ... The eyes still remain dried within the sockets, but the corneous extremity of the beak has perished, so that it scarcely exhibits that strongly hooked termination so conspicuous in all the original portraits. The deep transverse grooves are also visible, though less developed than in the paintings.”

Today, the specimen includes the head as well as the sclerotic ring (a bony feature found in the eyes of birds and lizards), a feather (which is mounted on a microscope slide), tissue samples, the foot skeleton, and scales from the foot. “Considering it’s been on display in collections and museums, pest eaten, dissected, sampled and handled by scientists for over 350 years,” Carnall says, “it’s in surprisingly good condition.”

 
 

There’s still much we don’t know about the dodo, and therefore a lot to learn. As the only soft tissue of a dodo known to exist, the head has been studied for centuries, and not always in ways that we would approve of today. “There was quite some consideration about dissecting the skin off of the head by Sir Henry Acland,” Carnall says. “Sadly there have also been some questionable permissions given, such as when [Melville] soaked the head in water to manipulate the skin and feel the bony structure. Excessive handling over the years has no doubt added to the wear of the specimen.”

Today, scientists who want to examine the head have to follow a standard protocol. “The first step is to get in touch with the museum with details about access requirements ... We deal with enquiries about our collections every single day,” Carnall says. “Depending on the study required, we try to mitigate damage and risk to specimens. For destructive sampling—where a tissue sample or bone sample is needed to be removed from the specimen and then destroyed for analysis—we weigh up the potential importance of the research and how it will be shared with the wider community.”

In other words: Do the potential scientific gains outweigh the risk to the specimen? “This,” Carnall says, “can be a tough decision to make.”

The head, which has been examined by evolutionary biologist Beth Shapiro and extinction expert Samuel Turvey as well as dodo experts Julian Hume and Jolyon Parish, has been key in many recent discoveries about the bird. “[It] has been used to understand what the dodo would have looked like, what it may have eaten, where it fits in with the bird evolutionary tree, island biogeography and of course, extinction,” Carnall says. In 2011, scientists took measurements from dodo remains—including the Oxford specimen—and revised the size of the bird from the iconic 50 pounder seen in paintings to an animal “similar to that of a large wild turkey.” DNA taken from specimen’s leg bone has shed light on how the dodo came to Mauritius and how it was related to other dodo-like birds on neighboring islands [PDF]. That DNA also revealed that the dodo’s closest living relative is the Nicobar pigeon [PDF].

A nicobar pigeon perched on a bowl of food.
A nicobar pigeon.
iStock

Even with those questions answered, there are a million more that scientists would like to answer about the dodo. “Were there other species—plants, parasites—that depended on the dodo?” Carnall asks. “What was the soft tissue like? ... How and when did the dodo and the related and also extinct Rodrigues solitaire colonize the Mascarene Islands? What were their brains like?”

 
 

Though it’s a rare specimen, and priceless by scientific standards, the dodo head is, in many ways, just like all the rest of the specimens in the museum’s collections. It’s stored in a standard archival quality box with acid-free tissue paper that’s changed regularly. (The box is getting upgraded to something that Carnall says is “slightly schmancier” because “it gets quite a bit of use, more so than the rest of the collection.”) “As for the specific storage, we store it in vault 249 and obviously turn the lasers off during the day,” Carnall jokes. “The passcode for the vault safe is 1234ABCD …”

According to Carnall, even though there are many scientific and cultural reasons why the dodo head is considered important, to him, it isn’t necessarily more important than any of the other 149,999 specimens he’s responsible for.

“Full disclosure: All museum specimens are equally important to collections managers,” he says. “It is a huge honor and a privilege to be responsible for this one particular specimen, but each and every specimen in the collection also has the power to contribute towards our knowledge of the natural world ... This week I was teaching about a species of Greek woodlouse and the molluscs of Oxfordshire. We know next to nothing about these animals—where they live, what they eat, the threats to them, and the predators that rely on them. The same is true of most living species, sadly. But on the upside, there’s so much work to be done!”

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