Norwegian broadcaster NRK presented this problem during its coverage of the 2021 FIDE World Chess Championship in Dubai. White is to give mate on the move. (Warning — there’s a trick.)

A hundred years ago, more than 3,000 athletes from 44 nations gathered in France for the Paris 1924 Summer Olympic Games (compared with 2024, when more than 10,000 athletes are expected from 206 nations). In 1924, competition took place in 126 events across 23 disciplines, at 17 Olympic venues in and around the city. Gathered below are images of the many events, athletes, and spectators at the 1924 Summer Olympics.

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Last week marked the second anniversary of when the first images were released from NASA’s James Webb Space Telescope (JWST). The telescope was launched in December 2021 and remains in orbit around a point in space about 1 million miles from Earth. In the past year, JWST has continued to fuel new discoveries, as well as returning more spectacular views of the universe around us. Gathered here is a collection of images from JWST’s second year in space.

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Following a landslide victory for the Labour Party, Britain has a new leader. The moment Keir Starmer is officially made prime minister of the United Kingdom, he will be given a flurry of briefings, piles of documents, and the urgent business to run the country. Lurking among those papers is a moral land mine.

Starmer will be given a pen and four pieces of paper. On each paper, he must handwrite identical top-secret orders that—hopefully—no other human being will ever see. The previous set of orders, written by outgoing Prime Minister Rishi Sunak, will then be destroyed, unopened. These top-secret papers are called the “letters of last resort.”

Since 1969, Britain’s nuclear deterrent has operated at sea, with nuclear missiles that could be launched from at least one continuously deployed submarine. Destroying those vessels would eliminate the United Kingdom’s nuclear deterrent, so the secrecy of the patrolling submarine’s location is paramount. Once deployed, the submarine may not transmit messages, only receive them, to maintain its crucial cloak of concealment.

Today, there are four submarines—one always on patrol—which is why there are four identical copies of the letters. Each handwritten letter is placed inside a safe, which is housed inside another safe, on board the nuclear-armed submarine. Right now, one of those submarines is patrolling the world’s oceans, its location known only to a tiny number of people at the highest levels of the British government.

[Read: The nuclear question America never answers]

During the Cold War, British authorities constantly feared that London could be wiped out in a surprise nuclear attack by the Soviet Union. If the British government ceased to exist in a blinding flash of atomic light, and everyone in the civilian chain of command was dead, who would have the authority to launch a counterattack? Without the credible threat of a “second strike” in response to a nuclear assault on the capital, Britain lacked a deterrent.

The letters of last resort are the solution to that dilemma: They allow the prime minister to issue orders for a counterattack from beyond the grave. If the submarine captain has reason to believe that London has been destroyed in a nuclear blast (one of the cues is said to be that the BBC has stopped broadcasting), then the captain is to make every attempt to verify that the British government no longer exists. Once satisfied that the worst has indeed taken place, only then may the captain open the two safes, unseal the letters, read their contents, and execute the order from the now-deceased prime minister. Should the United Kingdom release its nuclear arsenal and retaliate—or not?

The briefings with the prime minister are secret, but four main options are typically presented to the incoming leader: retaliate, don’t retaliate, put the submarine under the control of the United States Navy, or leave it to the commander of the submarine to decide. Because it’s impossible to forecast what has occurred, the letters must be elastic enough to respond to the annihilation of the British government, whether caused by Russia, North Korea, or a rogue terrorist group that has somehow acquired weapons of mass destruction. There is just one letter per submarine.

“The prime minister can write on that piece of paper anything that he likes,” Robin Butler (also known as the Right Honourable Lord Butler of Brockwell) told me when I met him in his flat in Westminster a few years ago. He had served as the private secretary to five prime ministers, briefing the newly elected ones on the responsibilities they’d assumed. During the Cold War, the very existence of the letters was top secret—nobody outside the highest echelons of the British government knew of them—so the need to draft them came as a shock to incoming prime ministers still riding the euphoria of being elected. Even though the letters are not a secret today, writing them is still daunting. A new prime minister must decide whether he or she is willing to engage in nuclear warfare. (Liz Truss may have failed to outlast a lettuce, but she did decide whether she would use nuclear weapons.)

[Read: Goodbye to Tory Britain]

After explaining the protocols, Lord Butler would tell incoming prime ministers to write down what they had decided. “All I did was to leave successive prime ministers with a piece of paper and a pen to write out what those instructions should be,” Butler told me. “But it must be, above everything else, the thing that brings home to them what the weight of their responsibility is.” Britain has, by accident, designed a protocol ensuring that new prime ministers cannot come to office thinking only of themselves, but must contend psychologically with the burden of power, too.

If the worst were to happen, the letter on board the patrolling submarine would be opened. If the prime minister had given orders to retaliate, the crew would immediately fire as many as eight Trident missiles comprising up to 40 warheads, with a payload that would make the Hiroshima blast look comparatively minor. The trigger mechanism incorporates a handle from a modified Colt 45 revolver. (The training trigger is black, whereas the real one is red.) It will operate only when the captain has turned a key to the “Fire” position, ensuring that two people are required to initiate a launch.

This weekend, Keir Starmer, like all prime ministers for the past five decades before him, will write his orders for what to do if the British government is wiped out. Unlike American presidents, who must only contemplate the terrifying nuclear power they control, British prime ministers must actually decide—definitively—whether they would use that power.

Prime ministers are hesitant to discuss the letters of last resort, and none of the handwritten orders has ever been seen. That’s understandable, because if the letters included any orders other than for a full-blown second strike, Britain’s adversaries would know that, and it could heighten the risk of a nuclear attack.

Nonetheless, former British Prime Minister Tony Blair spoke with me in 2020 about the letters of last resort. On taking office in 1997, Blair told me, “Whereas everyone else was euphoric, I really wasn’t. I was oppressed by the weight of the responsibility that was descending upon me and very conscious of it—very conscious of the fact that campaigning for office and governing in office are two very different things.”

The letters themselves didn’t weigh that heavily on Blair, however, because he took power during a period of comparative peace and prosperity, when the prospect of nuclear war seemed far-fetched. “Yes, of course, I paid a lot of attention deciding how I drafted the letters,” he said. “But it didn’t seem to be anything other than an extraordinarily remote possibility, so I can’t say it occupied my thoughts greatly.”

The same is unlikely to be true for Starmer, who takes office at a moment of global peril. Russia’s invasion of Ukraine has raised concerns that nuclear weapons could again be used in warfare. North Korea’s eccentric dictator continues to test his arsenal. Iran is more openly flirting with acquiring nuclear bombs. And one of the options prime ministers usually consider—turning over Britain’s nuclear arsenal to the United States Navy—could soon mean putting even more nuclear firepower in the hands of Donald Trump.

If the letters are opened, and they call for the awesome power of Britain’s nuclear arsenal to be unleashed, a deafening sound will follow—of missiles traveling at 18,000 miles an hour before exploding in a cacophony of death. This weekend, Keir Starmer must contemplate the destructive capability he now wields, while listening to a much quieter sound: the scratches of his pen on four pieces of paper that could determine the future of humanity.

So my wife took this picture in our garden yesterday, here in Kigali, Rwanda:



Take a close look.  This little bird — about the size of an American cardinal, or a European robin — is facing us.  It’s also facing the sun, though you can’t see that.  It is holding two twigs with its little claws, and… it’s puffing out its breast feathers in a very weird way.  It looks like a breeze is ruffling them.  But there is no breeze.

So we did a quick look-up and found: this is Colius Striatus, the Speckled Mousebird.  Long tail, “scruffy” crest, check.  Thin, rather hairlike breast feathers, check. Very common across tropical Africa, okay.  And then this:  

“Speckled mousebirds… can often be spotted roosting in groups where they’ll buff up their feathers. They do this to allow more sunlight to hit their bodies which helps speed up the fermentation process.”

Wait, what?


Okay so let’s talk for a moment about “folivory”, which is the fancy term for “eating leaves”.

If you want to eat leaves as your major food source, well, there’s good news and there’s bad news.  The good news is, leaves are everywhere!  And you don’t have to be stealthy or clever to sneak up on them, and they’re not going to run away.

The bad news is, leaves don’t have a lot of energy.  Your typical leaf is mostly water and cellulose with a bit of starch thrown in.  In terms of calories per kilogram, it’s one of the least energetic foods you can get.  So, an entire kilogram of lettuce only has about 150 calories.  That compares to about 550 calories for a kilogram of apples, or about 2,700 for a kilogram of beef. This is why we eat salads to lose weight, yes?

Now leaves as an *occasional* food source are fine.  They do tend to contain useful vitamins and minerals, which makes them very useful as part of a varied diet for an omnivorous animal.  But if leaves are all of your diet, or a lot of your diet, then you’re going to encounter some issues.  Leaves are such a low-energy food that either you have to eat a lot of leaves, or you have to just digest the heck out of the leaves that you do eat, so that you extract every possible calorie of nutrition.  Or both. 

This is why cows have four stomachs, yes?  They’re eating grass — grass is leaves — and then they’re sending it through a digestive system that is much more complex than yours or mine.  One thing they’ll do along the way is ferment the grass, which lets them crack energy out of all that cellulose, getting more calories per kilogram.  But fermentation is complicated:  it needs either a big complex digestive system filled with symbiotic bacteria, or heat and lots of time — preferably both.

Okay so if you’re an animal that’s going to eat a lot of leaves, that generally gives you three options.

— Be big.  Cows, elephants, gorillas.  You have lots of room for a massive digestive system that can extract all your caloric needs.
— Be cold-blooded.  Insects, slugs, iguanas.  Cold-blooded animals need a lot less energy!  So, it’s easier to get your energy needs from leaves.
— Be slow.  Sloths, koalas.  

There are some interesting edge cases and exceptions (ask me about rabbits, or better don’t) but broadly speaking those are the options.

Okay so if you think a moment you’ll realize that all of these make it challenging for a bird to survive on leaves.  Birds generally have blowtorch metabolisms, with base body temperatures quite a bit higher than most mammals’.  Birds are generally small.  And birds can’t usually afford to be slow.

There’s exactly one (1) bird that lives on leaves and nothing but leaves:  the hoatzin.



Briefly, the hoatzin is nature’s attempt to evolve a sloth again, starting with a bird.  They live in South American tropical forests and they spend most of their time crawling slowly through the tree canopy.  And they can fly, but just barely — it’s an awkward flapping scramble, just enough to escape a predator.  

Okay, so the hoatzin is the only bird that lives entirely on leaves.  What about birds that eat a lot of leaves, but other stuff too?

Well, that gets you geese.  Geese typically get between a quarter and half their calories from grass and other leaves.  But geese are also perfectly good fliers and they don’t lack energy; as anyone who’s ever confronted one can tell you, geese are horribly strong and quick.  They can make this work because they are (for a bird) quite big, so they have room for a relatively large digestive system.  That said, geese can’t live on grass alone, and will aggressively seek out other food sources — as anyone foolish enough to offer one bread will quickly realize.




All right then, some big birds can eat a lot of leaves.  But a small bird couldn’t manage it.  Right?

Until yesterday, I would have agreed.  But I would have been wrong.  The mousebird gets about half of its calories from leaves.  

How?

It cheats.  The mousebird uses a trick called “heterothermy” — the ability to change its resting body temperature.  A few warm-blooded animals can do this, usually for purposes of hibernation or torpor.  So for instance, when bears hibernate, their core body temperature drops from about 99 degrees F (38 Celsius) to around 88 degrees f (31 Celsius).  And it stays that way for months at a time.  Bears have a metabolic “low gear” that most mammals lack.

And so does the mousebird.  But the mousebird can drop its body temperature /much/ lower.  They can go all the way down to around 75 degrees F (24 C) — room temperature, more or less.  That is crazy low for a warm-blooded animal!  That’s reptile territory.  But the mousebird makes it work.  And that means the mousebird no longer has that hot, fast, high-energy bird metabolism that needs vast amounts of calories.  It can live on the limited energy it gets from leaves.

TBC, the mousebird doesn’t eat leaves alone.  They’ll eat all sorts of plant material — seeds, fruit.  In our garden, we see them drinking nectar from the flowers of the big yucca plant by the garden gate.  But the mousebird’s ability to drop its body temperature means that when high-quality plant food is scarce — for instance, during Rwanda’s dry season, which is right now — it can switch to eating low-energy leaves, slow down its metabolism, and survive just fine.

Okay, but there’s still one problem left: the mousebird wants to get maximum energy from its leafy diet.  To do that, it needs to ferment its food.  That requires either a big cow-like digestive system, or — at a minimum — heat and time.  But the mousebird has lowered its body temperature!  So, where will the heat for fermentation come from?

From the hot tropical sun, thank you very much.  The mousebird will eat a belly full of leaves, and then it will fly up to a sunny perch, and then it will just… bask.  It lowers its metabolism, conserving energy, but its body temperature stays high because of the sun.  Its belly is covered with long, hairy feathers that help it absorb solar heat.  So its digestive system stays hot and can ferment the leaves aggressively.  If you go back to the top of the post, that’s exactly what the mousebird in our garden is doing in the photo. 

Meanwhile the mousebird is torpid, but awake. Presumably it’s just alert enough to drop off its perch and flutter into cover if danger should approach.

“Basking in the sun to digest a meal” is a behavior that we associate with reptiles: a turtle on a log, a snake by the side of the road.  You really don’t expect to find it in a bird.  But the mousebird is pretty successful.  It’s not a weird one-off like the hoatzin.  There are half a dozen species of mousebird, spread all across Africa.

— That whole “cold blooded / warm blooded” thing we learned back in grade school?  Basically correct, but the details get infinitely complicated.  There are some reptiles that are kinda warm blooded.  There are birds and mammals that vary their body temperature.  There are insects that can warm themselves up.  Heck, there are a couple of plants that produce internal heat. 

The world is wide and full of wonders, you know?  And sometimes they’ll come and perch in your garden.