The two apes above might look very similar to the untrained eye, but they belong to two very different species. The one on the right is a bonobo; the one on the left is a chimpanzee. They are very closely related but the bonobo is slimmer, with a smaller skull, shorter canines and tufts of lighter fur. There are psychological differences too. Bonobos spend more time having sex, and playing with one another. They’re less sensitive to stress. They’re more sensitive to social cues. And they are far less aggressive than chimps.

Many years back, a young researcher called Brian Hare was listening to the Harvard anthropologist Richard Wrangham expound on this bizarre constellation of traits. “He was talking about how bonobos are an evolutionary puzzle,” recalls Hare. “They have all these weird traits relative to chimps and we have no idea how to explain them.”

But Hare had an idea. “I said, ‘Oh that’s like the silver foxes!’ Richard turned around and said, ‘What silver foxes?’”

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A cat-like animal explodes from the long grass and leaps onto an antelope. Its huge bulk drags the target to the ground and its muscled forelegs pin it down. With two long sabre-shaped canine teeth, it stabs its victim in the throat, just the once, severing its blood vessels and windpipe. Death comes quickly.

The hunter could be Smilodon, a sabre-toothed cat that lived throughout North and South America, around one or two million years ago.

Or, it could be a nimravid, another group of hunters that looked like cats, but belonged to a separate, closely-related family. Some of them had sabre-teeth too, and they wielded these weapons between 42 and 7 million years ago, well before Smilodon or its relatives did.

Or, it could be Barbourofelis, a member of yet another group of sabre-toothed not-quite-cats, which lived between 16 and 9 million years ago. Its long sabres slipped into long grooves in its lower jaw, which looked like it was about to melt away.

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In the Battle of Rorke’s Drift, 150 or so British troops defended a mission station against thousands of Zulu warriors. At the Battle of Thermopylae, around 7,000 Greeks successfully held back a Persian army of hundreds of thousands for seven days. Human history has many examples of a small force defeating or holding their own against a much larger one.

Among animals too, the underdogs often become the victors. One such example exists in the rainforests of Panama. There, capuchin monkeys live in large groups, each with its own territory. The monkeys often invade each other’s land. Numbers provide an obvious advantage in such conflicts, but small groups can often successfully defend their territory against big ones. Unlike human underdogs, they don’t win because of superior tactics or weapons. They win because their rivals are full of deserters.

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It turns out that elephants have a sixth toe. They’ve adapted one of their wrist bones into a strut that supports their giant squishy feet. I wrote about this for Nature (excerpt below), but here’s my full interview with John Hutchinson, the man behind the discovery, and the owner of (probably) the world’s largest collection of frozen elephant feet.

Elephants walk on the world’s biggest platform shoes. Now, John Hutchinson at the Royal Veterinary College in London and his team have found that their footwear also contains hidden stiletto heels.

Even though an elephant’s leg looks like a solid column, it actually stands on tip-toe like a horse or a dog. Its heel rests on a large pad of fat that gives it a flat-footed appearance. The pad hides a sixth toe — a backward-pointing strut that evolved from one of their sesamoids, a set of small tendon-anchoring bones in the animal’s ankle.

This extra digit, between 5 and 10 centimetres long, had been dismissed as an irrelevant piece of cartilage. Almost 300 years after it was first described, Hutchinson finally confirmed that it is a true bone that supports the squishy back of the elephant’s foot. The ones on the hindfeet even seem to have joints.

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You enter a room with two cages. One contains a friend, who is clearly distressed. The other contains a bar of chocolate, which clearly isn’t. What do you do? While a few people would probably go for the chocolate first (and you know who you are), most would choose to free the friend. And so, it seems, would a rat.

Inbal Ben-Ami Bartal from the University of Chicago found that rats will quickly learn to free a trapped cage-mate, even when they get nothing in return, or when there’s a tasty chocolate distraction around. Bartal thinks that the rats conduct their prison breaks because they empathise with one another. This ability to understand and share the feelings of another individual is found in humans, apes, elephants, dolphins and other intelligent animals. It seems that rats belong in this club too.

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The naked mole rat must be one of the strangest mammals in existence. They live in underground colonies like those of ants and bees, with a fertile queen lording over sterile workers. They feel no pain in their skin, they live unusually long lives, they can cope with chokingly low levels of oxygen, and they seem to be immune to cancer. Their sight is poor, they can’t control their body temperature very well, and their teeth jut out beyond their lips. And they look like wrinkled sausages.

Now, just when you thought they couldn’t get any weirder, we can add another bizarre trait to the naked mole rat’s extensive list: they have really rubbish sperm.

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Humans are remarkably fuel-efficient, or at least, our brains are. The lump of tissue inside our skulls is three times larger than that of a chimp, and it needs a lot more energy to run. But for our size, we burn about as much energy as a chimp. We’re no gas-guzzlers, so how did we compensate for the high energy demands of our brains? In 1995, Leslie Aiello and Peter Wheeler proposed an answer – we sacrificed guts for smarts.

The duo suggested that during our evolution, there was a trade-off between the sizes of two energetically expensive organs: our guts and our brain. We moved towards a more energy-rich diet of meat and tubers, and we took a lot of the digestive work away from our bowels by cooking our food before eating it. Our guts can afford to be much smaller than expected for a mammal of our size, and the energy freed up by these shrunken bowels can power our mighty brains.

This attractive and intuitive idea – the so-called “expensive tissue hypothesis” became a popular one. But Ana Navarrete from the University of Zurich thinks she has disproved it.

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In July 2002, an Italian man named Mr Francioni found something strange. Francioni owns a marble-cutting company in the Tuscan town of Pietrasanta, and he had just acquired a block of Egyptian marbleized limestone. After slicing the block into six slabs, he discovered the fossilised bones of an animal within. Fossils weaken the strength of cut stone and many people discard them outright. But Francioni was excited: he thought he had found a dinosaur, and contacted the nearby University of Pisa.

He got through to Giovanni Bianucci. “As soon as I saw the slabs, I realized that the finding was even more important than a dinosaur, at least for me. I study marine mammals,” he says. Bianucci realised that the bones belonged to an archaeocete, one of the predecessors of modern whales and dolphins. The story might have ended there: Francioni already had an offer from a foreign private collector, who wanted to slabs for his living room. Thankfully, Pisa’s local government intervened. They bought the fossils for the University’s Natural History Museum, where they are now on permanent display.

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To round off my brief stint at the Guardian, here’s a piece about a mastodon specimen with what looks like a spear-tip stuck in its rib. This specimen, the so-called “Manis mastodon” has been a source of controversy for several decades. Is that fragment man-made or simply one of the animal’s own bone splinters? Does it imply that humans hunted large mammals hundreds of years earlier than expected, or not?

Having re-analysed the rib in an “industrial-grade” CT scanner, Michael Waters thinks it’s definitely a man-made projectile. He even extracted DNA from the rib and the fragment and found that both belonged to mastodons. So these early hunters were killing mastodons and turning them into weapons for killing more mastodons. How poetically gittish.

Anyway, read the piece for more about why this matters. In the meantime, I want to draw your attention to this delicious tete-a-tete at the end between Waters and Gary Haynes, who doesn’t buy the interpretation. Note, in particular, the very last bit from Waters, which made my jaw drop.

But despite Waters’ efforts, the fragment in the Manis mastodon’s rib is still stoking debate. “It’s not definitely proven that it is a projectile point,” says Prof Gary Haynes from the University of Nevada, Reno. “Elephants today push each other all the time and break each other’s rib so it could be a bone splinter that the animal just rolled on.”

Waters does not credit this alternative hypothesis. “Ludicrous what-if stories are being made up to explain something people don’t want to believe,” he says. “We took the specimen to a bone pathologist, showed him the CT scans, and asked if there was any way it could be an internal injury. He said absolutely not.”

Waters adds, “If you break a bone, a splinter isn’t going to magically rotate its way through a muscle and inject itself into your rib bone. Something needed to come at this thing with a lot of force to get it into the rib.”

The spear-thrower must have had a powerful arm, for tThe fragment would have punctured through hair, skin and up to 30 centimetres of mastodon muscle. “A bone projectile point is a really lethal weapon,” says Waters. “It’s sharpened to a needle point and little greater than the diameter of a pencil. It’s like a bullet. It’s designed to get deep into the elephant and hit a vital organ.” He adds, “I’ve seen these thrown through old cars.”

A shark continually grows new teeth. Those at the front of its mouth fall away, only to be replaced by fresh rows that move forward like conveyor belts. By contrast, we humans only have two sets of teeth. The first falls away during childhood leaving a second set to last us for the rest of our lives. Most mammals are like us, but there are some notable exceptions.

The silvery mole rat of Kenya and Tanzania continually replaces its molars in an unsettlingly shark-like way. New ones sprout from the back of its jaw and slowly make their way forwards. The front ones, having been ground away, are absorbed.

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