Pigs roll in mud because they cannot sweat
Pigs have a reputation for being dirty, and it is mostly unfair. A PIG has almost no working sweat glands, so on a hot day it cannot cool down the way a person or a HORSE can. Rolling in mud covers its skin in a cool, wet layer that works like sweat, and dried mud also protects against sunburn and insects. Given the choice, pigs keep their sleeping area clean and use a separate spot as a toilet.
They are also clever. Pigs can learn to use a mirror to find hidden food, and in experiments they have learned to move a cursor on a screen with a joystick. A PIGLET knows its mother's grunts from the first days of life.
More chickens than any other bird
The CHICKEN is the most common bird on Earth, with tens of billions alive at any time — far more than any wild bird species. Every one descends from the red junglefowl, a wild bird of the forests of Southeast Asia that people began keeping thousands of years ago, possibly first for cockfighting and only later for eggs and meat.
A HEN lays an egg roughly every day or so in good conditions, and she does not need a ROOSTER to do it; the rooster only matters if the eggs are to hatch into chicks. The TURKEY, by contrast, comes from the Americas, and it ended up with the name of a country in Asia because Europeans confused it with another bird they bought through Turkish traders.
Sheep never forget a face
In 2017 scientists at the University of Cambridge trained SHEEP to recognise the faces of four famous people from photographs, and the sheep could still pick them out when shown the pictures from a different angle. Sheep also recognise each other's faces and can remember dozens of them for years. A LAMB and its mother find each other in a noisy flock by voice.
A GOAT has rectangular pupils, which give it a very wide view of the world with only a small blind spot behind — handy for spotting predators while grazing. Goats were among the first animals people farmed, around 10,000 years ago in the Middle East.
Cows, calves and a few newcomers
A COW has a stomach with four compartments. It swallows grass quickly, then brings it back up later to chew again, which is called chewing the cud, and it can spend around eight hours a day doing so. A young cow is a CALF, a male is a BULL, and it takes a great deal of grass to make a single glass of MILK.
The LLAMA and ALPACA come from the Andes. Alpacas are kept for their soft fleece, and llamas are sometimes put in fields with flocks of sheep as guards, because they will chase off foxes. The rest of the grid is farm life: the DUCK, GOOSE, DONKEY and FOAL, the TRACTOR, the BARN, the HAY, the FIELD, the FENCE, the STABLE, the WOOL and the FARMER. Our farm jobs page covers the people who look after them all.