Guinea Pig
Like people, it cannot make its own vitamin C and has to eat it. A herd animal from the Andes, and it whistles the moment it hears the fridge.
Overview
Guinea pigs cannot make their own vitamin C, exactly like people, and everything about feeding one follows from that. An adult needs somewhere between 10 and 25 milligrams per kilo of body weight a day, more if pregnant or ill, and without it they get scurvy: poor appetite, painful swollen joints, bleeding, and eventually death.
The catch is that the vitamin C in pellets degrades with heat, light and time, so a bag that has been open for months is not doing what the label says. Fresh vegetables and a stabilised pellet together are the reliable route. They are herd animals that live five to eight years, and they are prey animals to the bone, which shapes everything else about how they behave.
Temperament
Vocal, timid, and much more sociable with their own kind than people expect. Guinea pigs whistle for food, rumble at each other and shriek when startled, and a pair that has settled in will keep up a running conversation.
They should not live alone. Company is a need rather than a bonus, and a single guinea pig in a quiet room spends most of its time hiding. They also freeze rather than flee when frightened, and a guinea pig that has gone completely still is not calm.
Care and grooming
Unlimited grass hay is the base of the diet, then a stabilised pellet, then fresh vegetables every day for the vitamin C. Bell peppers and leafy greens do that job well.
Teeth grow continuously and hay is what wears them down, so a guinea pig off its hay is heading for dental trouble quickly. Long haired breeds need brushing several times a week, and every guinea pig needs its weight checked, since weight loss is usually the first sign of anything wrong.
Living and exercise
Floor space, not levels. They do not climb and they do not jump, so a tall cage is wasted on them, and roughly 7.5 square feet is the working minimum for a pair with more being better.
They need to be able to run in a straight line, and they need at least two hides so nobody gets cornered. Keep them out of damp and draughts, and out of anything over about 80 degrees F, because they overheat easily and cannot tell you until it is serious.
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