What a Guinea Pig's Body Expects

Part One · A grazer's design

On my first day volunteering at a small-animal rescue, the manager, Ms. Delgado, gave me a job that sounded too easy: sit by the guinea pig pen and watch for ten minutes. I expected to be bored. Instead I noticed that nothing in the pen ever stopped. The animals nibbled, wandered, nibbled again, and napped only in brief snatches. Later I learned why. Guinea pigs are active for up to twenty hours a day and sleep for short periods only, and an animal that spends that long awake is not idling. It is working, and the work, I came to understand, is eating, steadily, from morning until the lights go down.

A veterinary reference describes guinea pigs as obligate herbivores and hindgut fermenters, evolved to eat a high-fiber, low-nutrient, poorly digestible diet. In plainer words, their gut expects a steady flow of tough plant material that is processed slowly, so the guinea pig must keep grazing. That is why every care source I read puts hay first. The RSPCA says good hay should make up the majority of the diet and be available at all times, and the Merck Veterinary Manual suggests a daily serving of hay about the size of the guinea pig's own body. Pellets and fresh produce come after that, as supplements to the hay rather than as replacements for it.

Hay does a second job, and it shows up in the mouth. A guinea pig's incisors, the long front teeth, grow continually. Chewing fibrous hay and gnawing on untreated wood wears them down, and the wrong diet can bring on serious dental disease. This is why Ms. Delgado, who had run the rescue for many years, checked the front teeth and nails of every animal each week, as the RSPCA advises. It is also why she never let a volunteer trim anything: the RSPCA says only vets should correct overgrown or misaligned teeth, and the weekly check is meant to catch a problem early so that a vet can look at it.

Part Two · The missing enzyme

Then there is the vitamin that guinea pigs cannot make. They lack the enzyme, L-gulonolactone oxidase, that most mammals use to build vitamin C, which is also called ascorbic acid, from glucose. The RSPCA adds that they can store only a little at a time. Without a regular supply they can develop scurvy, a serious and painful condition. Ms. Delgado handed me a bell pepper and said, "This is vitamin C, and it is also lunch." Parsley and bell peppers are strong sources. Kale and spinach are trickier, because the RSPCA lists them as occasional treats, citing their sugar or calcium, while Merck counts them among the high-vitamin-C greens, so we followed the cautious list.

Pellets are a second source, but they come with a catch. Vitamin C breaks down over time, faster when air reaches it. Woodgreen, a British charity, says the vitamin C in an opened bag will halve within three months, and the RSPCA tells owners to give a fresh portion each day rather than topping up the bowl. The Merck manual notes that many pellets now use a stabilized form that holds up better. It also advises against adding vitamin C to the water, because the vitamin needs frequent replacing and the taste may put the guinea pig off drinking. Every fix has a cost, and a careful owner weighs them.

Variety may matter in a quieter way. In a 2025 study cited by the Merck manual, guinea pigs offered seven to ten, or more than ten, kinds of produce were significantly less likely to develop uroliths, which are urinary stones. I like that sentence for what it does not say. It is one study. It suggests a link between a varied diet and a healthier urinary tract, but it does not prove that ten vegetables will protect your own pet. Ms. Delgado's answer was practical: rotate the produce, change foods gradually, and leave the interpretation of studies to the vet, who can fit the evidence to the animal in front of them.

Part Three · A prey animal's secrets

Everything else about the pen made sense once I remembered that a guinea pig is prey. The RSPCA says guinea pigs feel vulnerable in open spaces and like to keep in contact with solid surfaces, so each pen had tunnels and houses, with hay piled deep enough to burrow into. Merck gives a rule that I now repeat to every new volunteer: more hiding spots than guinea pigs, so three animals need at least four. Many of their natural predators are birds, which is why a hand descending from above can set off alarm. I learned to crouch, approach from the front, and wait, and I learned to hold an adult with one hand supporting the hindquarters and the other around the chest.

The hardest fact is also the quietest. According to the RSPCA, guinea pigs feel pain but show no outward signs of it, so they may suffer before anyone notices they are sick. I suspect that a prey animal advertising weakness would be easy to hunt, although the sources record the behavior and not the reason. For a person, the practical lesson is that the evidence is not what the animal says but what it stops doing. Ms. Delgado kept a short list taped to the pen: not eating, quieter than normal, hiding more than usual. Any one of them meant telling a grown-up, a phone call, and a vet, because by the time a guinea pig looks obviously ill, the illness may have been under way for some time.

Part Four · Where the sources disagree

The honest surprise, once I started comparing pages, was how often reliable sources differ. On space, the Merck manual recommends a minimum of 7.5 square feet for one guinea pig and two to four more per additional animal, while the RSPCA gives no number and says only that the space must let the animals exercise and stand fully upright. On temperature, the RSPCA calls 17 to 20 degrees Celsius ideal and warns that above 26 can cause heatstroke, while Merck calls 18 to 26 a range most guinea pigs tolerate and most owners find comfortable. Ms. Delgado's rule was that when two good sources differ, we follow the more cautious reading and ask the vet, and that we write the disagreement down instead of pretending it is not there.

Bedding is another example. The RSPCA suggests dry, dust-free wood shavings with hay on top, while Merck advises avoiding shavings because aromatic oils may irritate the airways, and lists recycled paper products or fleece. Both want bedding free of loose strands that can tangle, and the RSPCA specifically rules out cotton wool. Even something as simple as water divides opinion: Merck found that guinea pigs drink more from a bottle, but Woodgreen notes that many prefer a bowl and that bottles can leak. None of this means the sources are bad or that the animals are fragile. It means that animal care is still an evidence-gathering job, and a careful owner reports what is known and what is not.

Part Five · What welfare means

Company came last, because it is the hardest to get right. The RSPCA says a guinea pig should live with at least one other friendly guinea pig, introduced gradually and under supervision. Introductions are never rushed. Pairs sort out a pecking order, and a pairing succeeds more by personality than by sex. Another species is not a substitute, and rabbits in particular are not, since the RSPCA says the two species have different needs and communicate differently. Ms. Delgado had a quiet corner for every shy animal, because the pen has to give each animal a way to be alone while still being together, and because there must be enough shelter and food for every animal at the same moment.

What does welfare mean, then? I think it means a good deal more than simply avoiding illness. It means living in a body and a home that match what the animal evolved to do: graze, hide, gnaw, tunnel, and stay with others. It also means some breeds carry exaggerated features, such as long coats that mat easily, that the RSPCA says can reduce quality of life. A grown-up decides about adoption, and the RSPCA suggests that rescues are worth considering. I washed my hands after every shift, because the NHS advises it after touching animals, their food, or their cages. Ask a grown-up to look at Peabody Pets with you. You will find it at pet.peabody.kids.