Since 1 September 2008, keeping a single guinea pig has been a violation of federal law in Switzerland. Not a vet association's guideline, not a shelter's adoption policy — an ordinance: the Tierschutzverordnung (SR 455.1), the country's Animal Welfare Ordinance.
The version that traveled around the world — Switzerland made it illegal to own one guinea pig — is the popular rendering, and it holds up. The mechanism underneath is drier, and better than the headline.
What the law actually says
No sentence in the ordinance mentions a lone guinea pig. Article 13 says: "Tiere soziallebender Arten müssen angemessene Sozialkontakte mit Artgenossen haben" — animals of social species must have appropriate social contact with their own kind. That one sentence does all the work.
The guinea pig part happens in an annex. The ordinance keeps a table — Anhang 2, Tabelle 1 — listing which species count as social, and guinea pigs are on it. General requirement plus table row: keeping one alone violates the ordinance. A law that made headlines worldwide is, structurally, a list entry.
Why the list includes guinea pigs
The science is older than the law. Sachser's 1998 review in Naturwissenschaften makes the case plainly: guinea pigs are social animals, and domestication did not breed it out of them — the domestic guinea pig kept the species' social behaviour repertoire.
The detail that makes it concrete is the talking. In 1976, Berryman published a catalogue of guinea pig vocalizations in Zeitschrift für Tierpsychologie and documented eleven distinct call types — whistles, purrs, chutts, whines, tweets, drrs — each with its own causation and its own function.
And the calls are for each other. Playback experiments published in Behavioral and Neural Biology in 1981 showed that guinea pigs respond to recordings of other guinea pigs' calls. A call is made to be answered. Keep one animal alone and it has an eleven-call repertoire with nobody on the other end.
The widow problem, and the rental market
The requirement has an obvious edge case. Guinea pigs are kept in pairs; when one dies, the owner is left holding exactly the arrangement the ordinance prohibits.
Which is how Switzerland ended up with rental guinea pigs. As the BBC and others reported in 2011, a Swiss breeder named Prisca Küng, in Hadlikon, runs a service that rents out companion animals, so a guinea pig that loses its partner is never left alone. That is the fact worth carrying out of this page: in Switzerland, you can rent a guinea pig so yours doesn't mourn alone. The state wrote loneliness into law, and the law produced a loaner-companion business.
Full disclosure of where this is coming from: I build Curio, an iOS app that serves five short curiosity stories a day, and the Swiss guinea pig law is exactly the kind of story it exists for. If that sounds like your sort of thing, it's here: Curio on the App Store. No pressure either way — but if you keep a guinea pig in Switzerland, it is legally entitled to a friend.