Guinea pigs are highly social animals whose welfare is seriously compromised by solitary housing. In Switzerland, it is illegal to keep a guinea pig alone. The UK has no such law, but welfare evidence is unambiguous.
A lone guinea pig experiences chronic psychological stress even in a technically enriched environment. Without conspecifics, guinea pigs exhibit stereotypic behaviours and elevated stress hormones. Rescue centres advocate for same-sex pairs or neutered mixed pairs as the minimum welfare standard for pet guinea pig housing.