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Guinea pigs thrive in groups of two or more; groups of sows, or groups of one or more sows and a neutered boar are common combinations, but boars can sometimes live together. Guinea pigs learn to recognize and bond with other individual pigs, and tests show that a boar's neuroendocrine stress response to a strange environment is significantly lowered in the presence of a bonded female, but not with unfamiliar females. Groups of boars may also get along, provided their cage has enough space, they are introduced at an early age, and no females are present. In Switzerland, where keeping a guinea pig without a companion is illegal, a service to rent guinea pigs (to temporarily replace a deceased cage-mate) is available. Sweden has similar laws against keeping a guinea pig by itself.