Housing systems are the physical and social arrangements in which captive animals are kept, characterised by enclosure type, flooring, space allowance, group size and composition, and provision of shade, ventilation and enrichment [1][2]. Because systems differ in several attributes, welfare is often compared between whole systems rather than single variables, and systems with similar attributes tend to produce similar outcomes: access to litter and soil raises exposure to disease and parasites, whereas enclosures that restrict movement favour osteoporosis [1]. Evidence from companion animals shows that design matters: communal housing of unfamiliar cats produces higher stress scores than discrete units [3], while shelter dogs housed with conspecifics show fewer displacement activities and lower faecal cortisol metabolites than dogs housed alone [4].