How tall should a mink cage be? Using animals' preferences for different ceiling heights to improve cage design

Regulations and guidelines assume that taller cages are better for mink, because they permit more diverse postures (e.g. standing upright) and freedom to move. New Canadian Codes of Practice therefore stipulate cage ceiling heights of at least 38 cm, while in Europe cages must be 46 cm or taller. However, minks' food is placed on the cage top. To eat, adults must therefore stand upright, and young mink must even climb (in great contrast to wild mink, who eat their prey on the ground). Furthermore, these new Codes overlook that some North American cages have 'drop-in' nest boxes that restrict vertical space inside the cage. We therefore investigated how ceiling heights affect welfare, testing the hypotheses that mink prefer lower cage ceilings to feed from (Experiment 1), but higher ceilings for performing other behaviours under (Experiment 2). Experiment 1 involved 64 2.5 month-old male-female pairs (in cages 75 L × 61 W cm). Cage height was 46 cm, except for a modified feeding area (15 L × 61 W cm) accommodating four heights: 25, 38, 46 or 53 cm (spanning the range used in Canada and Europe). After mink were habituated to feeding from each height, food was delivered onto all heights and feeding observed, a procedure repeated monthly until animals were adult body length (at 7 months old). Experiment 2 gave 32 adults (half of each sex) free access to a compartment with a moveable ceiling (with one small covered patch mimicking the underside of a nestbox). This was set to 13 cm (approximating the space under a drop-in nestbox) for 16 animals, being progressively raised every 3 weeks by 13 cm; and to 51 cm for 16 mink, being progressively lowered every 3 weeks by 13 cm. How ceiling height affected compartment use was assessed. Experiment 1's results were clear: the highest feeding heights (from which mink could not eat while sitting) were always avoided by females, and by males once 5+ months old (p < 0.01 for all significant contrasts); less food was also apparently consumed from highest heights (p < 0.01 for all significant contrasts). Turning to other activities, Experiment 2's results were more complex: mink showed no overall preferences for any ceiling height, but individual males who tended to stand upright often also used the compartment permitting this posture relatively more (F1,14 = 10.19; p < 0.01); whereas females preferred the lowest, covered heights (F1,14 = 7.66; p < 0.05). These findings provide valuable information for designing and assessing housing systems from an animal-based perspective: important for mink welfare and the social sustainability of fur farming.