Decreased litter size in inactive female mink (Neovison vison): Mediating variables and implications for overall productivity

Abstract

Farmed mink vary dramatically in activity: very inactive individuals rarely leave the nest-box, while others spend hours active daily, often performing stereotypic behaviour (SB). SB typically correlates with increased reproductive output, and inactivity, with decreased output. Our objectives were to determine whether SB or inactivity best predicted litter size (LS), and to test three hypothesized reasons for inactive dams' reduced LS: H1, excess fat; H2, chronic stress (potentially underlying inactivity because fear motivates hiding); and H3, health problems. We assessed time budgets prebreeding, scored body condition visually, conducted health exams, and assessed stress using faecal cortisol metabolites (FCM) and "glove tests" for fear. Results did not support H2 and H3: inactive females were no more fearful than active females (P>0.10), they excreted lower levels of FCM (P=0.033), and were considered healthy. As predicted by H1, inactive females had higher body condition scores (P=0.0001), which predicted decreased LS (P <0.040). However, path analysis determined this was unlikely to mediate the inactivity-LS relationship. Compared with SB, inactivity more consistently predicted both LS (negatively, P ≤ 0.038) and kit weight (positively, P≤0.037). Therefore, decreasing inactivity in farmed mink, rather than increasing their SB or decreasing their body condition should most improve productivity.