Physiological costs in monarch butterflies due to forest cover and visitors

The deleterious anthropogenic impact on animals is often hidden and not revealed in indexes of mortality in populations. Hence, animal monitoring should not only involve population numbers and survival, but also the individual physiological condition. Here we analyzed the physiological condition of the monarch butterfly Danaus plexippus in terms of protein content, immune response (proPhenoloxidase [proPO], phenoloxidase [PO] and lytic activity [LA]) and antioxidant defense (total antioxidant capacity [TAC] and superoxide dismutase [SOD]). We also evaluated survival rates after injecting Staphylococcus aureus. These parameters were compared between 5 different winter aggregations in Mexico (Chincua, Rosario, Malacatepec, Xoconusco and Albarranes). Chincua had the lowest forest cover (43.7%) and it is open for tourism, while Albarranes had the highest forest cover (78%) and had no tourist activities. We found no differences in the wing size, PO nor LA between sites. Albarranes showed the highest protein content. The proPO and TAC measurements were highest at Chincua. In addition, Chincua showed the lowest survival when injected with the bacteria. This suggests that the higher values of immune response and oxidative defense are not indicators of butterfly condition at the overwintering sites. However, forest cover seems to be more important for their survival and physiological condition. We highlight the importance to establish a regular monitoring program of the butterflies condition to detect po-tential hidden cost on its physiology, which may impact not only their survival during the overwintering season, but also during their north migration to their reproductive sites.