Long-term quantification of faecal glucocorticoid metabolite concentrations reveals that Mexican grey wolves may habituate to captivity

Mexican grey wolves are considered the most endangered wolf in the world. The aim of this longitudinal study was to assess the physiological stress level of 24 captive-born wolves confined at four zoos and two parks, by measuring faecal glucocorticoid (fGC) metabolite concentrations in 1005 samples, for 1 week in every season of the year over a 2-year period. Total overall mean fGC concentration of individual scats was 153.83 ± 8.16 standard error mean (SEM) ng/g dry matter (DM). Paired samples from the same individuals showed a decline in fGC levels between years (173.09 ± 12.15 and 135.94 ± 10.93 for 2010 and 2011, respectively; P = 0.02). Significant differences were also observed between reproductive and non-reproductive seasons (116.78 ± 12.48 vs. 173.20 ± 10.50, respectively; P < 0.0001), and between genders (males: 214.53 ± 13.43 vs. females: 121.51 ± 10.03; P < 0.0001). Tukey’s post hoc comparisons showed that elder wolves excreted higher levels than young adults and reproductive adults (P = 0.016). High-social-ranked wolves showed higher (P < 0.0001) fGC levels (202.03 ± 19.94; n = 229) than medium- (175.33 ± 19.94 ng/g DM; n = 214) and low- (162.06 ± 16.15; n = 377) ranked wolves did; however, no correlation was found between social rank and fGC levels. Anthropogenic acute stressors during husbandry procedures a day before sampling resulted in considerable elevations of fGC concentrations above 1270.34 ng/g DM, returning to baseline levels after 2 days. Our results suggest that wolves are becoming habituated to confinement at the zoos; these findings may contribute to a more comprehensive definition of confinement, which has traditionally been perceived as a stressful habitat for wild animals. Using fGC concentrations as an overall physiological state in long-term studies may provide crucial information on the resilience of captive animal populations