• Bonier, Frances
  • Martin, Paul R.
  • Moore, Ignacio T.
  • Wingfield, John C.


Dingemanse et al. suggest that the Cort-Fitness Hypothesis rests upon the idea that ‘the optimal level of resources allocated towards self-maintenance (immediate survival) versus long-term survival and/or reproduction differs across environments, with selection favoring individuals investing in self-maintenance when the environment becomes “challenging”’. This interpretation is incorrect. Instead, the Cort-Fitness Hypothesis rests upon the idea that difficult or challenging conditions lead to low fitness (i.e. low future survival and/or low future reproductive success), and that glucocorticoid (cort) levels can be used as a proxy for the degree of environmental challenge facing individuals and populations. Indeed, this proposed connection between high cort, difficult environmental conditions and low fitness is the main reason why conservation biologists use cort as a proxy for the health of individuals and populations.