Does cortisol provide an objective measure of subjective feelings? Maybe not (2007)

McCusker, C., Elwood, R. W., Smith, T. E.

Abstract

Although our results do not negate the use of cortisol as a welfare indicator in Lemur catta, they do highlight the myriad of behavioural and environmental factors in addition to stress that modify cortisol excretion in this species in captivity.