Life history and hormones - Why are hormone levels so different between and among individuals and species?

Individual birds show huge differences in the concentration of hormones such as testosterone or corticosterone. Which environmental, social and intrinsic factors drive the huge differences in hormone levels? And how do these differences affect survival, health, fecundity, and reproductive success of individuals?
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Selected publications of the Hormones and life histories project
12 (Suppl. 1), S7 (2015) DOI
Endocrine mechanisms, behavioral phenotypes and plasticity: Known relationships and open questions. Frontiers in Zoology25 (4), S. 685 - 699 (2014) DOI
Male-to-female testosterone ratios, dimorphism, and life history-what does it really tell us? Behavioral Ecology25 (4), S. 704 - 705 (2014) DOI
Correlated evolution of female and male testosterone-internal constraints or external determinants? A response to comments on Goymann and Wingfield. Behavioral Ecology27 (1), S. 66 - 80 (2013) DOI
The ecology of stress: Effects of the social environment. Functional Ecology63 (4), S. 625 - 633 (2013) DOI
Winning and losing in public: Audiences direct future success in Japanese quail. Hormones and Behavior25 (8), S. 1600 - 1613 (2012) DOI
Independence among physiological traits suggests flexibility in the face of ecological demands on phenotypes. Journal of Evolutionary Biology42 (6), S. 485 - 489 (2011) DOI
Testosterone and year-round territoriality in tropical and non-tropical songbirds. Journal of Avian Biology58 (2), S. 317 - 325 (2010) DOI
Impact of season and social challenge on testosterone and corticosterone levels in a year-round territorial bird. Hormones and Behavior163 (1-2), S. 149 - 157 (2009) DOI
Social modulation of androgens in male birds. General and Comparative Endocrinology157 (3), S. 241 - 248 (2008) DOI
Tropical field endocrinology: Ecology and evolution of testosterone concentrations in male birds. Invited review. General and Comparative Endocrinology51 (4), S. 463 - 476 (2007) DOI
Distinguishing seasonal androgen responses from male-male androgen responsiveness - Revisiting the Challenge Hypothesis. Hormones and Behavior80 (2), S. 228 - 240 (2007) DOI
Hormonal responses to male-male social challenge in the blue tit Cyanistes caeruleus: Single-broodedness as an explanatory variable. Physiological and Biochemical Zoology50 (5), S. 779 - 785 (2006) DOI
Testosterone and corticosterone during the breeding cycle of equatorial and European stonechats (Saxicola torquata axillaris and S. t. rubicola). Hormones and Behavior164 (3), S. 327 - 334 (2004) DOI
Testosterone in tropical birds: Effects of environmental and social factors. American Naturalist67 (3), S. 591 - 602 (2004) DOI
Allostatic load, social status and stress hormones: The costs of social status matter. Animal Behaviour