Fig. 2

Plasma concentrations of testosterone (c and d), triiodothyronine (e and f), and thyroxine (g and h) in ganders under two artificial photoperiods. Vertical bars indicate standard error of the mean. The artificial photoperiod treatment (a) consisted of a short photoperiod of 8 h (8L:16D) for 56 days from March 1, 2015 until April 25, 2015, followed by a long photoperiod of 12 h (12L:12D) for 218 days from April 26, 2015 until November 29, 2015, and finished with a long photoperiod of 16 h (16L:8D) from November 30, 2015 until the end of the experiment, on January 6, 2016. Another artificial photoperiod treatment (b) consisted of a short photoperiod of 8 h (8L:16D) that lasted for 86 days from March 1, 2015 until May 29, 2015. The second phase of a 12-hour daily photoperiod (12L:12D) lasted for 184 days from May 30, 2015 until November 29, 2015. The last photophase consisted of an 8-hour (8L:16D) short photoperiod for 38 days from November 30, 2015 until January 6, 2016. The long photoperiod treatment consisted of natural illumination during the daytime plus supplementary illumination (80–100 lux) by fluorescent tubes at times after sunset and before sunrise. Star: high hormone concentrations; Lace box: low hormone concentrations