Changes in the length of day throughout the year can be a marker for critical species functions such as migration, hibernation, and mating. These changes have been shown to affect an organism’s morphology and hormones as well as other biomarkers. For example, the thyroid hormones T3 and T4 have previously been demonstrated to have roles in the regulation of photoperiod response in birds. Birds that lack appropriate photoperiod response may not migrate or mate as they normally would.
In a January 2017 paper Mishra et al. examined hypothalamic gene expression along with thyroid hormone levels and morphological changes to track the response of male redheaded buntings to both acute (2 cycles) and chronic (12 cycles) long days.
In the acute experiments, photosensitive buntings that had acclimatized to short day cycles – 8h(L), 16h(D) – were exposed to 2 long day cycles (16h(L), 8h(D)). On day three these buntings were sampled and assessed for fat deposition, body mass, testis volume, plasma testosterone, plasma triiodothyronine (T3) and plasma thyroxine (T4). Further, hypothalamus tissue from the sampled buntings was used to measure gene expression for eya3, tsh, dio2, dio3, gnrh, gnih, cga, ar, cyp19, er and er. In the chronic experiments, 3 groups of buntings were assessed – Those on short days that remained on short days, those on short days who were moved to long day cycles for 12 days, and a group of photorefactory birds who had been on long days for some time and were kept on long days for the duration of the experiment. On day 13 the buntings were sampled and assessed for morphological features, hormone levels and gene expression as described for the acute experiment above.
As might be expected, the birds in the acute experiment showed no morphological changes after exposure to 2 long day cycles, but a significant rise in plasma T4 levels was noted (as measured using Arbor Assays T4 EIA Kit (cat.# K050-H). In the chronic experiment, the birds moved from short days to long days showed increased body mass and testes size as well as increased plasma levels of T4 and testosterone. These changes were absent in both the buntings that remained on the short day schedule, and the photorefactory buntings. Corresponding differences in gene expression were also noted in the chronic experiment.
Studies like Mishra et al above suggest circulating thyroid hormones have the potential to be early indicators of the shift from non-reproductive to reproductive state in birds as a response to lengthening periods of daylight. While it does seem thyroid hormones are involved in all bird species, the early indicator hormone seems to be species specific. Understanding the shift from non-reproductive to reproductive state on both the transcriptional and hormonal levels provides the tools to help us understand the stressors that can impact breeding success and species populations.