Many birds follow a seasonal reproductive cycle — they mate, nest, and raise chicks during specific seasons in a year. While birds that reproduce in spring and summer, like the Japanese quail, activate their sexual organs in response to long days (photoperiods), birds which reproduce in autumn or winter become sexually active in response to long nights (scotoperiods).
Seasonal breeders activate their sexual organs when environmental signals herald the approach of plentiful resources ideal for birthing and raising young ones. Between these bouts of amour and family time, seasonal breeders’ reproductive organs regress and remain inactive. To reactivate sexual organs during breeding season, a neurological and hormonal signaling cascade between the hypothalamus, pituitary gland, and sexual organs comes into play.
In birds whose reproduction is sensitive to long photoperiods, the mediobasal hypothalamus (MBH) in the brain measures photoperiod length. In the MBH, light activates the gene expression of Dio2 (type 2 iodothyronine deiodinase), an enzyme that converts thyroxine (T4) to triiodothyronine (T3). This MBH-produced T3 allows the hypothalamus to release gonadotrophin releasing hormone (GnRH), which stimulates the pituitary gland to produce gonadotropins — hormones that eventually activate the sexual organs. This system is called the ‘deiodinase-mediated pathway’.
But how does a non-photosensitive bird like the spotted munia control its seasonal reproductive cycle? In an effort to answer this question, Vinod Kumar’s group at the University of Delhi and Sangeeta Rani’s group from the University of Lucknow teamed up to investigate if the hypothalamic deiodinase pathway was a general mediator of seasonal reproductive responses, or whether it operated specifically in birds responsive to long photoperiods.
“We found a perfect model system in spotted munia, which, although an autumn breeding bird in nature, has been known to show gonadal stimulation under ultra-short days,” says Ila Mishra, first author of the publication that reports the team’s results.
When the researchers exposed male and female munias to ultra-short days (3 hours of light and 21 hours of dark) for four weeks, they found that the birds’ reproductive cycle was stimulated — sexual organs showed clear signs of revival with the female birds’ ovaries and male birds’ testes growing larger. Munias exposed to ultra-long days (21 hours of light and 3 hours of dark), however, showed no activation of neurological or hormonal pathways connected to reproduction, and their gonads remained shrunken and inactive.
Molecular tests revealed that GnRH levels were elevated in the hypothalamus of birds exposed to ultra-short days but not in those of birds exposed to ultra-long days. This indicated that sexual organ activation in the spotted munia followed the typical neurological and hormonal pathways known till date in photosensitive birds.
However, what came as a surprise, was that hypothalamic levels of Dio2 in these birds did not increase. Unlike birds which respond to long photoperiods, the hypothalamic deiodinase pathway did not seem to be involved in regulating seasonal reproduction in spotted munias.
The team’s work demonstrates that the hypothalamic deiodinase pathway can no longer be considered a universal mediator of reproductive seasonality, as was assumed until now. This raises a plethora of new questions on how scotosensitive birds use day/night lengths to time their reproductive seasons.
“This systematic study points towards an unknown mechanism in the avian neuroendocrine gonadal axis, and that there is much that is yet to be explored and unknown so far in this field,” says C. M. Chaturvedi, an avian neurobiologist from Banaras Hindu University, who is unconnected to this study.
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