(b) The termination of seasonal reproduction (refractoriness). A key aspect of the photoperiodic response is that long days (or short days in sheep) not only induce reproductive maturity but also end it. The gonads suddenly collapse and this has evolved as a means of ensuring each species has an optimal but limited time to breed each year. The term refractoriness is used since the animal becomes refractory to the prevailing photoperiod. The Bristol group's novel finding was that, quite counterintuitively, thyroid hormones are critical for refractoriness to develop and be maintained. This had been tentatively suggested in the Soviet Union prior to WWII but was developed much further by Trevor Nicholls, Arthur Goldsmith and Alistair Dawson. In simple terms removal of the thyroid glands stopped refractoriness developing in starlings (and other birds) as well as sheep, and the animals remained in breeding condition perpetually and were not photorefractory. Thyroid hormone replacement reinstates the refractory state. Importantly birds are hatched in a refractory state but this is ended by removing the thyroid glands (Tony Williams). The research group published many papers on the phenomenon as well as its neural basis across a range of species from sheep to starlings, and the concept has become established as an important part of the photoneuroendocrine pathway.