Thyroid involvement in the relative photorefractoriness of Japanese quail, Coturnix coturnix japonica

Abstract

AbstractThyroid involvement in the photosexual response was examined in photosensitive and relative photorefractory Japanese quail. Surgical thyroidectomy was carried out in gonadally regressed adult males held for several months on natural daylengths of 13.5–14.5 h. These birds were considered relative photorefractory (birds that are gonadally regressed at daylengths, 12–15 h, and initially stimulatory for gonadal growth in photosensitive quail). Following thyroidectomy and sham‐operation (controls), one‐half of each group were exposed for 30 days to light:dark (LD) 16:8 and then transferred to LD 13.5:10.5 for another 30 days. The remaining operated quail were exposed for the first 30 days to LD 8:16 and then transferred to LD 13.5:10.5 for another 30 days. In quail examined after the first 30 days, the left testis sizes were greatly increased in both thyroidectomized and control birds held on LD 16:8 but completely regressed in both groups held on LD 8:16. The photosexual responses are characteristic for both photosensitive and relative photorefractory quail held on such daylengths. After the second 30 days, the testes were strongly stimulated in both thyroidectomized and control quail that were transferred from LD 8:16 (first 30 days) to LD 13.5:10.5 (second 30 days). Apparently, photosensitivity had been reestablished during the first 30 days on short daylengths. In quail transferred from LD 16:8 (first 30 days) to LD 13.5:10.5 (second 30 days), the testis sizes were reduced in control birds indicating a relative photorefractory response to this intermediate daylength. However, the testis continued to grow in the thyroidectomized quail transferred from LD 16:8 to LD 13.5:10.5. The results indicate that the thyroid is important in the maintenance of relative photorefractoriness in Japanese quail.