Pharmacological doses of thyroxine simulate the effects of increased daylength, and thyroidectomy, decreased daylength on the reproductive system of European starlings
✍ Scribed by Dawson, A.
- Publisher
- John Wiley and Sons
- Year
- 1989
- Tongue
- English
- Weight
- 518 KB
- Volume
- 249
- Category
- Article
- ISSN
- 0022-104X
No coin nor oath required. For personal study only.
✦ Synopsis
Intact male starlings, thyroidectomized male starlings, and intact male starlings treated chronically with pharmacological doses of L-thyroxine were kept on four different daylengths, 8 hr of light : 16 hr of darkness per day (8L : 16D), 9.5L : 14.5D, 11L : 13D, or 13L : 11D. Changes in testicular size, molt, and plasma thyroxine concentrations were monitored for 18 weeks. In all three treatments, initial testicular growth rates increased with daylength. On all four daylengths, treatment with thyroxine increased initial testicular growth rates above those of intact birds. Thyroidectomy decreased testicular growth rate on 13L : 11D. Intact birds on 13L : 11D, and thyroxine-treated birds on 13L : 11D, 11L : 13D, and some on 9.5L : 14.5D, became photorefractory and molted. The results demonstrate that treatment with thyroxine simulates a long day and, conversely, that thyroidectomy simulates a short day. This may be due to an effect on the photoperiodic clock. However, the changes in plasma thyroxine concentration necessary for this far exceed the normal physiological range. Thus, photoperiodically induced changes in reproductive function are not mediated by thyroxine.