Restricted food intake and annual thyroid cycle of spotted munia

Abstract

The spotted munia shows a seasonal cyclicity in total daily food intake as well as in thyroid function. The seasonal decline in thyroid activity is preceded by a 50% reduction in food intake. In January, artificially restricting the food consumption to late summer/fall level (by about 50%; 2.5 g/bird/day over 4 days) led to a significant decrease in conversion of monoiodotyrosine (MIT) to diiodotyrosine (DIT), relative proportions of thyroxine (%T4) and triiodothyronine (%T3) in the gland, and plasma protein-bound iodine after the injection of a tracer dose of I at almost all points studied. The plasma radioiodohormone fraction (%Tx), as measured by Sephadex gel filtration, was also significantly less than that in the adlibitum-fed controls at all the points studied. These results were confirmed later (May) by direct measurement (radioimmunoassay) of hormone levels when plasma concentration of total and free T4 and T3 were found significantly low in similarly treated birds as compared to controls. A 30% restriction in diet (simulating late spring food intake) did not alter percentage thyroidal 131I uptake, MIT/DIT ratio, glandular %T4 plasma PBI, and %Tx but decreased %T3 in the gland. A constant fixed restricted diet (2.5 g/bird!day, ambient conditions) over 8 months suppressed the seasonal variation in some thyroid activity parameters (% 131I uptake, % thyroidal T4 and T3). The declining ambient temperature failed to restore the thyroid of underfed birds to normal winter efficiency. It is concluded that the seasonal lowering of thyroid activity in spotted munia may be partly due to minimal levels of food intake reached in late summer and fall. The increase in thyroid activity in winter may partly result from an increased food intake in response to the low ambient temperature.