ALIMENTARY IODINE SUPPLY AND THYROID META BOLISM IN 2 GERBILLIDS RODENTS, MERIONES-SHAWI-SHAWI AND MERIONES-LIBYCUS, IN THE FIELD AND IN THE LABORATORY

1. Thyroid function was studied in two rodents from south Tunisia livi ng in the same desertic environment, Meriones shawl and M. libycus, in the field and held in the laboratory on a normal iodine (control) or a low iodine diet. - 2. In M. shawl the low-iodine diet induced a 6 fo ld thyroid enlargement, large decreases in the thyroidal contents of i odine, T4 and T3 as well as a decrease in plasma T4. The same low-iodi ne diet increased thyroid and plasma T3/T4 ratios fourfold. Thyroid fu nction in animals in the field was similar to controls indicating an a dequate iodine supply. - 3. In M. libycus the effect of iodine deficie ncy was much less pronounced. Thyroid mass was only doubled and thyroi d metabolism was not significantly modified, except the plasma T3/T4 r atio which was doubled. This indicates a possible pre-adaptation to io dine deficiency. Thyroid function in animals in the field was similar to low-iodine laboratory animals. - 4. M. libycus, which is typically xerophilic, appears better adapted to a low iodine supply than the mor e hygrophilic M. shawl. As thyroid hormones appear essential to surviv al of desert rodent under conditions of hydric stress, this adaptation may be related to the better resistance to water deprivation observed in M. libycus compared with M. shawi.