Thyroid activity in desert rodents: a mechanism for lowered metabolic rate.

Desert rodents are known to exhibit three different levels of metabolic rate (MR) when related to the mouse-elephant equation. Several species have an MR that is either higher, lower, or similar to that predicted from the equation. The role of thyroid function as a possible mechanism associated with these different MR was examined in 12 species of desert rodents, representing 3 families and 3 different biotic communities. Thyroidal 131I uptake, ratio of neck-to-thigh 131I counts per minute (N/T ratio), thyroxine secretion rate (TSR), and plasma protein binding sites were used as indices of thyroid function. When relating thyroid function to MR, TSR was found to be low in species exhibiting low MR and vice versa. Other thyroid indices did not show a consistent relationship to MR. Also, TSR seems to be related to ecologic distribution: low TSR in desert areas and high TSR in arid woodland and montane forest areas. TSR was found to be correlated to body weight. The low TSR associated with reduced MR represents a useful adaptive mechanism for survival of desert rodents under their natural stressful environment.