Nephron size in Dipodomys: a comparison of the nephron tubule diameters of kangaroo rats from arid, semiarid, and coastal environments
During the past forty years, several papers have appeared in the literature discussing various aspects of water conservation by desert inhabiting rodents, Howell and Gersh (1935) reported that kangaroo rats (Dipodomys) are able to survive on a diet of dry seeds without drinking free water, in spite of the high diurnal temperatures and low relative humidity of their natural environment. A more extensive study of water metabolism in desert inhabiting species of kangaroo rats by Schmidt-Nielsen and Schmidt-Nielsen (1951) has shown that both behavioral and physiological adaptations are Involved in the conservation of water by Dipodomys. with the most important adaptation being the ability of these animals to produce a highly concentrated urine, thus retaining more water within the body. The kidneys of desert inhabiting kangaroo rats have thus been shown to be much more efficient in the use of water for the production of urine than kidneys of mammals from mesic and aquatic habitats. In attempts to correlate the structure of kidneys to this physiological adaptation for water conservation, Sperber (19^) and Schmidt-Nielsen and O'Dell (1961) have made measurements of the gross structures of kidneys of mammals from varying environments and correlated the measurements with physiological studies of urine concentrating ability. These investigations have shown that the kidneys of desert inhabiting rodents such as Dipodomys and Psammomys have a wider medulla and produce urine of higher concentration than the kidneys of species normally inhabiting mesic and aquatic environments. The wider medulla of the desert inhabiting species allows the nephron tubules to extend a greater distance into the medulla of the kidney, causing the intratubular material to travel a greater distance, thus allowing the counter current multiplier system of the kidney to remove a relatively greater amount of water from the intratubular material and return it to the body.