Structure and concentrating ability in the avian kidney.

We quantified various aspects of the morphology of the kidney in seven bird species and related these measures to urinary concentrating ability (the concentration of ureteral urine in dehydrated animals) for six of the species. Kidney mass, number of glomeruli, and number of medullary cones all tended to increase with body mass. Smaller birds, with smaller kidneys, had smaller nephrons with smaller glomeruli. Lengths of medullary cones tended to increase with body mass and were exceptionally long in the macaroni penguin. The proportion of nephrons that were mammalian-type (MT, with loops of Henle) ranged from 7% (ring-necked pheasant) to 30% (zebra finch, glaucous-winged gull) and was unrelated to kidney mass. The percent of kidney mass comprised of medullary cones varied from 5 to 13%, unrelated to kidney mass. The number of reptilian-type (without loops of Henle) nephrons associated with each medullary cone tended to increase with kidney mass. Of the variables examined, length of the medullary cones, and hence of the longest loops of Henle, was most strongly correlated with urinary concentrating ability; however, this correlation was negative. Concentrating ability was also strongly negatively correlated with body mass (small birds concentrate better). No significant relation existed between concentrating ability and proportion of MT nephrons. Our data suggest that, in a broad interspecific comparison, the quantitative extent of the renal medulla is not the primary determinant of urinary concentrating ability in birds.