Kidneys of the killerwhale and significance of reniculism

The kidneys of all Cetacea are composed of many small relatively independent kidneys (renicules) containing considerable interrenicular tissue. Although reniculism is not entirely confined to the Cetacea, it is desirable to consider the possible advantage of reniculism to mammals of gigantic size. The kidneys of the killerwhale, Orcinus orca, are compared from this standpoint to the kidneys of diverse mammals.

The specific renal parenchymal mass, glomerular counts, glomerular size, and specific glomerular mass of the killerwhale are measured and compared quantitatively (statistically) with similar data from numerous diverse mammals. Simultaneously, a method is described for enumerating the renicules of a cetacean kidney.

Specific parenchymal mass of a killerwhale adult's two kidneys (0.33%) is close to the expected value for mammals of its adult body mass (2,087 kg). The diameter of the adult's glomerular capsules (153 microm) is strikingly less than that expected from its body mass (regression equation and graph for mammals in general). However, the number of glomeruli per kidney (approximately 100 x 10[6]) is markedly greater than that for mammals of its body mass (regression equation and graph for mammals in general) and is the first such count for a cetacean. The total glomerular mass relative to parenchymal renal mass of the O. orca infant and adult is, nevertheless, 5.5% and 6.0%, respectively, and is thus close to the general mammalian value of approximately 5%.

Organization of a cetacean kidney into numerous renicules does not increase specific renal parenchymal mass or specific glomerular mass. The apparent advantage of numerous independent renicules is the limit that is afforded for length of tubules in the necessarily large kidneys of gigantic mammals.