The kidneys are large, and consist of many interconnected, closely packed lobes called "renculi". Such lobate kidneys are also found in seals, otters and bears, so it is not clear what adaptive value these may have for life in the water. The dolphin's kidneys also contain special structures which may aid with filtration during diving.
One might expect that dolphins would have a problem getting enough water to stay alive, living as most of them do in a saline environment, and that the kidneys would have a vital role to play. In fact, dolphins get most of the water they need from eating fish. On top of that, the dolphin's skin appears to act as an osmotic membrane, allowing water but not salt to enter its system. This may explain why the kidneys do not seem very specialized.
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