Seawater can keep a cat hydrated, but it will dehydrate a human. The difference lies in kidney design, not in sheer tolerance for salt. Feline kidneys can push salt concentration in urine beyond the level found in seawater, something human kidneys cannot match.
The key is renal medulla structure and the countercurrent multiplication system in the nephron. In cats, this architecture creates a steep osmotic gradient that drives intense water reabsorption from forming urine. As water moves back into the bloodstream, sodium and chloride remain trapped in the tubular fluid, producing urine that is denser than seawater. Human kidneys, with a lower maximal urine osmolality, hit a limit earlier, so excess salt must be excreted by pulling extra water out of the body, leading to net dehydration.
This renal advantage builds on the evolutionary history of cats as desert-adapted carnivores, whose baseline ability to conserve water already exceeds that of humans. Their glomerular filtration rate and tubular reabsorption patterns are tuned for water scarcity, making seawater just another extreme test of osmoregulation that their kidneys can pass where ours fail.
