What Is The Only Mammal That Can Not Swim?

　　Most baby mammals can't swim.  But assuming you only mean non-disabled adults.More than one species of mammal cannot swim.

　　Orangatangs for instance.  They simply don't have enough body fat, and their centre of gravity is too high in the bodies.  They sink if they try.  Same is true of Gorillas.Some chimpanzees can swim, but they usually hate it and have very low body fat, too.  So it's almost an individual thing with them; in general, you would say that chimpanzees can't swim.  Ditto for some species of monkeys.

　　Most armadillos appear to be unable to swim; unusually, the long-nosed armadillo of South America can swim fairly well.

　　Giraffes can't swim, either. You can imagine why they just never developed the ability, or lost any ability they ever had.Interestingly, most big whales don't seem capable of swimming backwards.Most other mammlas can swim, although reluctantly and badly in many cases.  Hamsters, Sloths, Koalas, All types of Cats, Kangaroos, Camels, most monkey species all come under the "reluctant but can do it" category.

　　Weirdly enough, rats are supposed to be good swimmers.  But my pet rats hate it!

Kangaroos can swim, and have been known to indulge in water play. There are many instances where kangaroos have been observed swimming to offshore islands, particularly in southern Australia.

Sloths can swim quite well.... They can move much faster in the water than in the trees
source - Planet earth DC

Orangutan

Leopards, lions, tigers, cheetahs, cat family