Answer Happy

There are lots of fishes that breathe air, but there are no mammals that breathe water. Why? Discuss the advantages and disadvantages of breathing either medium, and explain why air-breathing could evolve in water-breathers, but water-breathing would not evolve in obligatory air-breathers. Finally, donning the hat of an “intelligent designer", describe an imaginary water-breathing mammal, making sure that its gas exchanger function is sufficient to (quantitatively) match its demand for oxygen.