Sized by the wingbeat

The heaviest bat has a body mass of about 1.5 kg, which is about 10 times lower than the largest living bird species. Why this is so has puzzled scientists working on flight mechanics, since the power requirements increase approximately equally much for bats and birds. The solution lies in the muscular capacity in generating forces that beat the wings in active flight. While birds have one major depressor muscle responsible for a forceful downstroke, bats have several smaller muscles doing that same job. But the total muscle mass is smaller in bats, resulting in a lower maximum wingbeat frequency. When plotting scaling relationships for expected power required to fly, and power available from the flight muscles, it turns out that the power available curve (calculated on the basis of wingbeat frequency) does not increase as steeply as that of power required for flight. Where the two curves cross when plotted against body mass, you have the point of maximum mass for sustainable flight. In bats, this is about 1.5 kg, as shown in a new paper by Ulla Lindhe Norberg and Åke Norberg, of Gothenburg university, published in Journal of Experimental Biology. The same analysis was made earlier by Colin Pennycuick, which fixed the upper size sustained bird flight at about 12 kg. It seems as if birds have more muscle power allowing bigger size than bats. A question that follows is whether the basic bat design, having many flight muscles, prevent evolution of large size than about 1.5 kg, or if there are some additional factors limiting size?