3/4 and Kleiber's Law
The fraction three quarters has a particular interest to biologists because of its link to the research of Max Kleiber and metabolic rates in animals.
This video features Thomas Woolley from the University of Oxford.
Tom thanks @gameboygenius for pointing out the following: Small mistake. I think where you said they expected MR ∝ r^(2/3) you actually meant to say MR ∝ m^(2/3). If you take the r/V/A relations and solve for A, you get A ∝ V^(2/3). Mass is proportional to volume, so roughly A ∝ m^(2/3). If we then assume MR is proportional to area, the final assumption would be MR ∝ m^(2/3)."