A Theory of the Survival 

 Value of Motility 



FRANCIS D. CARLSON 



Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 



The widespread occurrence of motile forms strongly suggests that 

 the ability to move has, in some species at least, increased their chance 

 of survival. Certainly it can be argued that, in general, motile forms 

 can escape from predators more readily, collect food more rapidly, 

 and find a mate more frequently than their nonmotile counterparts. 

 As a consequence of these improved capabilities, their chances of 

 survival are greater. These are reasonable and plausible explanations 

 of the survival value of motility, but they lack decisiveness. I shall 

 present here a heuristic approach to the development of the relation- 

 ships which describe the energetics of motility of organisms which 

 move through liquids at Reynolds numbers considerably less than 

 one. The relationships will then be used as a basis for understanding 

 the survival value of motility. Since, for the most part organisms whose 

 movements are characterized by low Reynolds numbers are Protozoa 

 or microorganisms, they will be referred to as such in what follows. 



To begin with, we shall eliminate from further consideration all 

 hypotheses of the basis of the survival value of motility except the one 

 that motility increases the rate of food collection and, hence, the rate 

 of energy accumulation by the organism. This does not imply that the 

 other hypotheses are not applicable in some cases, for no doubt they 

 are. It is clear, however, that they lack generality. Many microorgan- 

 isms do not mate, and motility can hardly be expected to increase 

 the chances of mating if it never occurs. Escape from a predator or 

 an unfavorable environment presupposes that the individual is aware 

 of the fact that he is being chased or that he is in an unfavorable 

 environment. It is unlikely that all Protozoa and microorganisms can 



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