U.S. zoologist, one of the first scientists to study the behaviour of individual microorganisms and to experiment with genetic variations in single-celled organisms.

Jennings graduated from Harvard University (1896). He wrote his doctoral thesis on the morphogenesis of rotiferans (microscopic aquatic organisms), an area of scientific interest he pursued for the next 10 years. The culmination of his research and his primary contribution to zoology was his Behaviour of the Lower Organisms (1906). In this study of the reactions of individual organisms and individual response to stimuli, Jennings reported new experimental evidence of the similarity of activity and reactivity in all animals, from protozoans to man.

For the next 40 years of his career Jennings studied the mechanisms of inheritance and variation in single-celled organisms. His works from this period include Life and Death: Heredity and Evolution in Unicellular Organisms (1920), Genetics (1935), and The Biological Basis of Human Nature (1930).