Raymond Pearl is, also, known for his population biology work, as in his 1928 volume, "The Rate of Living: Being an Account of Some Experimental Studies on the Biology of Life Duration." In this book, he presents extensive research regarding population density effects on life duration in fruit flies, demonstrating that an optimal population density existed for that insect in his experimental model. This raised the question of whether or not the same effect might not occur in other species, including humans. His work demonstrating longer lifespans for flies with lower metabolic rates, also, raised the question of whether or not a similar phenomenon might be found in other species, including humans. Thus, he became a mentor to John B. Calhoun, famous for his ecological studies of rodent populations and their possible importance for modern humans. Population density effects on duration of life is thought, by population biologists, to be Raymond Pearl's greatest contribution to biological science.