Selection for increased safety factors of biological structures as environmental unpredictability increases

Theory predicts that selection should increase the ratio of the performance of a biological structure or system to the requirements placed upon it (that is, its safety factor) as conditions become increasingly unpredictable. Although conventional safety factors are rarely measurable, an alternative, truncation safety factor (the ratio of mean strength to maximum possible load), can be measured quantitatively for certain load-bearing structures. For intertidal limpet shells subject to prying forces, truncation safety factor was found to increase with increased variability in shell strength, thus providing direct support for the theory.