Predator lethality, optimal escape behavior, and autotomy

Flight initiation distance is the distance separating predator and prey when escape begins. The optimal flight initiation distance occurs where expected postencounter fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk. In current optimal escape theory, prey die when contacted by a predator. We explore effects of variable lethality, L, the probability of being killed on contact. Optimal flight initiation distance increases as lethality increases, matching expectations that prey should not flee when contact entails no fitness loss but should be increasingly wary as expected fitness loss on contact increases. Addition of lethality improves the ability of optimal escape theory to predict effects of factors affecting escape ability. Autotomy, the voluntary shedding of tails or other expendable parts as a last-ditch defense to permit escape, provides an example. After autotomy, running speed decreases in many prey, lethality increases because autotomy cannot be used again until the lost part has regenerated, and ability to obtain benefits may decrease due to reduced social status and foraging ability. These changes favor longer flight initiation distance but lowered initial fitness after autotomy has the opposite effect. Optimal escape theory including a lethality term clarifies how autotomy may lead to increase or decrease in flight initiation distance depending on the balance of its multiple effects. Effects of additional factors that may alter multiple parameters of the model, including age, sex, reproductive condition, injury, disease, and parasitism are discussed.