Dynamic state-dependent modelling predicts optimal usage patterns of responsive defences

Abstract

Chemical defences against predation often involve responses to specific predation events where the prey expels fluids, such as haemolymph or gut contents, which are aversive to the predator. The common link is that each predation attempt that is averted results in an energetic cost and a reduction in the chemical defences of the prey, which might leave the prey vulnerable if the next predation attempt occurs soon afterwards. Since prey appear to be able to control the magnitude of their responses, we should expect them to trade-off the need to repel the current threat against the need to preserve defences against future threats and conserve energy for other essential activities. Here we use dynamic state-dependent models to predict optimal strategies of defence deployment in the juvenile stage of an animal that has to survive to maturation. We explore the importance of resource level, predator density, and the costs of making defences on the magnitude of the responses and optimal age and size at maturation. We predict the patterns of investment and the magnitude of the deployment of defences to potentially multiple attacks over the juvenile period, and show that responses should be smaller when the costs of defences and/or predation risk are higher. The model enables us to predict that animals in which defences benefit the adult stage will employ different strategies than those that do not use the same defences as adults, and thereby experience a smaller reduction in body size as a result of repeated attacks. We also explore the effect of the importance of adult size, and find that the sex and mating system of the prey should also affect defensive strategies. Our work provides the first predictive theory of the adaptive use of responsive defences across taxa.

Florivory as an Opportunity Benefit of Aposematism

“…Prior state-dependent life-history models predicted that aposematic herbivores should grow more slowly and mature at a larger size than cryptic herbivores, as observed in a survey of Lepidoptera (Higginson and Ruxton 2010). Our model predicts this outcome when herbivores can also change location.…”

“…A small herbivore must use gathered energy and toxins to grow, defend itself, and mature at the size and time under its control so as to maximize its fitness, here defined as the product of the probability of survival to maturation and the reproductive value at maturation. That is, the herbivore trades off mortality during growth against the benefits of large size after maturation (Higginson and Ruxton 2010).…”

“…Indeed, color and location should often change at the same time. The increasing apparency of larger herbivores, combined with the effect of size on warningsignal effectiveness, implies that OCC is expected when the overall strategy is cryptic-when-small and aposematism-whenlarge (Higginson and Ruxton 2010;our fig. 1).…”

“…Previously, we showed how OCC can arise if size affects detectability and the effectiveness of warning signals (Higginson and Ruxton 2010), and we investigated how accumulating toxin content should influence microhabitat choice . However, how these two influences should combine to determine a combined strategy of microhabitat choice and color change has not been elucidated.…”

“…We use dynamic programming of insect life history to find the fitness advantages of a combination of florivory and aposematism relative to only crypsis and/or folivory. We characterize a herbivore, in line with Smallegange et al (2007), not as sequestering toxins but rather as storing them for responsive defenses (Higginson and Ruxton 2009), such as the regurgitation exhibited by many lepidopteran larvae (Grant 2006). For example, Pieris brassicae stores highly concentrated glucosinolate hydrolysis products in the foregut (Smallegange et al 2007) that are costly to use (Higginson et al 2011;Daly et al 2012).…”