Abstract
Predator-induced plasticity in life-history and antipredator traits during the larval period has been extensively studied in organisms with complex life-histories. However, it is unclear whether different levels of predation could induce warning signals in aposematic organisms. Here, we investigated whether predator-simulated handling affects warning coloration and life-history traits in the aposematic wood tiger moth larva, Arctia plantaginis. As juveniles, a larger orange patch on an otherwise black body signifies a more efficient warning signal against predators but this comes at the costs of conspicuousness and thermoregulation. Given this, one would expect that an increase in predation risk would induce flexible expression of the orange patch. Prior research in this system points to plastic effects being important as a response to environmental changes for life history traits, but we had yet to assess whether this was the case for predation risk, a key driver of this species evolution. Using a full-sib rearing design, in which individuals were reared in the presence and absence of a non-lethal simulated bird attack, we evaluated flexible responses of warning signal size (number of orange segments), growth, molting events, and development time in wood tiger moths. All measured traits except development time showed a significant response to predation. Larvae from the predation treatment developed a more melanized warning signal (smaller orange patch), reached a smaller body size, and molted more often. Our results suggest plasticity is indeed important in aposematic organisms, but in this case may be complicated by the trade-off between costly pigmentation and other life-history traits.
Highlights
Organisms live in a constantly changing environment, and this variation may have important effects on an individual’s fitness
In this study we investigate to what extent predator-induced plasticity explains the continuous variation in larval warning coloration, and how predation risk influences larval life-history traits
We examine any changes in the length of the orange patch, as well as larval growth and development in the presence versus absence of this predation risk
Summary
Organisms live in a constantly changing environment, and this variation may have important effects on an individual’s fitness. Locusts display a shift in coloration as a response to local population density: at low-densities, individuals are solitary and cryptic (green), whereas at high-densities individuals shift to a more gregarious morph with aposematic coloration in the longer wings (yellow and black form) (Sword, 1999) This plasticity is suggested to reduce the cost of conspicuousness when locusts are rare and facilitate predator avoidance learning at high densities. If predatorinduced plasticity plays a role in the size of the warning signal, we should expect differences in the length of the orange patch between the two predation treatments; this could be represented as a warning signal increase, as a more salient signal facilitates learning avoidance of avian predators, or as a warning signal decreases, as more melanic signals benefits from concealment when at high risk of detection or attack by naïve predators. We predict predation to have a negative effect on larval body size and developmental time (shorter developmental time with fewer instars) as risky environments seem to promote negative life-history shifts in many insects (e.g., Duong and McCauley, 2016)
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