Opposing selection pressures on insect coloration
By Johanna Mappes.
Insect coloration is a result of several selection pressures among which predation and temperature are the most obvious ones. These selection pressures are typically studied in isolation and possible trade-offs between different selection pressures are not often taken into consideration. Parasemia plantaginis –moth larvae have a phenotypically and genetically variable warning signal: an orange patch on their otherwise black body. Thus, larvae with a small orange signal are darker i.e. more melanic compared to larvae with larger signals.
We hypothesized that although predators select large signalling larvae due to faster avoidance learning, thermoregulation may constrain the signal size in colder habitats. With a full sib rearing experiment with two selection lines (small and large orange signal) and with two temperature manipulations (high and low radiation environment) we showed that temperature constrained the size and the brightness of the warning signal. Individuals developed smaller and darker signals in the low radiation treatment. The development time of the individuals from the small signal selection line in the low radiation treatment was shorter than in the large signal selection line, suggesting that producing a large signal is costly in cold environments.
Thus, temperature can cause environmental variation in the signal size, supporting the idea that the cost:benefit ratio of the warning signals varies spatiotemporally. We also show that the benefits of aposematism as an anti-predatory strategy change seasonally according to the age structure (and thus experience level) of the predator community. Conspicuous warningly coloured prey suffer higher mortality than more inconspicuous prey when naïve predators (fledglings) are common, while the reverse is true before and after the fledgling period. Thus, opposing selection pressures and seasonally changing predation can maintain variation in antipredatory strategies.Other information
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