Signal honesty and predation risk among a closely related group of aposematic species

Abstract

Many animals have bright colours to warn predators that they have defences and are not worth attacking. However, it remains unclear whether the strength of warning colours reliably indicate levels of defence. Few studies have unambiguously established if warning signals are honest, and have rarely considered predator vision or conspicuousness against the background. Importantly, little data exists either on how differences in signal strength translate into survival advantages. Ladybirds exhibit impressive variation in coloration both among and within species. Here we demonstrate that different levels of toxicity exist among and within ladybird species, and that signal contrast against the background is a good predictor of toxicity, showing that the colours are honest signals. Furthermore, field experiments with ladybird models created with regards to predator vision show that models with lower conspicuousness were attacked more frequently. This provides one of the most comprehensive studies on signal honesty in warning coloration to date.

Figure 1. Toxicity of the different ladybird species analysed in this study.

There were significant differences between all species tested, while controlling for the effects of size and weight. Both the water and the Methanol controls were less lethal to the Daphnia individuals than any of the toxin extracts analysed. Larch ladybirds are the least toxic species followed by the yellow 14-spot ladybirds. Pine ladybirds are more toxic than those previous two species. The two morphs of 2-spot ladybirds are indistinguishable in their toxicity, but less toxic than the orange ladybird, which has the highest toxicity. The notches on each boxplot represent the 95% confidence interval for each species.

Figure 2. Contrast against the species’ preferred background in terms of colour contrast (just noticeable differences; JNDs +/− SD) and its relation to toxicity.

Species with higher toxicity are more contrasting than less toxic species. The only exception is the orange ladybird, which despite being the most toxic species is of intermediate conspicuous.

Figure 3. Correlation between saturation and toxicity within species, the more saturated individuals in each species are more toxic.

These results are true for all species except the orange ladybird. The shaded grey area shows the standard deviation of the general trend line.

Figure 4. Survival probability in wild environments for each of the five colours of models used in this experiment.

Red, orange, and yellow models have higher survival rates than brown and black models. The average predation time across all models and all transects was 28 hours.

Figure 5. Conspicuousness of the artificial ladybird models against natural backgrounds.

Brown models are the least conspicuous, while bright colours such as orange and yellow are highly contrasting. The red and black models showed no differences in their contrast despite their different survival rates.