Abstract
It is widely believed that aposematic signals should be conspicuous, but in nature, they vary from highly conspicuous to near cryptic. Current theory, including the honest signal or trade‐off hypotheses of the toxicity–conspicuousness relationship, cannot explain why adequately toxic species vary substantially in their conspicuousness. Through a study of similarly toxic Danainae (Nymphalidae) butterflies and their mimics that vary remarkably in their conspicuousness, we show that the benefits of conspicuousness vary along a gradient of predation pressure. Highly conspicuous butterflies experienced lower avian attack rates when background predation pressure was low, but attack rates increased rapidly as background predation pressure increased. Conversely, the least conspicuous butterflies experienced higher attack rates at low predation pressures, but at high predation pressures, they appeared to benefit from crypsis. Attack rates of intermediately conspicuous butterflies remained moderate and constant along the predation pressure gradient. Mimics had a similar pattern but higher attack rates than their models and mimics tended to imitate the signal of less attacked model species along the predation pressure gradient. Predation pressure modulated signal fitness provides a possible mechanism for the maintenance of variation in conspicuousness of aposematic signals, as well as the initial survival of conspicuous signals in cryptic populations in the process of aposematic signal evolution, and an alternative explanation for the evolutionary gain and loss of mimicry.
Highlights
Animal coloration is a classic example of the power of natural selection in action
We found that background predation pressure contributed significantly to the variance in attack rates on butterflies belonging to the focal mimicry rings
The study provides the first empirical evidence for a possible mechanism that can explain the maintenance of aposematic signal diversity independent of variation in toxicity
Summary
Animal coloration is a classic example of the power of natural selection in action. Conspicuous aposematic (warning) signals are used by toxic animals to communicate their unprofitability to potential predators, while cryptic coloration (camouflage) is used by prey to conceal themselves from predators. We predicted that attack rates on highly conspicuous species would be low compared to less conspicuous species when background predation pressures were low, as a result of innate avoidance of aposematic signals and quick learning by predators (Gamberale-Stille & Guilford, 2003; Lindstedt, Lindström, & Mappes, 2009; Rowe & Guilford, 1996; Ruxton, Sherratt, & Speed, 2004; Schuler & Hesse, 1985). Species in the selected Danainae mimicry rings vary substantially in their degree of conspicuousness These butterflies can be visually separated based on differences in color, pattern, and overall conspicuousness (Figure 2), but share the same habitats, including roost and forage sites (STA’s personal observations), are similar in size (with the wingspan of 70– 100 mm) and have a similar characteristic slow flight. The evolution of their conspicuousness can be reasonably attributed to aposematism, independent of sexual selection
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