Additive genetic variation, but not temperature, influences warning signal expression in Amata nigriceps moths (Lepidoptera: Arctiinae)

Abstract

Many aposematic species show variation in their color patterns even though selection by predators is expected to stabilize warning signals toward a common phenotype. Warning signal variability can be explained by trade‐offs with other functions of coloration, such as thermoregulation, that may constrain warning signal expression by favoring darker individuals. Here, we investigated the effect of temperature on warning signal expression in aposematic Amata nigriceps moths that vary in their black and orange wing patterns. We sampled moths from two flight seasons that differed in the environmental temperatures and also reared different families under controlled conditions at three different temperatures. Against our prediction that lower developmental temperatures would reduce the warning signal size of the adult moths, we found no effect of temperature on warning signal expression in either wild or laboratory‐reared moths. Instead, we found sex‐ and population‐level differences in wing patterns. Our rearing experiment indicated that ~70% of the variability in the trait is genetic but understanding what signaling and non‐signaling functions of wing coloration maintain the genetic variation requires further work. Our results emphasize the importance of considering both genetic and plastic components of warning signal expression when studying intraspecific variation in aposematic species.

Predator selection on multicomponent warning signals in an aposematic moth

“…We designed signals to match the lower and upper quartiles of this variation, resulting in 15.5% of the wing area orange in the small and 22.1% of the wing area orange in the large spot ( Figure 2a ). Similar variation in wing signals is observed in the moth population on Macquarie University campus ( Binns et al 2022 ) and we expect to find both signal types also in our other study location at Newholme Research Station, although this has not been quantified. The model wings were created from images of real A. nigriceps moths that represented small (15.5.% orange) and large (22.1% orange) wing spots, using Adobe After Effects CS4 ( Christiansen 2013 ).…”

“…Our study demonstrates that different warning signal elements may vary in their salience to predators, and understanding selection pressures for prey warning coloration requires investigating the function of each individual element ( Winters et al 2017 ). We show that the orange abdominal stripes of A. nigriceps are an important warning signal for noisy miners, which could lead to relaxed selection on orange wing spots and provide one explanation for the variation in the wing spot size ( Binns et al 2022 ). However, predators used the wing signal in their foraging decisions when the stripes were not visible, and future work should aim to quantify the visibility of each warning signal element in different contexts to understand their role in predator attack decisions.…”

“…However, in many aposematic species these are combined with other signal modalities, such as odors or chemical secretions ( Rowe and Haplin 2013 ), which can have interactive effects that change predator responses to visual signals ( Rojas et al 2019 ). This might be the case also in A. nigriceps that secrete defensive neck fluids when attacked ( Binns et al 2022 ). These secretions could include odor cues, and further research is needed to understand the potential interactions between different signal modalities, and whether this changes predator responses to different visual elements of A. nigriceps warning signals.…”

“…However, there is no evidence of an association between wing coloration and toxicity in A. nigriceps (Hämäläinen et al in preparation). Similarly, there is no evidence of toxicity differences between the sexes (Binns et al, in preparation), even though females have larger orange wing signals than males ( Binns et al 2022 ). Females and males also differ in the abdominal patterns, with males having one more orange stripe (six stripes) than females (five stripes), but this does not influence the proportion of orange on the abdomen that is similar in both sexes (Binns et al, in preparation).…”

“…The two color patterns also differ in visibility: when the moths are resting, the wings can cover a large proportion of the abdomen, making the stripes invisible. The size of the orange spots on the wings is highly heritable and varies from 10 to 30% of the wing area within and across populations, and females typically have larger orange spots than males ( Binns et al 2022 ). Orange stripes on the abdomen, in contrast, are more consistent, with each sex having a fixed number of stripes in abdominal segments (females: five orange stripes; males: six orange stripes) and low variation in stripe width (Binns et al in preparation).…”