How Color Morph Frequency Affects Predation Risk in an Aposematic Moth

By: Abby Campbell, Molly Cannon, and Marissa Freitas (Stonehill College, BIO323: Evolution, Spring 2022)

Safety in numbers: How Color Morph Frequency Affects Predation Risk in an Aposematic Moth is an article with the main purpose of understanding polymorphic warning signals in aposematic systems. This article was published by the American Naturalist in July 2021, written by Swanne P. Gordon, Emily Burdfield-Steel, Jimi Kirvesoja and Johanna Mappes. Aposematic is defined as a defense strategy that combines a primary warning signal (aposematic) with a secondary chemical defense. This study evaluated how bird behavior influenced the survival of three morphs of the aposematic wood tiger moth (Arctia plantaginis) that coexist in the same environment to a predator located from the same area. The three morphs of the moth were white, yellow, and red/ orange coloration. It calls attention to the need to understand predator foraging in natural environments with variable prey defenses to better examine how behavioral interactions between predator and prey affect evolutionary change.

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It is theoretically expected that populations with aposematic signaling should drive the population to have no variation in morphology. Instead, we see without selective pressure, morphs from small populations are expected to be removed through genetic drift. However, there are many examples of warning signal variation or polymorphism across a variety of populations. This contradiction could explain the selective forces behind diversity in nature, especially in more specific cases where warning signal variation exists at the population level. There were three main questions addressed in this study: 1) Do birds bias their decision to attack prey based on differences in morph frequency on a small spatial scale, 2) what predator characteristics affect these decisions, and 3) does the order of attack matter (that is, does preying on a particular morph influence subsequent choices)? The questions being answered in this study would provide evidence to whether polymorphism in warning coloration is a contradiction or if it should be considered a normalcy.

Figure 1 from the article, showing (A) a schematic of the experimental setup of a control trial and (B) a picture of the actual grid setup before the experiment in the aviary.

To perform this experiment, freeze-killed moths were separated into four different treatments and placed on the floor of an aviary as bait. The control treatment contained eight white males, eight yellow males, and eight orange/red females. The red bias treatment contained six white males, six yellow males, and twelve red females. The white bias contained twelve white males, six yellow males, and six red females. Finally, the yellow bias contained six white males, twelve yellow males, and six red females. Great tits (Parus major) a specific bird species was trained to eat out of Petri dishes prior to being in the aviary with the moths. Before observation, the birds were deprived of food for 1-2 hours beforehand. When in the aviary, the birds’ timing, order of moths eaten, and rejection or acceptance of moths were recorded. This behavior was observed after the bird had attacked twelve moths or when two hours had passed. Overall, there were ten trials for each treatment, totaling forty trials, and no birds were used twice in these trials. To better understand the results, having a positive frequency-dependent selection means there is a strong relationship between the moth’s morphs and survivability. Using real individuals allowed for clear observations of predator attacks and behavior. Now diving into the results, evidence for frequency dependent selection was found in the wood tiger moths’ but the results were not consistent between the three morphs. Analyzing each treatment separately, in the control treatment the red morph moths had a higher probability to survive while the white moths were eaten more frequently. In the red bias treatment, the results were remarkably similar with the yellow morph moths slightly surviving more and the red morph moths slightly being eaten more. The white bias treatment resulted in the white morph moth’s probability of surviving being extremely high and the red morph moths were eaten more frequently. Lastly, in the yellow bias treatment, the yellow morph moths resulted in having a higher probability of surviving while the red morph moths were eaten more frequently by the great tits.

Figure 2 from the article, showing the probability of surviving (not being attacked) to the end of a trial by moth morph and treatment. Error bars represent standard errors. R = red; W = white; Y = yellow.

An important side note of the study that was taken into consideration was the bird characteristics measured. These bird characteristics measured did not affect the overall moth survival score. The bird’s characteristics also did not affect what was the first moth chosen to be eaten by the bird. However, the moths in the experiment were recently frozen; hence, they could not attempt to avoid a predation attempt behaviorally, which could have skewed the results.

The theoretical assumption of warning signal polymorphisms (the moths having multiple warning colorations) is that predators should learn to avoid common colorations in a population of their prey. This study assessed this assumption of positive frequency-dependent selection, resulting in evidence for this assumption but also showing that other mechanisms allowed the ability of multiple colors or morphs to be present in aposematic populations. The study found great tits show behavioral differences according to both age and sex. The behavioral differences included the male great tits at adult and juvenile stages and females at juvenile at a juvenile stage did have some hesitation in attacking certain aposematic prey compared to female mature great tits. Although variation in frequency did influence the attack risk on the different morphs, the survival of assorted color morphs did not respond equally to these changes in frequency.

This study highlights that positive frequency-dependent selection is important in aposematic systems, but it is not the only process that shapes survival outcomes. Characterizing frequency-dependent selection under more conditions is therefore an important necessity toward understanding the evolution of warning signals under dynamic environments.

Article: Gordon, S.P., E. Burdfield-Steel, J. Kirvesoja, and J. Mappes. 2021. Safety in numbers: How color morph frequency affects predation risk in an aposematic moth. The American Naturalist: 198: 128–141.

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