Batesian Mimicry

Overview

In Batesian mimicry, a harmless species (the mimic) evolves to resemble a harmful or unpalatable species (the model) in order to avoid predation. The strategy works because predators that have had a bad experience with the model learn to avoid anything that looks like it. But there's a catch: the effectiveness of mimicry depends on the relative frequency of mimics to models.

If mimics become too common relative to models, predators encounter enough palatable prey in the mimetic pattern that they stop avoiding it. The protection breaks down. This frequency dependence is the key prediction of Batesian mimicry theory, and it's what you'll be testing here.

What You'll Do

You'll take on the role of a field researcher conducting a salamander survey in an Appalachian hardwood forest. Pick a survey date, set up your transects, and start flipping cover objects -- rocks, logs, boards, bark -- along the forest floor. Under each one you might find nothing, a common Red-backed Salamander, or if you're lucky, a Red Salamander or a Red Eft.

When you find a salamander, you'll need to identify it. The Red Salamander (Pseudotriton ruber) closely resembles the toxic Red Eft stage of the Eastern Newt (Notophthalmus viridescens) -- that resemblance is the mimicry, and telling them apart requires careful observation of skin texture, spot patterns, tail shape, and costal grooves. Record your measurements in a field notebook, check your data before leaving the site, and analyze your results to test whether the mimic-to-model ratio in your data supports the predictions of frequency-dependent selection theory.

Learning Objectives

1. Explain the frequency-dependent nature of Batesian mimicry and why mimic rarity is essential
2. Practice systematic field survey protocols: transect setup, cover object surveys, specimen handling, and data recording
3. Distinguish between similar-looking species using morphological characters under realistic conditions
4. Calculate mimic-to-model ratios and apply chi-square tests to evaluate frequency-dependent selection
5. Evaluate data quality, recognize sources of error, and understand detection probability

Animal Systems

• Red Salamander / Eastern Newt -- Forest floor; the red salamander mimics the toxic red-spotted newt
• Viceroy / Monarch Butterfly -- Open meadow; the classic textbook example (though the story is more complicated than most textbooks admit)
• Scarlet Kingsnake / Eastern Coral Snake -- Forest; "red touches yellow, kills a fellow" ring patterns
• Hoverflies / Yellowjacket Wasps -- Garden; hoverflies gain protection from their wasp-like coloration
• Zone-tailed Hawk / Turkey Vulture -- Sky; the hawk mimics the vulture's flight profile to approach prey undetected

Background

Henry Walter Bates described this form of mimicry in 1862 based on his observations of butterflies in the Amazon basin. He noticed that palatable species in certain butterfly families closely resembled unpalatable species from other families, and proposed that natural selection favored the resemblance because predators avoided both equally.

The frequency-dependent aspect was formalized later. If models are common and mimics are rare, most predator encounters with the shared color pattern result in a negative experience (the model is distasteful), so avoidance is maintained. As mimics become more common, the average encounter becomes less aversive, and predators start sampling the pattern again. This creates negative frequency-dependent selection on the mimic phenotype --the rarer you are, the better off you are.

Models can also evolve in response to mimics. If a model population experiences increased predation because mimics have diluted the effectiveness of the warning signal, selection may favor models that look less like the mimics --driving an evolutionary arms race. These dynamics make Batesian mimicry one of the richest topics in evolutionary ecology.