What Is a Mimicry Ring? Several Species Sharing One Warning Sign
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Several unrelated species that are all genuinely unpleasant to eat sometimes converge on the same colour pattern, and each one benefits from the others. That cooperation between species that never interact is one of the tidier results in evolutionary biology.
How the arrangement works
A predator learns to avoid a distasteful prey species by sampling it and associating the taste with its appearance, which costs the prey population a number of individuals eaten during the learning process. If two distasteful species share a pattern, that learning cost is divided between them, since a predator educated by either one avoids both, and the more species share the pattern the smaller the per-species cost becomes. Selection therefore favours individuals resembling the common pattern, which drives convergence, and the result is a set of species with no close relationship wearing nearly identical warning colours. This is mutualism rather than deception, since every participant is genuinely defended and the signal is honest. It is distinguished from the arrangement where an undefended species copies a defended one, which is parasitic, since the mimic gains protection while diluting the reliability of the signal for the model.
What makes it work
Several conditions have to hold for the pattern to be maintained:
- •Genuine unpalatability or toxicity in all participants, otherwise the signal degrades
- •Shared predators that learn by experience rather than by instinct
- •Overlapping ranges and flight times, since a predator only generalises across what it encounters together
- •Conspicuous and simple patterns, since a signal must be easy to learn and remember
- •Enough encounters for learning to occur, which means the species must be reasonably common
- •Selection against intermediates, since an individual with a pattern between two rings is protected by neither
The classic examples
Neotropical butterflies supply the best-studied cases and have become a standard system in evolutionary genetics. Several distantly related groups share wing patterns across large areas of South and Central America, with the same pattern appearing in species from different families, and the patterns change across geography so that a set of species wears one pattern in one region and a different shared pattern elsewhere, with narrow zones where the patterns meet. That geographic mosaic is what makes the system so useful, since it presents the same evolutionary problem solved repeatedly in different places. The genetic work has identified a small number of genes controlling the patterns and has shown that the same genetic regions are involved across species, sometimes through the transfer of genetic material between species by hybridisation, which is a striking result. Bees, wasps and their mimics form comparable rings, as do several groups of beetles and some snakes.
The other kind of mimicry
The contrasting arrangement, where a harmless species resembles a defended one, works by a different logic and has different consequences. The imitator gains protection without paying the cost of being toxic, which is a substantial saving, and it therefore spreads. It also degrades the signal, since a predator that samples an imitator and finds it palatable learns the opposite of what the model needs it to learn. That creates a frequency dependence, since the arrangement works only while imitators remain rare relative to models, and an imitator becoming too common undermines its own protection. Selection therefore pushes the model to change its pattern and the imitator to follow, which produces an evolutionary chase rather than the stable convergence seen among genuinely defended species. Distinguishing the two arrangements in a real system requires testing palatability directly, and species long assumed to be harmless imitators have turned out to be defended.
Where it gets complicated
Real systems are messier than the clean version and the complications are interesting. Many rings contain both genuinely defended species and undefended imitators, so the arrangement is partly mutualistic and partly parasitic, and the proportion of cheats determines how reliable the signal remains. Palatability is a spectrum rather than a binary, with species varying in how unpleasant they are and individuals varying with what they ate, so a ring can contain a gradient. Predators differ in what they have learned and in how much they generalise, and a naive individual bird must learn afresh. Some predators specialise on defended prey and are undeterred. And the geographic structure means a species can belong to different rings in different parts of its range, which complicates the notion of a species having a single strategy. The systems are studied precisely because these complications are tractable.
The takeaway
Sharing one warning pattern divides the cost of educating predators between several defended species, so selection drives unrelated species onto the same colours and every participant gains. That is mutualism with an honest signal, unlike an undefended species copying a defended one. Neotropical butterflies wear different shared patterns in different regions, which is why the system is so heavily studied.