14.5.3. Mimicry as a continuum
The strict differentiation of defensive mimicry into two forms — Müllerian and Batesian — can be questioned, although each gives a different interpretation of the ecology and evolution of the components, and makes dissimilar predictions concerning life histories of the participants. For example, mimicry theory predicts that in aposematic species there should be:
- limited numbers of co-modeled aposematic patterns, reducing the number that a predator has to learn;
- behavioral modifications to “expose” the pattern to potential predators, such as conspicuous display rather than crypsis, and diurnal rather than nocturnal activity;
- long post-reproductive life, with prominent exposure to encourage the naïve predator to learn of the distaste-fulness on a post-reproductive individual.
All of these predictions appear to be true in some or most systems studied. Furthermore, theoretically there should be variation in polymorphism with selection enforcing aposematic uniformity (monomorphism) in Müllerian cases, but encouraging divergence (mimetic polymorphism) in Batesian cases (section 14.5.1). Sex-limited (female-only) mimicry and divergence of the model’s pattern away from that of the mimic (evolutionary escape) are also predicted in Batesian mimicry. Although these predictions are met in some mimetic species, there are exceptions to all of them. Polymorphism certainly occurs in Batesian mimetic swallowtails (Papilionidae), but is much rarer elsewhere, even within other butterflies; furthermore, there are polymorphic Müllerian mimics such as the viceroy. It is suggested now that some relatively undefended mimics may be fairly abundant relative to the distasteful model and need not have attained abundance via polymorphism. It is argued that this can arise and be maintained if the major predator is a generalist that requires only to be deterred relative to other more palatable species.
A complex range of mimetic relationships are based on mimicry of lycid beetles (see Plate 6.5), which are often aposematically odoriferous and warningly colored. The black and orange Australian lycid Metriorrhynchus rhipidius is protected chemically by odorous methoxy-alkylpyrazine, and by bitter-tasting compounds and acetylenic antifeedants. Species of Metriorrhynchus provide models for mimetic beetles from at least six distantly related families (Buprestidae, Pythidae, Meloidae, Oedemeridae, Cerambycidae, and Belidae) and at least one moth. All these mimics are convergent in color; some have nearly identical alkylpyrazines and distasteful chemicals; others share the alkylpyrazines but have different distasteful chemicals; and some have the odorous chemical but appear to lack any distasteful chemicals. These aposematically colored insects form a mimetic series. The oedemerids clearly are Müllerian mimics, modeled precisely on the local Metriorrhynchus species and differing only in using cantharidin as an antifeedant. The cerambycid mimics use different repellent odors, whereas the buprestids lack warning odor but are chemically protected by buprestins. Finally, pythids and belids are Batesian mimics, apparently lacking any chemical defenses. After careful chemical examination, what appears to be a model with many Batesian mimics, or perhaps a Müllerian ring, is revealed to demonstrate a complete range between the extremes of Müllerian and Batesian mimicry.
Although the extremes of the two prominent mimicry systems are well studied, and in some texts appear to be the only systems described, they are but two of the possible permutations involving the interactions of model, mimic, and observer. Further complexity ensues if model and mimic are the same species, as in automimicry, or in cases where sexual dimorphism and polymorphism exist. All mimicry systems are complex, interactive, and never static, because population sizes change and relative abundances of mimetic species fluctuate so that density-dependent factors play an important role. Furthermore, the defense offered by shared aposematic coloring, and even shared distaste-fulness, can be circumvented by specialized predators able to learn and locate the warning, overcome the defenses and eat selected species in the mimicry complex. Evidently, consideration of mimicry theory demands recognition of the role of predators as flexible, learning, discriminatory, coevolving, and coexisting agents in the system (Box 14.1).