14.5.1. Batesian mimicry

In these mimicry triangles, each component has a positive or negative effect on each of the others. In Batesian mimicry an aposematic inedible model has an edible mimic. The model suffers by the mimic’s presence because the aposematic signal aimed at the observer is diluted as the chances increase that the observer will taste an edible individual and fail to learn the association between aposematism and distastefulness. The mimic gains both from the presence of the protected model and the deception of the observer. As the mimic’s presence disadvantages the model, interaction with the model is negative. The observer benefits by avoiding the noxious model, but misses a meal through failing to recognize the mimic as edible.

These Batesian mimicry relationships hold up only if the mimic remains relatively rare. However, should the model decline or the mimic become abundant, then the protection given to the mimic by the model will wane because the naïve observer increasingly encounters and tastes edible mimics. Evidently, some palatable but- terfly mimics adopt different models throughout their range. For example, the mocker swallowtail, Papilio dardanus, is highly polymorphic with up to five mimetic morphs in Uganda (central Africa) and several more throughout its wide range. This polymorphism allows a larger total population of P. dardanus without prejudicing (by dilution) the successful mimetic system, as each morph can remain rare relative to its Batesian model. In this case, and for many other mimetic poly- morphisms, males retain the basic color pattern of the species and only amongst females in some populations does mimicry of such a variety of models occur. The conservative male pattern may result from sexual selection to ensure recognition of the male by con- specific females of all morphs for mating, or by other conspecific males in territorial contests. An additional consideration concerns the effects of differential predation pressure on females (by virtue of their slower flight and conspicuousness at host plants) — meaning females may gain more by mimicry relative to the differently behaving males.

Larvae of the Old World tropical butterfly Danaus chrysippus (Nymphalidae: Danainae) feed predominantly on milkweeds (Asclepiadaceae) from which they can sequester cardenolides, which are retained to the aposematic, chemically protected adult stage. A variable but often high proportion of D. chrysippus develop on milkweeds lacking bitter and emetic chemicals, and the resulting adult is unprotected. These are intra-specific Batesian automimics of their protected relatives. Where there is an unexpectedly high proportion of unprotected individuals, this situation may be maintained by parasitoids that preferentially parasitize noxious individuals, perhaps using their cardenolides as kairomones in host finding. The situation is complicated further, because unprotected adults, as in many Danaus species, actively seek out sources of pyrrolizidine alkaloids from plants to use in production of sex pheromones; these alkaloids also may render the adult less palatable.

Chapter 14