14.5. Defense by mimicry

The theory of mimicry, an interpretation of the close resemblances of unrelated species, was an early application of the theory of Darwinian evolution. Henry Bates, a naturalist studying in the Amazon in the mid-19th century, observed that many similar butterflies, all slow-flying and brightly marked, seemed to be immune from predators. Although many species were common and related to each other, some were rare, and belonged to fairly distantly related families (see Plate 6.4). Bates believed that the common species were chemically protected from attack, and this was advertised by their aposematism — high apparency (behavioral conspicuousness) through bright color and slow flight. The rarer species, he thought, probably were not distasteful, but gained protection by their superficial resemblance to the protected ones. On reading the views that Charles Darwin had proposed newly in 1859, Bates realized that his own theory of mimicry involved evolution through natural selection. Poorly protected species gain increased protection from predation by differential survival of subtle variants that more resembled protected species in appearance, smell, taste, feel, or sound. The selective agent is the predator, which preferentially eats the inexact mimic. Since that time, mimicry has been interpreted in the light of evolutionary theory, and studies of insects, particularly butterflies, have remained central to mimicry theory and manipulation.

An understanding of the defensive systems of mimicry (and crypsis; section 14.1) can be gained by recognizing three basic components: the model, the mimic, and an observer that acts as a selective agent. These components are related to each other through signal generating and receiving systems, of which the basic association is the warning signal given by the model (e.g. aposematic color that warns of a sting or bad taste) and perceived by the observer (e.g. a hungry predator). The naïve predator must associate aposematism and consequent pain or distaste. When learnt, the predator subsequently will avoid the model. The model clearly benefits from this coevolved system, in which the predator can be seen to gain by not wasting time and energy chasing inedible prey.

Once such a mutually beneficial system has evolved, it is open to manipulation by others. The third component is the mimic: an organism that parasitizes the signaling system through deluding the observer, for example by false warning coloration. If this provokes a reaction from the observer similar to true aposematic coloration, the mimic is dismissed as unacceptable food. It is important to realize that the mimic need not be perfect, but only must elicit the appropriate avoidance response from the observer. Thus, only a limited subset of the signals given by the model may be required. For example, the black and yellow banding of venomous wasps is an aposematic color pattern that is displayed by countless species from amongst many orders of insects. The exactness of the match, at least to our eyes, varies considerably. This may be due to subtle differences between several different venomous models, or it may reflect the inability of the observer to discriminate: if only yellow and black banding is required to deter a predator there may be little or no selection to refine the mimicry more fully.

Chapter 14