IS AN EYE TO A BIRD WORTH TWO IN A BUSH? BOTH EYESPOTS AND OBSCURED BRANCH PLACEMENT REQUIRED FOR PROTECTION IN MODEL CATERPILLARS

Abstract

Many moths, butterflies, caterpillars, and pupae exhibit small paired symmetrical circular markings commonly called eyespots, which are hypothesized to provide significant protection by mimicking the eyes of a deadly predator. A growing body of experiments has demonstrated that these markings do confer protection, in adult as well as larval lepidoptera, against predation from birds. Eyespots are thought to be particularly effective in combination with a startle effect, meaning that they are more effective when displayed suddenly to exploit a predator’s immediate reaction. It has been hypothesized, though never tested, that eyespots are more prevalent in caterpillars which spend most of their time in obscure microlocations, due to this startle effect. To investigate the protective effects and interaction of eyespots and obscure microlocation, four treatments of artificial clay caterpillar were placed in a 2x2 factorial design and exposed to predation by wild birds in the Area de Conservacíon Guanacaste, in Costa Rica. The survival of each model caterpillar was monitored for 96 hours in four trials. The data showed that both eyespots and obscured microlocation were both required to see a major reduction in hazard rate, suggesting that caterpillar eyespots are more effective when a caterpillar is not plainly visible from far away. Furthermore, for a subsample of these models, branch size and caterpillar placement along the branch were correlated and led to increased predation rates. These results support the hypothesis that eyespots are less effective when constantly displayed and more effective when displayed suddenly, and that an effective means of generating this effect for caterpillars is to live in an obscure microlocation.