In the world of insects, high risk of attack has led to the development of camouflage as a means for survival, especially in the larval stage. One caterpillar may look like a stick, while another disguises itself as bird droppings. Though crypsis may have its advantages, University of Florida researchers uncovered some of the most extensive evidence of caterpillars using another strategy previously best-known in adult butterflies: mimicry.
Insects use camouflage to protect themselves by looking like inanimate or inedible objects, while mimicry involves one species evolving similar warning color patterns to another.
The study in the current issue of The Annals of the Entomological Society of America helps scientists better understand how organisms depend upon one another, an important factor in predicting how disturbance of natural habitats may lead to species extinctions and loss of biodiversity.
"Mimicry in general is one of the best and earliest-studied examples of natural selection, and it can help us learn where evolutionary adaptations come from," said UF lepidopterist Keith Willmott, lead author of the study and an associate curator at the Florida Museum of Natural History on the UF campus.
Bright warning coloration has evolved in many insects with physical or chemical defenses and further research into how insects metabolize plant toxins for their own benefit has potential use in the medical field.
"It's very interesting how caterpillars can detoxify a plant's poisonous chemicals and resynthesize them for their own chemical defense or for pheromones," said Florida Museum collection coordinator and study co-author Andrei Sourakov. "We can look at the caterpillars' metabolic systems to understand how they deal with secondary plant compounds, the toxic plant substances used for centuries as tonics, spices, medicine and recreational drugs."
Based on the number of eggs laid by a single female butterfly, scientists estimate about 99 percent of caterpillars die before reaching the pupal stage. Survival tactics include sharp spines, toxic chemicals and hairs accompanied by bright warning coloration.
The study focuses on two groups of Neotropical caterpillars: Danaini of the Caribbean Island of Hispaniola and Ithomiini of the upper Amazon in eastern Ecuador. Sourakov raised and observed danaine caterpillars, including the monarch butterfly and its relatives. These species apparently form Müllerian mimicry rings, in which toxic species adopt the same warning color patterns so a predator will more quickly learn which species to avoid.
In Ecuador, Willmott and study co-author Marianne Elias, from the Muséum National d'Histoire Naturelle in Paris, found that 22 of 41 ithomiine caterpillars displayed some kind of warning coloration. Five exhibited a previously undocumented pattern with a bright yellow body and blue tips, and four were likely Batesian mimics, in which edible species adopt the coloration of an unpalatable model species for protection. These "freeloaders" only appear to have the defense mechanisms of the model species.
"They act almost like parasites, because the mimics are actually edible and therefore deceive predators without having to invest in costly resources to maintain toxicity," Willmott said. "Such a system can only be stable when the mimics are relatively rare, otherwise predators will learn the trick and attack more individuals of both mimics and models, driving models to evolve novel color patterns to escape the predators."
Mimicry may be relatively rare in caterpillars because it is more difficult for them to establish bright coloration, Willmott said. A brightly colored caterpillar has less chance of evading predators than a mobile adult butterfly.
"In adults, bright coloration may be favored by sexual selection for signaling to males and females," Willmott said. "Bright colors may be disadvantageous since they attract predators, but advantageous for attracting mates. Once established, bright colors might then be modified by natural selection for mimicry, another possible reason why mimicry seems to evolve much more frequently in adults than in caterpillars."
However, Sourakov believes mimicry is more common in caterpillars than scientists realize, but may receive less attention because larvae must be raised to adulthood to identify mimicry complexes, a process that takes weeks of lab work. Also, few collections of immature stages are maintained, and colors are not as well preserved in caterpillars.
"We know mimicry is an important ecological process for several species of animals, and I hope this study will give people incentive to further research immature stages of insects," said Andre Victor Lucci Freitas, a professor in the Instituto de Biologia at Universidade Estadual de Campinas. "We need to remember in most insects, immature stages are the most abundant."
- Keith R. Willmott, Marianne Elias, Andrei Sourakov. Two Possible Caterpillar Mimicry Complexes in Neotropical Danaine Butterflies (Lepidoptera: Nymphalidae). Annals of the Entomological Society of America, 2011; 104 (6): 1108 DOI: 10.1603/AN10086
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