Shape-shifters in the natural world

Posted: Sunday, September 30, 2007

Shape-shifters are part of the mythology and lore of many cultures around the world.

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The lusty Greek god Zeus changed his form in order to copulate, in disguise, with young women: a bull for Europa, a shower of gold coins for Danae, a swan for Leda. The soul-eaters of Nigeria were cannibalistic witches in animal form.

Closer to home, Raven of Tlingit lore changed his shape for various nefarious or magical purposes.

Shape-shifters are also part of the natural world, especially among animals that are prey.

When an animal is attacked by a predator, it may defend itself by fleeing, fighting, ducking for cover, or rolling up into a protective ball. More subtle responses are found in creatures that respond to a perceived threat of predation. Among the most interesting of these are the shape-shifters.

Many aquatic organisms respond to the actual or perceived presence of a predator by changing shape. For example, some aquatic invertebrates develop crests, helmets, spines, or more elongated bodies when a predator is present. All such changes increase the probability of escaping capture by certain predators.

Shape changes are induced, not merely by the presence of the predator, but also by chemical cues given off (unwittingly) by the predator or its feeding activities. The cues are released into the water and sensed by the prey, even when the predator is no longer present. The cues tend to be especially strong if the predator has previously been eating this (or similar) kinds of prey. Such cues may originate from the previous prey as it was being eaten or from the digested remains of the prey that passed through the predator.

The tadpoles of frogs and toads have an array of responses to predators, both behavioral and shape-changing. And they have many kinds of predators, including birds, salamanders, fish, crayfish, diving beetles, and dragonfly larvae. The most extensive array of experiments has been done with dragonfly larvae (of the type known as 'darners') as the predators. In these experiments, the dragonfly larva is caged, so it cannot attack the tadpoles in the same pool. Usually the dragonfly larva has been fed on tadpoles (from a different pool) or on other prey.

Tadpoles of many species alter their behavior when dragonfly larvae are present, seeking cover and becoming more inactive. The cost of such behavioral shifts is usually lower feeding rates and slower growth. Some species, especially those that whose time as a tadpole is limited by their habitat drying up, then metamorphose into adult-shaped froglets at a smaller size and are more subject to desiccation and predation by terrestrial predators. This is a potential cost of the behavioral change.

Shape-changing by tadpoles exposed to the caged dragonfly larvae typically involves developing a deeper, sometimes more colorful, tail fin, and sometimes also changing body shape. This response is generally stronger if the previous food of the dragonfly was other tadpoles than if the previous food was insects. Tadpoles with deeper tail fins don't swim any better than those with more slender tail fins. Instead, the enlarged tail fin appears to deflect predator attacks. Most tail damage is concentrated at the tip of the tail, especially if the modified tail has color marks there. The predator attacks the tail, so the head and body of the tadpole is more likely to escape damage. As a result, tadpoles with modified tail fins escape better and survive longer than those with slender tails. The benefit of the deeper tail fin is clear; less clear are any possible costs of that tail fin.

Tadpoles can vary their response according to the number of predators present, or the number of tadpoles previously eaten by the predator, or the kind of predator. Deepened tail fins work best against dragonfly larvae and less well against fish or salamanders.

Shape-shifting in mythology often is ascribed to fearsome, evil creatures, such as werewolves. And in the natural world, shape-shifting occurs in some predatory amphibians. For example, some salamanders have two morphological larval types, a so-called normal larval form and one with a bigger head. The big-headed larvae are cannibalistic, eating others of the same species (especially when they are crowded in the rearing pool and competition for food is fierce). The cannibalistic larvae of one salamander species also eat frog tadpoles and, in this species, the presence of tadpoles in the same pool induces development of more big-headed larvae. These salamanders live in cold ponds with limited food resources and a short growing season, and frog tadpoles provide an additional source of food. However, the tadpoles are not as nutritious as larval salamanders, so the big-headed form does not grow as large as the normal form and probably has lower survival or reproductive success after it metamorphoses into the adult.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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