Rainbow Hues of Amphibian "Worms" Demystified

Many animals are colorful to demonstrate fitness and health to one another—the robin's red breast is a sterling example. But pigment, like everything else in the body, comes at an energy cost.

For animals that live in the dark, such an expense is unnecessary, so most are pale and colorless. Yet caecilians, amphibians with earthworm-like bodies that spend most of their lives underground, are bafflingly colorful.

Combining information about caecilian behavior and evolutionary history, researchers Katharina Wollenberg at the Technical University of Braunschweig in Germany and John Measey at the South African National Biodiversity Institute suggest that just a little bit of light exposure was all it took for color to evolve in these animals.

Above, the caecilian Boulengerula boulengeri, found in the cloud forests of Tanzania, is blue—and blind. Since its bright colors are not demonstrating fitness to blind kin, researchers think color might have evolved when its distant ancestors spent time on the surface.

The research will appear in an upcoming issue of the Journal of Evolutionary Biology.



Many ocean-dwelling animals have dark colors on top of their bodies to help them blend in with the black ocean depths as predators pass overhead.

The caecilian Ichthyophis kohtaoensis's coloring may serve a similar purpose. When the subterranean creature occasionally moves above ground, the brown on top of its body may help it hide from predatory birds that attack from above.



The wormlike amphibian Scolecomorphus vittatus sometimes moves over leaf litter in the forests of Tanzania. Since color in amphibians is usually a sign of toxicity, researchers think the caecilian's bright stripe could be a warning that, if eaten, the species creates a burning taste in the mouth.



The pale stomach, yellow striped sides, and brown back of the caecilian Ichthyophis tricolor may be evolutionary adaptations, study authors Wollenberg and Measey suggest. The brown back helps hide the animal from aerial predators while the yellow sides warn terrestrial predators of the possibility of a potential bad taste.



Yellow is a color usually found on the most toxic amphibians. Wollenberg and Measey report that it has evolved independently three times in caecilians, including in Schistometopum thomense, referred to as the cobra bobo, or yellow snake, by the locals of São Tomé Island in the Gulf of Guinea.

Researchers suggest that bright-colored caecilians may also be highly toxic, but this still needs to be investigated.



A native to Kenya's Taita Hills, the caecilian Boulengerula taitanus looks a dull blue-grey from a distance. But up close and from the sides, bright vertical stripes become noticeable.