SOME CATERPILLARS ARE SCAVENGERS

by Whit Gibbons

February 28, 2016

Wild animals lead precarious lives, walking on the cliff ledge of ample supplies of food. One southern moth is walking on one of the narrowest ledges ever discovered. Its caterpillars eat the carcass of a species protected under the Endangered Species Act. The caterpillars of the gopher tortoise moth dine on the shells of dead gopher tortoises.

Gopher tortoises are fascinating turtles that construct subterranean tunnels, can live more than 30 years and, unlike most turtles, can drown if they fall in a lake.

From western Alabama to Louisiana the species is federally threatened and in Florida, Georgia and South Carolina has been recommended by some conservationists as a candidate for the federal endangered species list.

Rare indigo snakes and gopher frogs, along with more than 300 different kinds of insects have been documented to inhabit gopher tortoise burrows as safe havens from winter cold, drought and predators. The gopher tortoise moth has one of the most unusual relationships with tortoises yet discovered.

This moth qualifies as a scavenger specialist. More than 90 percent of moth and butterfly caterpillars are vegetarians, aka herbivores. The diversity of plants they eat and the ways in which they feed on them seem near endless.

A small proportion of the world's caterpillars are carnivorous, subduing and eating other animals, including flies and spiders. Some predatory caterpillars even eat other caterpillars, and a Hawaiian species discovered in 2005 eats snails after capturing them in a silk trap. However, aside from herbivores, the most common caterpillar feeding category is scavengers that make a living by eating dead animals.

The gopher tortoise moth belongs to an infamous family of scavenging moths with which most people are familiar. These are the clothes moths whose caterpillars eat wool and fur.

Members of the clothes moth family were a stimulus for the development of mothballs to deter their laying eggs in coat closets. More than 3,000 species within the clothes moth family have been described worldwide. The gopher tortoise moth belongs to a group within the family whose caterpillars are known to eat animal hooves and horns, feathers and hair.

The female gopher tortoise moths are among the last scavengers to arrive when a tortoise dies and the shell is lying on the ground; the muscles, bone and major organs have already been scavenged by flies, beetles and vertebrates. The moths lay their eggs at night on or near the carcass, to which the hatching caterpillars are attracted.

The material of interest to the caterpillars is the protein known as keratin (our fingernails are keratin), which binds the tortoise's large shell plates together. Keratin provides the food for the caterpillars. But what keeps these little soft-bodied critters from becoming a meal to predatory invertebrates, such as ants or spiders, roaming the woods?

Each gopher tortoise caterpillar secretes a protective tube made mostly of silk and covered with grains of sand that is long enough to extend from below the ground to the various seams on the tortoise shell.

The tube protects the creature from predators and buffers it from hot or cold weather as it crawls to one end of the tube to eat and to the other to retreat underground.

All animals that are food specialists live in a state of double jeopardy. They face the usual challenges that most animals do of finding food while avoiding becoming food themselves in at least some stage of their life.

The need for the gopher tortoise moth to find a dead individual of a reptile that lives a long time and is becoming rarer and rarer would presumably be a major liability.

An environmental cascade is created when one species is vital to the survival of another, for if the latter disappears the former is sure to follow. We now know that we must protect the gopher tortoise not only for its own sake but for that of a little moth that depends on it for survival.

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