Vertebrate animals are those with backbones — fish, amphibians, reptiles, birds and mammals. As a group, they are much less abundant than invertebrates such as insects, spiders, and mollusks.
But there is one vertebrate that occurs in stunning numbers, often quite close to home.
Among common vertebrate species, chipmunk population densities can range from 10 to 18 per acre, garter snakes might reach 35 per acre, vole densities can reach 300 per acre. The vertebrate population champ, however, is the red-backed salamander.
Red-backed salamanders spend about 90 percent of their time under rocks, logs, and leaf litter, so they are seldom seen… unless you look for them.
I flip rocks and roll logs when I walk in the woods. (I always return them to their original position.) I never know exactly what I’ll find — grubs, slugs, earthworms, beetles, millipedes, slugs, snails, snakes, — but I almost always find red-backed salamanders. Often I find two or three under a single rock or log
Population densities of red-backed salamanders can range from 800 to 8,000 per acre. In some Appalachian forests, the biomass of red-backed salamanders exceeds that of all the birds and mammals that occupy the same area.
Red-backed salamanders measure three to four inches from snout to tip of tail and are found from Minnesota eastward to the Maritime Canadian provinces and south to North Carolina.
Curiously, they belong to a family of lungless salamanders. They respire directly across their moist skin.
Two color forms or “morphs” occur in most red-back populations. The red-backed morph has a broad reddish-orange stripe extending from the neck to the base of the tail.
The lead-back morph lacks the red stripe, but is often found with red-backed morphs.
Like most salamanders, red-backs are active at night and commonly roam the forest floor on rainy nights, so they’re seldom seen except by rock flippers and log rollers. And though red-backs are easy to find under rocks and logs, on any given day most are below ground.
In four consecutive weekly populations surveys in Michigan, for example, the number of red-backs collected each time remained constant. This suggests that most of the time most red-backs are under ground.
Because red-backs are so abundant, they play an important role in forest food chains. They’re active predators and eat just about anything they can catch. Their menu includes ants, termites, beetles, earthworms, spiders, snails, slugs, mites, springtails, centipedes, and millipedes. One study estimated that red-backs eat nearly three-quarter million prey items per acre per year.
On the other hand, red-backs are also important food for small forest snakes, shrews, voles, chipmunks, and birds that forage in the leaf litter. Towhees come immediately to mind.
Surprisingly, red-back salamanders usually breed in the fall. Though males breed every year, females in the northern part of their range breed every other year.
Apparently it takes northern females more than a full year to generate energy-rich yolks for a full clutch of eggs.
Males find females by following pheromone trails. After some ritualistic posturing, the male deposits a spermatophore — a package of sperm — on the ground.
The female then straddles the spermatophore, picks up the sperm, and stores it in a special chamber called the spermatheca, where she can store the sperm for months.
In the spring or early summer when the female lays her eggs, she releases the sperm and fertilizes the eggs as they are laid.
The eggs, usually two to 14, are laid in grape-like clusters in natural cavities and crevices under rocks and logs. They are pale, less than a quarter-inch in diameter, and enveloped in gelatinous material. The female coils around the eggs to protect them from predators and prevent them from drying out during the six week incubation period.
After hatching, juveniles remain in the nest with the female for several weeks before dispersing.
To observe red-backed salamanders in the field, flip a few rocks and roll a few logs. But be careful where you put your hands if you live in rattlesnake or copperhead country. Use a walking stick or an old golf putter to keep your hands out of harm’s way.