During last week’s warm dry spell, I finally got around to attacking the Bermuda sorrel. (You never quite get it all, but you can make a dent; and in theory it will die out if sufficiently discouraged.) In the process, I was gratified to find two California slender salamanders (Batrachoseps attenuatus) in the front bed. Ron reported a couple more in the back yard. We hadn’t seen any of these odd amphibians for a while; good to know they’re still around.
As the accompanying photo shows, slender salamanders resemble chunky earthworms with eyes and legs. They live under logs, bark, rocks, and the cardboard box you forgot to recycle; in leaf litter and various crevices and crannies. With almost vestigial limbs that are useless for burrowing, slender salamanders use worm tunnels to move through the soil, feeding on mites and other small creatures. Sticky skin secretions protect them against would-be predators like garter snakes: one snake still had its jaws gummed together 48 hours after an encounter with a B. attenuatus.
They don’t get around much. One study in the Berkeley Hills found a typical cruising range of 5.5 feet. A Batrachoseps can spend its entire live under the same log. Populations can be dense: UC-Berkeley emeritus professor Robert C. Stebbins reported that attenuatus was the most abundant vertebrate in the redwoods around Canyon, exceeding 700 per acre. Otherwise asocial, several females may lay their eggs at the same site. There’s no larval stage: the young are miniatures of the adults.
Some of their peculiarities are shared with other kinds of small salamanders. Their genomes tend to be huge—larger than those of mammals, birds, or reptiles—and their cells are correspondingly large. No one seems to know why. Even allowing for size, their brains are small and their neural wiring is stripped down; some species have fewer neurons than a honeybee. They may be able to get away with this because of their low metabolic rates and limited need for food. Salamanders, writes David B. Wake, another Cal emeritus professor and a salamander specialist, “are much smaller than they appear to be.”
When Stebbins published the first edition of his Field Guide to Western Reptiles and Amphibians in 1966, taxonomists recognized three species of slender salamander, mostly in California with one extending into southern Oregon. Now there are twenty. Some turned up in remote corners of the southern Sierra; others were identified through genetic analysis in Wake’s lab at UC Berkeley’s Museum of Vertebrate Zoology.
Twenty species could be conservative. Wake and two biologists at the University of Connecticut sampled mitochondrial DNA across the range of the California slender salamander and described five genetically distinct lineages. The authors raise the possibility that B. attenuatus “is actually a complex of several cryptic species.” Genetic diversity was unusually high even at the local population level. Elsewhere, Wake has characterized the diversity of small, direct-developing salamander species as “fractal.”
Wake calls the slender salamanders’ evolutionary trajectory a “nonadaptive radiation.” An adaptive radiation is what happens when a species colonizes a new environment and its descendants
diverge in ways that allow them to exploit different ecological niches. The classic example would be the Darwin’s finches of the Galapagos, in which the generalized beak of an ancestral bird became modified in descendant species for feeding on large or small seeds or insects. A more spectacular adaptive radiation occurred in the Hawai’ian islands, where another ancestral finch
population diversified into species with beaks adapted for seed-cracking, nectaring, prying buds open, and extracting insect grubs from wood. In the African Great Lakes, the jaws of founding species of cichlid fish morphed into specialized tool kits for algae-grazing, mollusk-crunching, and ripping off the scales of other fish.
None of that happened with the slender salamanders. The twenty species vary somewhat in size, shape, and habitat preferences, but they all look and behave very much alike. In 40 million years, there have been no radical innovations in jaw morphology (or anything else) and no exploration of new niches. But there has been plenty of genetic divergence. These forms behave like “good” biological species; where their ranges overlap, they don’t hybridize.
The key to this apparent paradox may lie in what Wake calls Batrachoseps’ “fidelity to pieces of the planetary crust.” Our state, as readers of John McPhee’s Assembling California will be aware, is a geological jigsaw puzzle, an amalgamation of micro-plates called terranes. It appears that at least some slender salamander species became geographically isolated as their home terranes shifted into new positions. The salamanders didn’t so much colonize vacant habitats as go along for the ride. This is vicariance with a vengeance. Cut off from neighboring populations, the salamanders accumulated enough genetic differences that they could no longer interbreed with them, or at least were not interested.
The case of the slender salamanders is a valuable counter to the common notion that evolution has to be “for” something—that most physical features are adaptations that allow an organism to leave behind more copies of its genes. The surviving salamander species didn’t outcompete less fit rivals. Even more than most of us, they’re the creations of time and chance.