Boas in Berkeley? Unlikely as it seems, we do have a native species of boa. But this isn’t one of the giant constrictors of the tropics, big enough to swallow a deer. Rubber boas max out around 26 inches in length, prey on small vertebrates, and are among the most inoffensive of serpents.
One of two North American boa species, the rubber boa was discovered by Paulo Emilio Botta, one of those polymath 19th-century naturalists. Botta was the surgeon about the French merchant ship Heros when it docked in Central California in 1827. In addition to collecting the rubber boa, he was the first European to describe the Anna’s hummingbird and introduced the roadrunner to Parisian science. Botta went on to become personal physician to the Egyptian ruler Mohammed Ali and French consul in Iraq (then part of the Ottoman Empire), where he excavated the Assyrian ruins of Khorsabad. He’s immortalized in the snake’s Latin binomial, Charina bottae.
My only encounter with a wild rubber boa wasn’t in Berkeley. Some years ago I met a young one on a field trip in the Sierra, near UC’s Sagehen Creek research station. It was basking on a rockpile in the late afternoon sun. On first impression, it was hard to tell whether the snake was coming or going: it had a narrow head, small eyes, no neck to speak of (yes, snakes have necks) and a blunt-tipped tail. And its smooth skin was in fact rubbery to the touch. When Roger, the trip leader, picked it up, it coiled around his wrist and seemed reluctant to disengage. But there was no hostile intent; it was just getting a grip.
It’s not surprising that I haven’t seen one since. Rubber boas are secretive snakes, hiding out under rocks, fallen logs, and tree bark and hunting at night or in twilight. Unlike some of their kin, which are ambush predators, our local boas actively seek out their prey, exploring the burrows and tunnel systems of mice, shrews, and pocket gophers. They’ll also eat salamanders, lizards, and (lacking a sense of professional courtesy) smaller snakes, as well as eggs. When they invade rodent nests, they’re reported to use their clublike tails to fend off parental counterattacks. Wild-caught rubber boas often bear the scars of mouse bites on their tails. If they’re attacked by a larger predator, they roll up in a ball with the tail-tip protruding; they may even feint with the tail, faking a strike.
Boas and their close relatives, the pythons, are an ancient snake lineage, one of the first groups to diverge from the ancestral lizard stock. They’ve retained some anatomical vestiges, like paired lungs (typical snakes have only one) and rudimentary hind legs, which males use as claspers when mating. But it would be a mistake to consider them primitive—a loaded word, anyway; scientists prefer “basal”—since members of the group have diversified to exploit a wide range of environments, from rain forests to deserts.
What’s the difference between a boa and a python? One is anatomical: pythons have teeth on their premaxillary bones, directly under the snout; boas don’t. The other involves reproductive strategy: all pythons bear living young, while most boas (the exception being the Calabar burrowing boa of West Africa) lay eggs. Collectively, they’re an old Gondwana family that evolved in the southern supercontinent during the time of the dinosaurs. Pythons seem to have originated in Australia, then spread to southern Asia and Africa. Typical boas are most diverse in the Central and South American tropics, with outliers in Madagascar and the islands of the Southwest Pacific. The closest relatives of our rubber boa and the southwestern rosy boa are the desert-dwelling sand boas of the Middle East and North Africa, plus that one oddball West African species.
All boas and pythons are constrictors, a trait that has evolved independently in several snake families. Lacking venom, they need to subdue their prey quickly by brute force, looping their coils around the victim and squeezing to block its breathing and blood circulation. With their slow, thermally driven metabolism, they can get by with widely spaced meals. One study of rubber boas found that their rate of digestion varies with ambient temperature; it can take them twice as long to digest a mouse in spring as in summer.
Rubber boas mate in spring, but the offspring (as few as two, as many as eight) aren’t hatched until late summer or fall. Although I haven’t been able to find any references documenting parental care, I wouldn’t want to rule it out; snakes in general are more attentive mothers than you might think. After winter hibernation, the young boas disperse to find their own territories.
The snake literature consistently describes the rubber boa as gentle and easy to handle, like the one I met at Sagehen Creek. One web site suggests it as an ideal snake for therapeutic desensitization of the ophidiophobic. But if the fear of snakes is as deep-seated as some (E. O. Wilson, for one) speculate, I doubt that even a therapy boa would help with some people.
Interestingly, there’s evidence that snake fear is partly innate and partly acquired. In a study by behavioral psychologist Susan Mineka, lab-reared monkeys had no aversive reaction to snakes until they saw wild-caught monkeys freaking out. Then they displayed fright the next time they were exposed to a snake. However, showing them doctored film of a monkey recoiling in fear from a flower did not induce fear of flowers. The snake response seems to be latent until triggered by the behavior of a role model. Remember that if you meet a rubber boa on your next family outing.