The Social Lives of Snakes

If the word snake pops into your mind in social situations, you’re probably not thinking of a legless reptile. Indeed, the prevailing opinion among animal behaviorists for years was “very dogmatic that snakes weren’t particularly social,” says Harry Greene. “They courted, they mated, and that was it. Mothers abandoned the babies.” Although Cornell University herpetologist Greene describes himself as a “total snakeophile,” he says, “I was as blinkered as anybody else.” But his view began to change one morning in 1995.

“I was sitting in my house in Berkeley reading the newspaper when the phone rang,” he begins. It was David Hardy, a retired Arizona anesthesiologist who worked with Greene on radio tracking black-tailed rattlesnakes. “His voice was practically quivering,” Greene remembers. Hardy described a rare sighting of a rattler, accompanied by newborns. Even more surprising, the mother and young ones would remain together for more than a week.

The behavior of this radio-tagged mom, known to the scientists as superfemale 21, started Greene and Hardy toward revising their view of snake parenthood. They focus on pit vipers, the group that includes rattlesnakes and their relatives. Suddenly, old anecdotes and a rare study or two scattered throughout the literature became relevant.

Other snake watchers in the 1990s also began devising experiments to test interactions that earlier herpetologists never dreamed of, such as sisterly companionship. Snakes aren’t planning cotillions, but many species seem to care for their young, hang out together when pregnant, and to associate with relatives, these researchers say.

Slow start

Some of the old-fashioned view of the snake as the ultimate cold-blooded loner came from the difficulty of studying such swift and cryptic beings. “You could catch a snake and mark it, and you’d never see it again,” says herpetologist Rick Shine of the University of Sydney in Australia.

Radio tracking has opened a new world, in which a biologist stands a decent chance of finding the same snake time and again, Shine says. During the 1980s, transmitters shrank to snake-friendly sizes, and biologists refined the technique for quickly implanting them in an animal’s body cavity.

The elusiveness of the study subjects doesn’t fully explain scientists’ earlier disregard for snake interactions, according to Shine. “One of the problems we have as human beings is this intuitive feeling that smaller and very different kinds of life forms are really very stupid,” he says.

Snake interactions shouldn’t come as a big surprise, considering what’s known about other small nonmammalian creatures, says Shine. Biologists know that a frog can recognize other individual frogs, a precondition for a lot of social interaction. When a male distinguishes a neighbor’s ribit from the call of a stranger, it can save its energy for the fights that matter. “The male frog doesn’t race across and beat up a neighbor because he knows, ‘Ah, that’s Harry, who lives across the road,'” says Shine.

Also, lizards, which are evolutionarily closer to snakes than frogs are, have shown much more variety and sophistication in social organization than researchers had expected. Until a decade ago, scientists were satisfied that the extent of the lizard social scene was “territorial males that beat up the other guys and have a harem of girls in the defended area,” says Shine. Then, Michael Bull of Flinders University in Adelaide, Australia, reported that some of Australia’s big, live-bearing skinks are monogamous. In a typical Tiligua skink love story, a pair goes to some trouble to relocate each other every mating season for years.

Shine and Sydney colleague Dave O’Connor published an account in the March 2003 Molecular Ecology of black rock skinks (Egernia saxatilis) as the first example of a reptilian nuclear family. “Most of the big, live-bearing skinks are turning out to have extraordinarily complex social lives,” says Shine.

Mommy dearesssst

Definitions of parental behavior differ, but Shine points out that mother snakes seem to go to some trouble for their offspring. For instance, python moms will often stay coiled around their pile of eggs for about 2 months, even though they haven’t had anything to eat for 6 or 7 months. At first glance, it might seem hopeless for a cold-blooded animal to try to incubate its eggs. When the temperature drops sufficiently, though, the python shivers, thereby warming the clutch with heat derived from muscle activity.

Many rattlesnakes and their pit viper cousins don’t lay eggs but instead give birth to ready-to-wriggle offspring. Back in the Chiricahua foothills of Arizona, the black-tailed rattler mother that so excited Hardy and Greene stayed near her youngsters and the sheltering rocks of the birth site for more than 9 days. The scenes that the researchers described in 2002 might apply as well to a mother dog and her pups.

On day 4 after the birth, Hardy observed superfemale 21 near the birth site as five of her newborns crawled around. They had worked their way out of the shelter’s entrance, over the mother’s body, and a little way into the surrounding grass. An hour later, several youngsters had piled on top of her. When one wriggled over her head, she tolerantly rearranged her coils.

Thus, the days went by with the family basking just outside its rocky den. About 9 days after birth, the little snakes shed their skins as their mother watched from a few inches away. The youngsters then disappeared, presumably crawling off on their own.

Greene and Hardy’s detailed monitoring of black-tailed rattler life had convinced them that the females typically don’t eat during winter hibernation or the spring pregnancies that follow. Greene paints a heroic picture of the mother, who further delays her return to hunting. “She hasn’t eaten for about 10 months, but she stays around for 10 more days,” he says.

He and Hardy have since observed similar behavior in several females.

Does the mother’s presence help the newborns? Because the little snakes don’t see well through their soon-to-be-shed skin and can’t muster the striking force of a grown-up, the lingering mother might be deterring predators. Also, her large body might keep the young ones warm.

Despite the plausibility of such benefits, Greene doesn’t use the term “parental care” for the mother snake’s apparent solicitude; she calls it “maternal attendance.” Also, the benefits of the mother’s vigilance haven’t been tested, he points out.

Nevertheless, Greene decries the old explanations for the occasional finds of female snakes and a litter of young. They had been regarded as reflecting just the happy timing of a biologist who happened by right after a litter’s birth or while independent snakes converged in a good spot.

If the sightings were just lucky timing, Greene argues, then the reports should cover a wide range of snakes. He’s found mention of mothers near litters of young for all 14 U.S. species of pit vipers but none for the 36 or so live-bearing garter snakes and their relatives, even though these species are relatively big, abundant, and well studied.

The question of whether the rattlers might just be hanging around a good spot or whether mothers were too warn out to move inspired a Florida team to devise a test of maternal behavior in Carolina pygmy rattlesnakes (Sistrurus miliarius). Articles going back to the 1980s had reported mother rattlers turning up with newborns. Peter May of Stetson University in De Land, Fla., and his colleagues wanted to know, however, whether the mothers and young were actually attracted to each other.

The researchers tested 16 rattlesnake families by putting the mother in one side of a terrarium and the offspring in the other. To see whether the youngsters were drawn to the mother, the researchers connected the compartments with a tube lined with rows of nails. The little snakes could fit between the rows of nails, but the mothers couldn’t. By the third afternoon, 84 percent of the newborns were in the mother’s compartment, May and his colleagues reported in 2002.

To see whether mothers were attracted to their young, May and his colleagues divided a terrarium in two with a cardboard partition low enough for an adult snake to cross but too high for the babies to manage. In the experiment, more than half the adult snakes climbed over to their young. Eight stayed there throughout the experiment, and four others made multiple trips. Of the 23 barrier crossings, 18 went toward the side of the babies.

Yes, there is a mutual attraction, the researchers concluded.

The group also looked at the fiercer side of motherhood to see whether a female is extra-likely to respond to predators when her young are nearby. The researchers let females in outdoor cages see a tethered southern black racer (Coluber constrictor priapus). This common snake doesn’t inject venom, but it eats pygmy rattlers in the wild. Among pygmy rattlers that had recently given birth, 83 percent gave some response, such as puffing up their bodies or rattling. In contrast, reactions occurred among only 33 percent of the nonreproductive snakes used for comparison.

The new evidence fits with a novel test of rattlesnake maternal behavior from the 1980s. Brent Graves of Northern Michigan University in Marquette tried a simple experiment: walking up to female rattlesnakes and seeing what happened.

Pregnant females typically let him get close, sometimes close enough to touch them, before they started rattling or taking strikes at him. Also, if they had a shelter nearby, they typically slid into it. Females with young nearby, however, started rattling when he was 3 or 4 meters away. These females delayed longer in diving into their dens, and during these delays he saw the young snakes slip into hiding.

He’s not surprised, he says, that a lot of vipers seem to have evolved maternal care. For one thing, he says, they can offer genuine defense for their young. In contrast, he says, “How much can a garter snake do?”

Sisterhood

Maternal attendance isn’t the only interaction observed among snakes. Rulon Clark of Cornell University reports evidence of sisterly togetherness. He started wondering about social behavior in snakes when he came across a pile of pregnant timber rattlesnakes basking in the sun.

Clark started his investigation with a classic question about chummy animals: Do they recognize their kin? Clark put pairs of female timber rattlesnakes in cages and watched their interactions. Non-sibs stay, on average, more than twice as far apart as siblings do, he says in an upcoming Biology Letters of the Royal Society of London. The siblings also spent significantly more time touching or twined around each other.

The males in his study tended to remain far apart, regardless of whether they were related. Clark suggests that laboratory conditions, such as abundant food and warm temperatures, might have caused the males to go into a state of reproductive readiness, in which they wouldn’t tolerate other males nearby.

Clark is planning experiments in which he’ll analyze the genetics of wild populations and observe their behavior. Now, says Clark, he’s wondering whether kin recognition plays a role in communal winter denning. Timber rattlesnakes, like many other species in cold climates, get through the winter by piling into some crevice or cave that sinks deeply enough into the ground to stay unfrozen.

Shine calls the spring emergence of the garter snakes of Manitoba “one the world’s best wildlife sights,” as tens of thousands of snakes slide out of their refuge. He crosses the Pacific every year to watch them.

Herpetologists have been unsure whether such aggregations are social gatherings. Perhaps many snakes just select the same winter getaway, each indifferent to the presence of its fellows.

Snakes that spend the winter tangled in a gigantic living yarn ball don’t necessarily pal around once everybody warms up in spring. There may, however, be interactions that people haven’t noticed yet. That’s the new frontier, according to Shine. “I suspect complex social systems are very widespread in snakes, but they’re very subtle,” he says.

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