Nature Notes | In West Texas, Summer Brings the Tarantula “Swarm”

Texas is home to as many as 14 tarantula species, but the most common in the state, and in our region, is the “Texas brown,” Aphonopelma hentzi.
Texas is home to as many as 14 tarantula species, but the most common in the state, and in our region, is the “Texas brown,” Aphonopelma hentzi.

In West Texas, Summer Brings the Tarantula “Swarm”

By Andrew Stuart

Henry Thoreau is a touchstone in environmental thinking, and he pioneered a form of political resistance still employed today. But for the last decade of his life, he was devoted to a quieter endeavor: “phenology,” the study of cyclic and seasonal natural phenomena. If he were living in West Texas, he’d be on the lookout now for one of the region’s strangest natural spectacles.

For this episode, we’re teaming up with “West Texas Wonders” – a reporting series where listeners ask questions and Marfa Public Radio finds answers. This question comes from listener Liz Lambert, of Marfa and Austin.

“I’ve noticed a lot of times, particularly in the summer after a rain, you can be driving down some stretch of asphalt in Far West Texas, and you see just this mass of tarantulas,” Lambert said, “not the one or two you see in the grass or dirt somewhere, but hundreds of tarantulas, and they seem to be undulating, popping up and down. Why is that? Why do a bunch of tarantulas run out to the middle of road? How do they all get the message? And what in the hell are they doing undulating in the middle of the road?”

Call it “the running of the tarantulas,” the tarantula “surge.” There’s apt to be an afternoon in early summer when the roads here swarm with the big spiders. Lambert has a hunch for what it’s about, and she’s on the right track.

“I feel, for whatever reason, that these are males tarantulas,” she said.

Dr. Christopher Ritzi is a biologist at Sul Ross State University.

“Tarantulas in general tend to be pretty solitary, and sedentary, arthropods,” Ritzi said. “An average tarantula would love to spend all its time sitting inside a nice little silk-lined burrow underground, where it’s nice, comfortable and a little cool. But the males don’t have that option and privilege if they want to find a female to mate with.”

The days lengthen, temperatures warm – for male tarantulas, these are cues that it’s time to mate. Then, if fortune smiles, rain comes in late June or early July.

For the males, the stakes couldn’t be higher – they’re leaving the safety of the burrow for a search with few signposts. In the wake of a storm, they have a better chance of avoiding dehydration and overheating. The moment is now.

Tarantulas may be top dog among arthropods – they eat not only beetles and other spiders, but lizards and baby rodents and birds – but they’re also vulnerable. Tarantulas use their venom to immobilize prey, but they have bristles, or “urticating hairs,” on their abdomens, which they can remove and direct at a potential predator. These irritating bristles are often effective against mammals – like coyotes. They’re not much help against raptors.

As the males abandon their reclusive ways to run the gauntlet, the female tarantulas are safe at home. Some female spiders leave silk threads laden with pheromones in the landscape, as a trail for would-be mates. Female tarantulas don’t do much to help males locate them.

Man-made roads provide a ready corridor as the males wander, alert to the faint chemical scent of a female. They’re all hunting the same thing – and can end up massing on patches of roadway.

The undulating among these males may look like a “rave,” as Lambert said – but it’s not a celebration.

“When they start coming in contact with each other, they could be kind of bristling and sizing each other up,” Ritzi said, “to determine if they need to potentially fight, or if somebody’s about to become food, or if somebody could become their food. They’re largely trying to find mates for reproduction, but I wouldn’t put it past any animal, given the opportunity, to try to get a free meal.”

Cannibalism is a live question any time tarantulas meet, and when a male does locate a female’s burrow, he knows better than to enter. Females are bigger, and they’ll make a meal of an uninvited guest.

He tries to lure her out. He may “knock,” by tugging at the web at the opening of her burrow.

Then, it’s showtime. If she emerges, the male must make an impression. The details of the display are mysterious, Ritzi said, but males have been known to pulsate or “dance.” Tarantulas have appendages near their fangs called pedipalps, and the males may shake these palps, or drum them to create a beat.

If he passes muster, the female may allow him to place sperm in her genital opening with his pedipalp. Just as often, the male wraps his sperm in a packet of silk, leaves it with the female and departs. It’s sufficient intimacy for these solitary, venomous creatures.

In captivity, female tarantulas have been known to eat males after mating. But Ritzi said such “sexual cannibalism” is likely rare in the wild.

It’s not a concern for most males. The success rate in locating females is probably low. Males are expendable in this arrangement, and many don’t survive their sojourn through the summer dusk. The search is undertaken by “mature” males – which in our region could mean a 6-, 7- or 8-year-old tarantula – and some males may live to attempt the quest again.

And, as Lambert notes, the difficulty of the journey serves an evolutionary function.

“It’s just crazy – it really is survival of the fittest,” Lambert said. “The tarantula we see today is the brightest and best, I would guess. I really have wondered this for a long time – so I’m really happy to know.”

When the first rains of summer come, keep your eyes on the road for the tarantula surge – and avoid crushing the big spiders if you can: the odds they face are long enough.

Nature Notes is supported by the Shield-Ayres Foundation and produced by Marfa Public Radio with the Sibley Nature Center. The program can be heard each Tuesday and Thursday, at 7:45 a.m. and 4:45 p.m., Central time, on KRTS Marfa, 93.5 FM, and KXWT Odessa/Midland, 91.3 FM. This episode was written by Andrew Stuart.