New Mexicans are used to seeing tarantulas on roadways and trails in the early fall, when males leave the safety of their burrows on a grueling march for a mate. They can travel miles before finding any females, which typically are larger than males. Even when they manage to stumble across a potential mate, they risk being eaten by her.
Although many New Mexicans call this annual fall ritual a migration, tarantulas aren’t really migrating, according to Peter Lipscomb, Park Manager at Cerrillos Hills State Park. Some birds migrate south or to lower elevations for the winter. But the species of tarantula we see here in Northern New Mexico, commonly called the desert tarantula, sticks around all year despite often frigid temperatures and frequent snowfall. Unlike many spiders that lay eggs and then die before cold weather hits, tarantulas can live for years, particularly females. Female tarantulas have been known to live for twenty years or more. So how do they do it?
No one knows for sure. Professor and Arthropod Museum Curator Emeritus David B. Richman of New Mexico State University says that female tarantulas live underground most of the year, and “certainly all winter.” There, they lay several hundred eggs, and the spiderlings emerge in spring.
But why don’t tarantulas freeze? Like most spiders, tarantulas rely on a special system in their bodies to move nutrients and oxygen, called a hemolymph system. (People and other vertebrates, such as dogs and cats, rely on blood moving through blood vessels to do this work.) Hemolymph in tarantulas is a combination of water and other substances that spread throughout their bodies, filling in the spaces between organs. And like water in an enclosed space, hemolymph can be compressed. In fact, that’s how tarantulas move their legs: by compressing their hemolymph (much like a hydraulic system), as well as by moving their muscles. Hemolymph has a lower freezing point than water, according to Dr. Richman, which can help tarantulas survive cold temperatures.
Professor Cara Shillington, an expert on tarantulas at Eastern Michigan University, notes that tarantula burrows can be quite deep. If they are deep enough, tarantulas might be able to avoid freezing temperatures above the surface. And they might slow down their metabolism to survive harsh winter months with little food. “We do know that tarantulas have a low metabolic rate even in warm temperatures,” she says, “but no one has measured their metabolic rate in winter.”
As Halloween approaches and temperatures drop, male tarantulas have likely ended their search for a mate and are tucked safely in their winter burrows (if they survived the treacherous trek for a female, that is). But Lipscomb has seen a male tarantula as late as November 3. So if you’re lucky, and weather permits, some Halloween you just might see a straggler marching onward, seeking immortality or maybe just a place to ride out the winter storms.