Question of the Day: Why did the tarantula cross the road?

If you’ve been to the Wilderness Park recently, you may have spotted tarantulas out on the fire trails, and you might well wonder:

Why are so many tarantulas out now?
Why are they crossing the roads?

The tarantulas you see out on the roads this time of year are all adult males, and they are out looking for females. They really have only one thing on their little tarantula minds, and they are pretty oblivious to anything else.

The females are found in burrows that are scattered across the landscape, and the males search for them by alternating between walking in straight lines and walking in concentric circles until a female’s scent is detected, suggesting that a burrow is nearby. The males may travel as much as 3/4 of a mile looking for a mate, and if a road is in their path, they just trundle on across it.

Once a male has found a female’s burrow, he signals his presence by doing some fancy drumming on the ground at the burrow’s entrance. If the female likes his drumming, she will emerge from her burrow, and mating can commence. The male uses special hooks on its front legs (which you can see in photo above) to immobilize the female’s fangs and position her during mating.

Autumn is the mating season for tarantulas, so that’s when you’ll most likely spot the males out on their mating missions. If we look at the distribution of tarantula sightings reported on our iNaturalist project, you can see that over 80% of the observations were made in August – November.

It’s not clear exactly how many tarantula species reside in the Wilderness Park. Only one species – California Ebony Tarantula (Aphonopelma eutylenum) – has iNaturalist observations labeled as “Research Grade”, meaning that the identification has been confirmed by a consensus of other identifiers. Steindacher’s Ebony Tarantula (Aphonopelma steindachneri) has also been reported, but the identifications have not been confirmed.

Different Aphonopelma species look very similar and can be extremely difficult to distinguish without molecular data (see Hamilton et al., 2016), so we may never know whether one or two species are present in the Park. You can see all the reported iNaturalist observations of tarantulas in the Park here.

If you’d like to find out more about the tarantula’s life cycle, check out Ralph Washington, Jr.’s article in Bay Nature magazine.