Carleton Student Studies “Junior High” Phase of Sierra Dome Spiders

For two-and-a-half months last summer, Carleton College senior Rachel Bercovitz of Des Plaines, Ill., studied the “junior high” phase of sierra dome spiders, observing the species’ mating rituals-from avoidance of the opposite sex to two boys fighting over one girl to ultimate consummation. Bercovitz conducted lab experiments to test an established hypothesis linking a female’s weight to the length of her “associative” period, and along the way developed a new hypothesis that a male sierra dome spider will fight harder for a female who is about to mature, versus one who is farther from her sexual prime.

With help from a Kresge Science Fellowship, Bercovitz conducted her research at Flathead Lake Biological Station in Polson, Mont., where she
was an assistant to Paul Watson, a research assistant professor at the University of New Mexico who has been studying sierra dome spiders for
more than 20 years. Bercovitz is currently at work as the first author of a paper describing her observations, titled “Female nutritional status, the
efficiency intrasexual selection, and investments in the incitation of male-male combat by female sierra dome spiders.”

Bercovitz spent her time at Flathead in a lab observing the behavioral patterns of the spiders. A female sierra dome spider goes through five
different stages of growth, called instars. Bercovitz’s study involved the penultimate instar, when the female is in “spider junior high.” Male sierra
dome spiders mature before the females, all at the same time early in July, within about two to three days of each other. A mature male is nomadic
and spends his time searching for females and food.

For the first 12 to 16 days of this instar, the female is “avoidant” and doesn’t interact with the males. Bercovitz observed that when a male wanders onto the web of an avoidant female, she will run out onto the supporting lines or even drop out of the web altogether. After about an hour, the male will get the message and go off in search of a more receptive female.

Bercovitz focused the majority of her data collection on the next stage of development-what is called the “associative” period. She tested Watson’s
original hypothesis that stated a female’s nutritional status, or weight, determined the length of this associative period, which happens when the
female spider instantaneously loses her dislike for male spiders several days before becoming sexually mature. When a male spider wanders onto
her web, instead of running away, the female now hangs upside down in the apex of her dome-shaped web and waits for the male to approach.

Watson noted that this “friendly” behavior assures the male that the female is within six or seven days of sexual maturation, so the male sticks
around and begins the waiting game. While he’s waiting, and since he’s larger, faster, and better at subduing prey, the male out-competes the female for his nutritional source (essentially eating all of her food). It’s this kleptoparasitic behavior that keeps the females from having an infinitely long associative period. According to Bercovitz, the loss of her prey introduces for the female the cost involved with being associative.

Also during the waiting period, it’s likely that one of the male’s nomadic competitors will wander onto the same web. When that happens, the two
males fight.

While collecting data to prove the nutritional status/length of associative period hypothesis, Bercovitz noticed an increase in the ferocity of the
males the closer the female got to maturation, so she ran a statistical analysis to see if there was a true correlation between the intensity of the
fighting and the date of maturity. To her satisfaction, the statistical p-values resulting from her test were convincing.

Bercovitz described the male-male combat as consisting of three ritualized phases. In phase one, the males stretch out their legs and vigorously
shake the strands of the web to show their size and strength. If it’s clear that one spider is bigger and stronger, Bercovitz said, the weaker male
slinks away unharmed and the better spider gets the girl.

If neither male gets scared away after the web shaking, the fighting progresses to phase two. The males begin to wrestle using their pedipalps-two short antennae-like extensions with balls on the end attached to the spider’s head. The males try to shove each other up through the dome of the web. Bercovitz compared this to arm wrestling-a contest to prove physical strength with little danger of physical harm.

Usually these two phases are enough to decide who gets to mate with the female. Bercovitz noticed that the fighting rarely progresses to phase
three, a ferocious free-for-all where the goal is to bite without getting bitten. A sierra dome spider is susceptible to its own venom, so getting
bitten means injuries or death.

The statistical tests Bercovitz ran showed that males are more likely to escalate fights beyond phase one most often the day the female matures,
somewhat less one to two days before, and least often three or more days before. “You don’t sprint at the beginning of a race,” she said. “You
sprint when you know you’re getting close to the finish line.”

A male is willing to escalate fights because a female sierra dome spider will mate with the first available male immediately after she matures, and
at least 60 percent of the sperm that fertilize her egg sac come from that first mate. A male who wants his genes passed on to the next generation
will work as hard as he can to be the only male on a female’s web the moment she matures.

According to Watson, Bercovitz also noticed a “hopping” behavior of the associative female during the male-male fights that Watson had
previously only seen in sexually mature females. “This behavior may function as a signal in addition to her associative behavior to let males know she is within two to three days of maturation and therefore worth fighting harder for,” he said. He also said that Bercovitz’s observations suggest the need for further data collection to clarify exactly what that sign is.

Watson has catalogued all the egg sacs produced from this year’s matings, and will study the inherited characteristics of these spiders next year.
Bercovitz’s observations will be essential to him as he continues working to document the intricate behaviorial patterns of the sierra dome spider.