Eight Legs, Six Eyes, and an Aggressive Personality

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Spiders: they are creepy as fuck. Who knows what they’re thinking as they skulk through the dark corners of our world? At best, they are mindless killing robots methodically stalking their prey. At worst, they are malicious schemers, looking for a way to crawl all over you and sink their venomous fangs into your flesh. Who among us has not lived in fear of experiencing the shower scene from Arachnophobia? Or spent the midnight hour cowering while an eight-legged interloper scuttled across the bedroom ceiling? I am probably way more tolerant of these crawlies than the next guy; the mere mention Charlotte’s Web makes me weep. I also know that few species are actually dangerous and widespread aversion to spiders is highly irrational. But even to me, there’s something about them—the excess of legs, the double fangs, the surfeit of eyeballs, and the calculated spinning of intricate webs—that makes them seem inherently sinister.

Thankfully, we have science to puncture our hyperbolic nightmares. First of all, spiders, unlike mosquitos, have no taste for human blood. And, according to biologist Jonathan Pruitt of the University of Pittsburgh, who published the results of a seven-year field study in the science journal Ecology Letters this month, individual spiders have observable, differing personalities. What’s more, the variable personalities of social spiders have a direct effect on their chances of survival. What that means is that not all spiders are hostile killing machines, but some are.

Surprisingly, that's a good thing for understanding how jerk animals help—or hurt—threatened ecosystems.

After researchers discovered that individual spiders of the Anelosimus studiosus species— a social type of spider that thrives in groups rather than as solitary web spinners—can be observed as having either an “aggressive” or “docile” personality Jonathan decided to conduct some tests of his own.

In order to test how individual spider personality affects the whole colony, Pruitt and his team observed what happens when social spiders are placed in a box together. They found that spiders will respond in one of two ways: the “aggressives” (or in this case passive-aggressives) will settle as far apart as the web allows. Meanwhile, the “dociles,” who don’t mind physical contact, will nestle right up against each other in a cute spider-spoon.

To see how personality variables affected overall colony health, Jonathan divided 180 spiders (1,440 legs for those keeping score at home) into three different pair combinations: aggressive/aggressive; docile/docile; aggressive/docile.

The spider’s traits are inherited by their progeny, so docile pairs produce exclusively docile offspring, aggressive pairs create aggressive offspring, and mixed pairs result in both personality types.

After breeding 90 colonies, the researchers took the golf-ball-sized cobwebs into the woods of Tennessee, and left them to flourish—or die.

As a control for the experiment, the research team guarded half of the colonies, removing any invasive species that might exploit the social spiders. At the end of the study, these colonies showed minimal variation in reproductive rates and overall colony health.

Meanwhile, the untended colonies had a variety of outcomes. After six years, the docile pairs had been totally decimated by social parasites since they did little to defend themselves or their colony. Although population boomed at first, as they focused all of their energy on reproduction and feeding, interlopers soon took advantage of their industriousness, feasting off the prey caught in their cobwebs, and gobbling up the spiders themselves. They moved on to their offspring colonies until their entire genetic lineage was extinguished.

Both aggressive pairs and mixed pairs fared better. The aggressive ones spent most of their time fighting with each other and intruders, and spent much less time making sweet spider love. Group tasks like web weaving or prey capture were often derailed by intercolony squabbles. Jonathan calls the oversaturation of aggressive personalities the “Three Stooges Effect,” wherein all spiders are too busy hitting each other over the head and poking each other in the eye to get to the real item on their docket—survival. At the end of the experiment, there were a good number of hardy colonies thriving in the woods.

In a mixed group, the docile types defer to their aggressive counterparts on some tasks, but proactively attended to other matter that the former neglects, such as mating. At the end of the study, more aggressive colonies than mixed colonies remained, suggesting that since both personality types are present in nature, optimal colony health depends on a specific but not equitable balance between aggression and docility. I spoke to Jonathan about hanging out with spiders, what colony health says about personality, and the larger implications of his research.

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VICE: In terms of behavior, what are the big differences between aggressive and docile personality types?
Jonathan Pruitt: Aggressive individuals are attacked by social parasites and predators one-tenth of the time as docile individuals. They were more active in general, constantly grooming and moving around colony. They are likely to attack a predator than run away. Docile individuals just run away and hide. Aggressives will chase off a wasp. They’ll do something like sending webbing over it, just mobbing it. It’s high-risk behavior because the wasp could just grab one of them in the chaos. Sometimes it works really well and sometimes not at all. The dociles will build up their colony so much that that wasp can’t get through the maze to get them, which works for a while but not in the long run.

But why would it be advantageous at all to have any docile members in a colony?
It turns out that having a colony of all aggressive individuals results in what I call the “Three Stooges Effect.” Every individual in the colony will attack the prey, or predator, or repair a web, and then when they touch each other—because they are so intolerant of touch, they spend more time fighting each other and chasing one another off so that stuff goes unnoticed and undone.

Docile individuals tend to do stuff slowly. They tend to join in the task after the aggressive individuals have initiated it. They are very tolerant. For example, they will position themselves on the opposite side of the prey from the aggressives; they will approach the action slowly, almost imperceptibly. Basically their presence waters down the intensity of the aggressive individuals. They still do stuff for the colony. They are better at rearing offspring. There is a division of labor. Too many aggressive individuals are just not so effective. Individually they are highly efficient but when they work in a group they are pretty impotent at most tasks.

How do you think this data helps us better understand ecology and conservation?
Our experiment allows us to watch extinction events happening and know what the major drivers are—it’s these social parasites. The problem with extinction in general is that by the time some animal is at risk, we can’t do experiments on it, because there aren’t that many of them left. But extinction happens all the time with these spiders. And it happens only after a couple of years. Most of our colonies were dead after two or three years. So we can manipulate various stressors, like disease or predation, or manipulate the number of mates, or any sort of other variables, observe all the possible interactions, and see how it influences the extinction risk. I can take individuals from aggressive colonies and put them into docile colonies and see if I can save docile colonies from extinction—see how many individuals have to show up, find out if there is a point when it’s too late to save the colony. These are things that we can test in the lab, that can tell us what should happen.

So you think understanding personality will help you engineer a less hostile environment for threatened species.
My research, although its not on an endangered species, could help us identify when a certain species might be at risk, what are some viable options to save them, and in general, how you can decrease the probabilities of extinction. This is a real life animal that’s super abundant but that has colony extinction happening all the time that you can manipulate experimentally. We can stage gene flow, taking individuals from non-dead-end populations and putting them in dead end populations, so that we can see if we can save them. We already do things like that in conservation—we don’t do experiments but we just try things because we’re desperate. We’ll do things like take the mountain lion and introduce it in Florida in order to save the Florida panther. That actually didn’t work very well, but one of the reasons might be because it was already too late to save the Florida panther by the time we brought the mountain lion in.

But it’s also as likely that we will want to create extinction, for diseases, or invasive species that have moved across the world, or pest species in agriculture. We want to get rid of those things; we want them to go extinct at least locally, so perhaps it is useful to manipulate extinction so we can see how that otherwise wouldn’t happen.

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