by Joseph DeSisto
Should you ever find yourself in the Kalahari Desert in southern Africa, keep an eye out for small spiders with plump, velvety bodies, black-banded legs, and a thick white stripe running down the abdomen. These spiders are likely to be Stegodyphus dumicola, the social velvet spider.
Now capture as many as you can and put them in a jar. If these were ordinary spiders, the jar would immediately become the stage for a bloodbath: most spiders are fiercely cannibalistic. But velvet spiders are not ordinary spiders. Put them in a jar, and they will begin to work together, filling the jar with silk. All of this happens with virtually no disagreement, bullying, or typical spider humorlessness.
Recently I had the chance to talk with Colin Wright, a PhD student studying social spiders at the University of Pittsburgh. As it happens, he has been to the Kalahari and seen firsthand what happens when these spiders team up. He told me, “These spiders are highly tolerant of one another. In fact, they are so tolerant that they will even get along with other closely related Stegodyphus spiders being in their colonies.”
Outside the jar, social spiders work together to construct large webs. These webs contain shelters in which the spiders can hide, and sheet-like portions used to capture insects. Although each spider alone is small and unlikely to succeed in tackling dangerous prey, together these spiders can take down animals much larger than themselves. I asked Wright what the record was for largest prey in a social spider’s web:
“Believe it or not, we have found the carcasses of mice in some of [dumicola‘s] capture webs. We have yet to see any active feeding on small mammals, but I don’t doubt for a moment that, in larger colonies, they will devour a mouse. We have fed these things giant locusts, while it’s a struggle, they don’t have that much difficulty taking them down.”
Working together, a team of social spiders can be as formidable as any tarantula, but not all spiders participate equally in prey capture. If a moth tumbles into the web, no problem, but when a locust or wasp gets stuck, only a brave spider wants to take the first bite. Here’s where things get complicated — some spiders are more cautious spiders than others, and when a colony is dominated by cautious spiders, life becomes much more difficult.
In any society, two major factors affecting success are a) the number of individuals and b) the personalities of those individuals, whether they be spiders, bees, or humans. But which is more important? You might think that since spiders aren’t especially bright, numbers matter more than something as abstract as personality, but Keiser and Pruitt (2014) would disagree. They did just what I told you to do on your hypothetical Kalahari vacation, only instead of putting spiders in jars at random, they tested each spider to determine whether it was “bold” or “shy.”
When reading peer-reviewed papers, it’s tempting to skim the “Methods” section, since often the methods are barely comprehensible to a non-expert. This paper, however, was worth a closer read. The authors tested spider bravery — or in their words, conducted “individual personality assays” — by puffing air on them using “an infant ear-cleaning bulb.” Arachnologists like to have fun.
When so accosted, the spiders stopped moving, and the time it took for them to start moving again was used to decide whether the spider was bold, shy, or in-between. Keiser and Pruitt then assembled these spiders into colonies, either with only bold individuals, only shy, only in-between, or a mix. They also made some colonies larger than others, to see whether colony size was more important than personality.
Next, Keiser and Pruitt wanted to see how quickly the spiders in each colony responded to a prey item. Naturally everything had to be standardized, so instead of using live insects they simulated a prey item in each web using “a battery-powered handheld vibratory device.” As expected, colonies with more bold spiders responded the fastest to the simulated prey, since bolder spiders have fewer qualms about leading the attack.
Personality had a much bigger role than colony size in deciding which colonies responded the fastest to prey, so much so that “colonies of just 10 bold spiders would attack prey with as many attackers as colonies of 110 ‘average’ spiders” (Keiser and Pruitt 2014). Even more revealing, a separate experiment showed that if you add just one bold spider to a colony of timid spiders, the whole colony becomes more aggressive (Pruitt and Keiser 2014).
The bolder spiders tend to be the largest (Wright et al. 2015), and because spiders don’t take turns eating, smaller or slower individuals sometimes have to skip meals. Colonies don’t always harvest enough insects to keep everyone plump and happy, but if a web isn’t very productive, spiders always have the option of leaving to start their own colonies. When things get tough, a social spider can make a balloon out of silk and float away on a draft (Schneider et al. 2001).
We can learn a lot about the evolution of social behavior by studying social spiders. There’s still a lot left to learn, but one thing is clear: the next Spiderman movie must feature a superhero who can amass a spidery army to take down even the biggest, baddest villains, and then balloon his way out of tense social situations.
A big thank you is owed to Colin Wright for answering some of my questions about Stegodyphus dumicola. Wright is a student in Dr. Jonathan Pruitt’s lab at the University of Pittsburgh: you can learn more about that lab’s work here.
Several months ago I wrote an article called “Spiders Only Love Once” about another Stegodyphus species, the lined velvet spider (S. lineata). Lined velvet spiders don’t form colonies, but they do have amazing lives, which you can learn about here.
Keiser C.N. and J.N. Pruitt. 2014. Personality composition is more important than group size in determining collective foraging behaviour in the wild. Proceedings of the Royal Society B 281(1796), doi: 10.1098/rspb.2014.1424
Miller J.A., C.E. Griswold, N. Scharff, M. Řezáč, T. Szűts, and M. Marhabaie. 2012. The velvet spiders: an atlas of the Eresidae (Arachnida, Araneae). ZooKeys 195: 1-144.
Pruitt J.N. and C.N. Keiser. 2014. The personality types of key catalytic individuals shape colonies’ collective behaviour and success. Animal Behavior 93: 87-95.
Schneider J.M., J. Roos, Y. Lubin, and J.R. Henschel. 2001. Dispersal of Stegodyphus dumicola (Araneae, Eresidae): They do balloon after all! The Journal of Arachnology 29: 114-116.
Wright C.M., C.N. Keiser, and J.N. Pruitt. 2015. Personality and morphology shape task participation, collective foraging and escape behavior in the social spider Stegodyphus dumicola. Animal Behavior 105: 47-54.