Science-- there's something for everyone

Tuesday, August 27, 2013

Spiders have personalities

We’re all familiar with the amazing social behavior of ants and bees. But did you know that certain spiders also live in colonies? Not only that, but according to Lena Grinsted and her colleagues from Aarhus University, the social spiders Stegodyphus sarasinorum have distinct personalities.

In order for any large social colony to function, members must perform several different types of jobs. For example, some group members might be responsible for gathering or preparing food whereas others might defend the nest. Honeybees sort their tasks by age with each individual progressing through a set schedule of jobs as she matures. However, in Stegodyphus colonies, all the spiders tend to be the same age. How do they decide which among them will be responsible for capturing prey and which for warding off trespassers?

One possibility is that the spiders divide up jobs by body size. That is, spiders that happen to be larger than their mates might gravitate to certain jobs. Another is that the spiders have individual preferences or personalities. 

To test these possibilities, the scientists selected forty spiders out of each of sixteen different nests and painted them, possibly using a technique similar to this method of painting ants. You can see the lovely result below.
Color-coding spiders helps scientists study their personalities. 
Photo: Lena Grinsted.

Those forty spiders were tested for boldness (how quickly they recovered from a perceived threat), aggression (how they reacted to being poked with a stick) and eagerness to capture prey (how quickly they emerged from the nest to investigate a possible meal and whether they then attacked that prey).

The boldest spiders were the first to rush out of the nest at any sign of prey. They were also the first ones to attack and subdue that prey. Larger spiders were also quick to emerge, but they didn’t necessarily attack first. In fact, size and developmental stage didn’t really make that much difference in determining the spiders’ behavior. Some of them seemed to be inherently bolder or more aggressive. 

Because of these differences, the authors conclude that S. sarasinorum have some rudimentary personality traits, though it's not clear what causes the variation in behaviors. Interestingly, there were also colony-wide differences in how the entire spider communities reacted to stimuli. As S. sarasinorum colonies are highly inbred (siblings tend to remain in the same nest together after having consumed their mother), this could indicate a genetic component to spider personality.

Obviously, no one is suggesting that arachnologists begin administering the Myers-Briggs test to their little charges. However, these data do show that spiders do not react as automatons and that there are differences between individual spiders. Needless to say, this was a rather unexpected result. We don't tend to think of such creatures as individuals. This will definitely give me pause the next time I sweep out my house. 

Lena Grinsted, Jonathan N. Pruitt, Virginia Settepani, & Trine Bilde (2013). Individual personalities shape task differentiation in a social spider Proceedings of the Royal Society B, 280 (1767) DOI: 10.1098/rspb.2013.1407.

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