Nocturnal encounters: spider colonies of the Kalahari

On the odd occasion during my night-time wanderings of the Kalahari desert, I would come across something very, very interesting. Something that always managed to distract me utterly from the task at hand (yes, regular readers are by now figuring out I'm easily distracted) due to my ongoing fascination with all things arachnid. Sometimes, in the night, I'd run into one of these:

The nest of the colonial Stegodyphus spider

Yes that is a gigantic nest full of spiders

Living in colonies of up to 2000 individuals, Stegodyphus are unusual among arachnids in that they are able to co-exist in these vast social groups. For most spiders, if you put a bunch them together in a small space you're going to end up with... Well, one fat spider. But these ones have learned a neat trick for maximizing fitness. They've learned to co-operate.

From how they hunt to which individuals breed and perform other tasks, Stegodyphus colonies parcel out jobs to different individuals. How members of the colonies get assigned to tasks turns out to have a lot to do with both size and body condition, according to a study by scientists at Pittsburgh university. Smaller individuals, for example, are more likely to work at tasks like web building. Spiders in good body condition were found to actually be less likely to participate in prey capture, but this was not dependent on other factors like their size. Or their personality.

What do you mean personality?

Yes, personality. Are we going to have a problem here?

Spiders, as it turns out have personality, just like nearly everything else we've tested, from sea anemones to elephants. Usually, in order to measure this, animals will be presented with an experimental situation such as encountering a novel object or being given a puzzle to solve. Factors such as their latency to approach the object, willingness to interact with it and time taken to solve or give up on a puzzle are then scored to place an individual somewhere along a spectrum of boldness/shyness thus allowing scientists to compare individuals in terms of groups of linked traits (i.e. Some may be bold AND aggressive).

In the case of the spiders (and the sea anemones incidentally) the test used to determine personality was how long it took individuals to move again after being threatened by a predator. The predator for the spiders was simulated by a puff of air being blown across the their bodies. Bolder, more aggressive individuals would then take a shorter amount of time to get going again.

The birds are coming! I'm gtf out of here!

The really fascinating part of these studies is how the personality of individual spiders influences the colony. As it turns out, spiders in the group follow the lead of the boldest among them. So if the leader attacks, many of the others follow suit. Interestingly, the roles particular spiders play have been found to be influenced by how long they've known one another. Spiders which had been together in a particular group for a longer time, settled more easily into their roles as either bold or shy.

To add a little twist, the Pittsburgh university scientists trained spiders to identify particular vibrations with either the presence of an ant (a dangerous enemy) or an edible moth. These spiders were then introduced into groups where all other spiders were naive (i.e. Had never experienced the training vibrations). When presented with the food vibration, the bolder trainees mobilized their forces and attacked rapidly.

The ant vibration however was now backed up with either an ant or with a tasty insect. In both cases, bold individuals refused to attack and somehow prevented their web-mates from doing so as well! Shy spiders on the other hand never seemed to learn what either signal meant. Unsurprising, as one of the traits associated with being bold in animal personality terms is greater curiosity.

The influence of bold spiders was so great in these tests, that some colonies actually starved rather than making a move contrary to what their 'Fearless Leader' had determined was correct.

Arachnophilia

Now while I'm sure this whole thing gives some of you the absolute creeps, think for a second about the astounding complexity of it all. For sure, when I'd find one of these groups in a desert bush I could happily spend hours simply watching them go about their tasks in such a structured, careful way. Yes, they're spiders. But they're very clever ones.

Till next time people,
Happy Steeming,
The Wise Fox

Sources:
https://www.livescience.com/47566-social-spider-personalities-colony.html
http://www.iflscience.com/plants-and-animals/misinformed-spiders-lead-colonies-disaster/
http://www.springer.com/about+springer/media/springer+select?SGWID=0-11001-6-1458255-0
https://phys.org/news/2014-03-size-personality-kalahari-social-spiders.html
https://link.springer.com/article/10.1007%2Fs00265-014-1696-9

Images: all images are from wikimedia commons