A reader sent me the link to an old scientific article (from 1995) about a “mathematical spider” living in the Namibian desert. It turns out that the adjective “mathematical” is pretty misleading, but since the article was interesting I thought I’d give it a brief shout-out.
The paper, published by G. Costa et al. in the Journal of Arid Environments, (reference below, can join ResearchGate and get free copy, or ask me), is about a new species in the family of tube-dwelling spiders, Segrestriidae; the species is named Ariadna sp. (the “sp.” means “species not identified”, although it may well have been in the last 18 years).
24 specimens of this ground-dwelling spider were studied near Gobabeb, a well-known research station in the desert of Namibia. The spiders dig burrows in the ground from whence they venture to get prey. Here’s a picture of the spider:
Here’s its bleak habitat. Life is nearly everywhere on this planet:
Some gypsum casts of its burrows. There are about 2.5 cm to the inch, so the burrows are about five inches long. The spiders line them with silk.
Now for the “mathematical” part. For reasons yet unknown, the spiders pile stones around the entrance of their burrows. The stones are fairly uniform in size, and there are usually about seven, though the number ranges from five to nine. The centimeter scale is at the top, along with a Namibian five-cent coin for extra scale (useful only to Namibians!)
The interesting thing about the stones is that they are usually placed radially, with the narrowest parts near the burrow, and of fairly uniform size. This makes the burrow look a bit like a flower. Stones are clearly selected for size, as you can see by the surrounding stones. Perhaps the spiders use the biggest stones that they’re able to carry. The photo below shows a typical array of 7 stones.
The “mathematical” part, which the authors make a great deal of, is that the mean number of stones (and the mode) is seven, with other numbers distributed fairly symmetrically around that. Here’s the table showing the percentage of stones falling in each category:
Well, that’s interesting, but hardly mathematical. It doesn’t show at all that the spiders can count, and it’s not surprising in any way that the distribution is a bell-shaped curve. What may be going on here is simply that the spiders heft the largest stones they can carry, that their burrows are of a relatively fixed size because spiders are of a relatively fixed size, and the average number of spider-heft-able stones that can surround a burrow happens to be seven. The spiders do apparently exercise a preference for quartz stones, though again this preference isn’t documented statistically.
What is more interesting to me is that the stones appear to be placed radially (though I’d like to see more photos and measurements), and, especially, that the spiders even bother to highlight their burrow this way. Why? The authors raise three possibilities:
a. Detection or attraction of prey. The authors suggest that “the stone ring could perhaps attract prey or facilitate the detection of prey by the spider waiting inside its burrow.” Well, maybe, though the attraction hypothesis seems more viable than the detection one. At any rate, this could be tested, even in the lab, by removing the stones and seeing fewer prey approach the burrow. But neither of these hypotheses seem really convincing.
b. Strengthening the burrow and making it impervious to sand or debris. This seems more likely to me. The raised stones could keep dirt or sand blowing along the ground from entering and clogging the burrow. Again, this could be tested fairlly easily.
c. Deterrence of predators. Here’s what the authors say:
The stone ring might be a way of reducing predatory risk. The evenly lighted circle around the burrow’s entrance could make this appear like, [sic] a black stone or a shaded area. On the other hand, spider holes may simulate the little black stones that are scattered over the gravel plain. The characteristic alternation of light and dark areas on the gravel ground complicates the detection of real burrows by predators.
That’s possible, but again requires testing. Another possibility, which the authors don’t mention, is that the symmetrical pattern may help the spider find its burrow in a complicated patchwork of stones and ground. In other words, the stones could act as landmarks, much the same way that some ground-dwelling “digger” wasps recognize their burrows by the patterns of debris on the ground nearby. (That work, a classic study of animal behavior by Tingergen and Kruyt, showed that the wasps could be confused by simply moving the landmarks around a burrow.) This “recognition” hypothesis may not be likely, though, if the spiders’ vision is poor.
At any rate, it’s a cute behavior whose significance is not yet determined, but would seem to be tractable to easy experiments.
Costa, G., A. Petralia, E. Conti, and C. Hanel. 1995. A mathematical spider living on gravel plains of the Namib Desert. Journal of Arid Environments 29:485-494.