From coal-tit to cannibalistic spiders

By Eric Crampton 22/05/2015


XKCD’s spider explainer has been making the rounds. And one part reminded me of Gordon Tullock.

In 2009, the Back River Wastewater Treatment Plant found themselves dealing with what they called an “extreme spider situation.” An estimated 80 million orb-weaving spiders had colonized the plant, covering every surface with heavy sheets of web.[10] The whole thing is detailed in a fascinating and horrifying article published by the Entomological Society of America.[12]
What was the total force of gravity from all those spiders? First we need their mass; according to a paper titled Sexual Cannibalism in Orb-Weaving Spiders: An Economic Model, it’s about 20 grams for males and several times that for females.[11] So even if you were standing next to the Black River Wastewater Treatment Plant in 2009, the pull of all the spiders inside would still be only 1/50,000,000th that of the Sun.

The paper models when the female spider will eat the male rather than mate as a function of number of potential males around for mating and other food sources. So opportunity costs and relative prices.

Gordon Tullock founded the field of bioeconomics when he observed that the Coal Tit seemed to apply rational choice in its food search, or at least that one could improve on an avian ecologist’s modelling by putting it into that framework. He later founded the journal Bioeconomics.

Here’s Janet Landa’s summary:

Tullock’s (1971) first published bioeconomics paper titled, “The coal tit as a careful shopper”, was published in The American Naturalist, a scientific journal sponsored by The American Society of Naturalists.

According to Tullock, the inspiration for his coal tit article was provided when he read a book by David Lack (1966)—an avian ecologist—in which Lack summarized the work of J. Gibb (1958) on the coal tits’ consumption of eucosmid moth (Ernarmonia conicolana). Because Gibb used a diagram which, to Tullock, looked somewhat like the economist’s demand and supply diagram, this led Tullock to develop Gibb’s idea by explicitly formulating coal tits’ foraging behavior as an economic optimizing problem by comparing the coal tits’ behavior to that of a careful housewife comparison-shopping in the cheapest market: the coal tit would seek its grubs in those areas where the energy cost would be lowest; in other words, coal tits:

… are maximizing the return to their labor in searching out food supplies… . Presumably, they have inherited an efficient pattern of behavior resulting from natural selection which would eliminate inefficient heritable behavior patterns (p. 77).

Tullock (pp. 79–80) ended the article by saying:

It may surprise biologists and certainly will surprise economists to learn that it is possible to use segments of economic theory to explain biological phenomena. Nevertheless, it seems to me that the problems of biologists are difficult enough so that they should seek help wherever it seems to them that their particular competence may be of value. This essay is, then, an effort, to establish a minor link between two fields that at one time were very closely connected, but have grown apart.

Alas, the 1991 spider paper doesn’t cite Tullock.

Next time you see a member of the Swedish Academy who failed to vote for a Tullock Nobel, kick it in the shins for me.