The bell shaped Hubbert curve is commonly observed when a non renewable, or slowly renewable, resource is exploited in a free market. But what is the origin of the curve? Ugo Bardi and Alessandro Lavacchi have recently published a simple "mind sized" model of the Hubbert behavior in "Energies" . Fat cows and lean cows are not included in the equations of the model, but come as a logical consequence of it.

(Note: I made the figure above using two cow images that I had downloaded from the internet long ago. If someone owns these images, please alert me or the editors. We'll be happy to give due credit or to remove the images if so requested.)