The Gathering Return Bound: Energy per Hour and the Distance at Which Gathering Stops Paying for Twigs, Leaf Litter, and Fallen Branches

Gathered fuel is usually costed, if at all, by the weight carried home. This paper costs it by the hour and by the kilometre, for three fuels gathered in a stated headload of 0.06 cubic metres: twigs, leaf litter and fallen branches, each with stated energy content, moisture, bulk density and gathering rate. At a stated gathering site 1 km away, fallen branches return 75.97 MJ of fuel energy per hour of total labour, twigs return 57.71 MJ per hour and leaf litter returns 23.78 MJ per hour. A break-even condition is then derived in which the fuel gathered must exceed the gatherer's metabolic cost of the whole trip by a stated factor of 10.0, reflecting that food energy is far more valuable per megajoule than fuel energy. Under that condition, leaf litter stops paying at 4.87 km, while twigs and fallen branches remain within the condition out to 17.15 km and 25.03 km respectively and are instead limited by a stated eight-hour trip cap, at 12.69 km and 12.41 km. Without the stated margin, metabolic break-even alone would allow absurd distances of 58.4 km for leaf litter and 266.7 km for branches, which shows that a bare metabolic comparison cannot govern gathering decisions. The common practice of carrying leaf litter home over distances beyond about 5 km does not pay for itself under the stated margin. This study is simulated, not measured. SIMULATED, NOT MEASURED.