The limits of endowment effects in great apes (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus)

The endowment effect describes the bias that people often value things that they possess more than things they do not possess. Thus, they are often reluctant to trade items in their possession for items of equivalent value. Some nonhuman primates appear to share this bias with humans, but it remains an open question whether they show endowment effects to the same extent as humans do. We investigated endowment effects in all four great ape species (Pan paniscus, Pan troglodytes, Gorilla gorilla, Pongo pygmaeus) by varying whether apes were endowed with food items (Experiment 1, N = 22) or tools that were instrumental in retrieving food (Experiment 2, N = 23). We first assessed apes' preferences for items of a pair and their willingness to trade items in their possession. We then endowed apes with one item of a pair and offered them to trade for the other item. Apes showed endowment effects for food, but not for tools. In Experiment 3, we endowed bonobos (N = 4) and orangutans (N = 5) with either one or 12 food items. Endowment effects did not differ between species and were not influenced by the number of endowed food items. Our findings suggest that endowment effects in great apes are restricted to immediate food gratification and remain unaffected by the quantity of food rewards. However, endowment effects do not seem to extend to other, nonconsumable possessions even when they are instrumental in retrieving food. In general, apes do not show endowment effects across a range of different commodities as humans typically do.