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Gustafsson et al.’s data support their hypotheses. Domestic and crossbred pigs’ behavior followed optimal foraging theory. Both groups spent less time in patches with less food and visited fewer patches when there was a cost to move to a different patch. As predicted, the crossbred pigs used a costlier foraging strategy by moving between patches more frequently than the domestic pigs ( [link] ).
The finding that domesticated animals use a less costly foraging strategy than wild ones is not limited to pigs—for example, it has been found in comparisons of domesticated and crossbred chickens (Andersson, 2001; Schutz, 2000)—and may be characteristic of the domestication process (Jensen and Gustafsson, 1997).
Domestication is the process by which animals adapt to humans and their environment (Price, 1984). The main feature of domestication that separates it from taming is that for domestication to occur, humans must control which individuals reproduce (Mignon-Grasteau et al., 2005). This process, in which humans select for desired traits and may produce new breeds, is called artificial selection.
Resource allocation theory argues that an animal’s resources are balanced between traits for breeding and production (Mignon-Grasteau et al., 2005). Thus, when humans select for a particular trait, there will be a decrease in another trait unless the animal’s resources increase. As a consequence of this, humans selecting for one trait may unintentionally change an unrelated trait.
For some traits, the pressures of artificial selection and natural selection may bring about the same result. For example, both natural and artificial pressures favor large pigs. Traits for self-defense or predatory behavior are not selected by humans, and often decrease or are lost in the process of domestication (Mignon-Grasteau et al., 2005). For instance, natural selection has resulted in wild boars with long, sharp tusks while selective breeding has resulted in domestic pigs with less conspicuous tusks.
Wild and domestic pigs are highly social animals (Grandin, 2009, Graves 1984). As piglets, they fight viciously over their mother’s teats before setting up a hierarchy that determines which sibling receives the most milk (Fraser, 1991; Fig. 4).. Even after they are weaned, pigs’ feeding behavior is complex and rarely solitary. Social foraging theory is used to study these types of interactions, in which one forager’s actions affect another’s (Giraldeau and Caraco, 2000). Social foraging theory and optimal foraging theory are not competing theories. Optimal foraging theory predicts the behavior of individual foragers while social foraging theory predicts the behavior of foragers in groups.
Adult pigs also establish dominance hierarchies that help quell competition for food (Nielsen, 1996), but this does not mean that aggression motivated by food resources disappears entirely. Wild boars fight more often in the winter when food is scarce than in the summer when it is more plentiful (Graves, 1984). Thomsen et al. hypothesized that this may be due to the spatial distribution of food as well as its availability (Thomsen et al., 2010). They predicted that the number of aggressive interactions between domestic pigs would increase when buckets of food were clustered together versus spread out to a medium or far distance from each other, even if the total amount of food were the same in each condition. As shown in [link] , this hypothesis was supported.
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