Social resource foraging is guided by the principles of the Marginal Value Theorem

Abstract

Optimality principles guide how animals adapt to changing environments. During foraging for nonsocial resources such as food and water, species across taxa obey a strategy that maximizes resource harvest rate. However, it remains unknown whether foraging for social resources also obeys such a strategic principle. We investigated how primates forage for social information conveyed by conspecific facial expressions using the framework of optimal foraging theory. We found that the canonical principle of Marginal Value Theorem (MVT) also applies to social resources. Consistent with MVT, rhesus macaques (Macaca mulatta) spent more time foraging for social information when alternative sources of information were farther away compared to when they were closer by. A comparison of four models of patch-leaving behavior confirmed that the MVT framework provided the best fit to the observed foraging behavior. This analysis further demonstrated that patch-leaving decisions were not driven simply by the declining value of the images in the patch, but instead were dependent upon both the instantaneous social value intake rate and current time in the patch.Optimal foraging theory describes the behavior of animals seeking out resources in a patchy environment according to an energy-maximizing strategy. As animals use the supply of resources within a patch, patch value naturally declines and animals must decide when to leave the current patch in search of a new one. The Marginal Value Theorem (MVT) is a broadly applied optimality model that predicts foraging behavior in a variety of taxa [1][2][3] . MVT predicts that animals should leave the current patch when the energy intake rate within the patch diminishes to the average energy-harvesting rate in the environment 4,5 . Thus, the time that animals spend within a patch (i.e., patch-residence time) depends upon a variety of factors, including the value of the current patch (in terms of the resource being consumed), the value of other patches in the environment, and the time it would take to travel to the next closest patch (i.e., travel time).The optimality of the strategies animals use to forage for primary resources such as food and water has been studied broadly. Across taxa, animals seek out primary resources in accordance with MVT, spending relatively more time in high quality patches that are farther from other patches in the environment and less time in patches that are low quality and nearer to other patches 1,3,[6][7][8] . This optimal foraging model has also been applied to describe foraging for nonsocial information. Human subjects foraged for scholarly publications 9 ; written content in online web searches (e.g., ref. 10 ); and even their own memories 11 . These studies used a patch foraging framework to model humans' strategic decision processes focused on exploiting nonsocial information. Although information is inherently difficult to quantify relative to primary resources, individuals' nonsocial information-foraging decisions could be described using an optimal f...

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