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Edited by Dorothy L. Cheney, University of Pennsylvania, Philadelphia, PA, and approved May 18, 2017 (received for review January 28, 2017)
Significance
Many animals, especially humans, carry out activities collectively because the benefits of doing so exceed those that can be achieved individually. But how can collective action evolve when individuals receive the benefits of cooperation regardless of whether they pay the costs of participation? Collective action may be especially difficult to achieve when groups are large, because each individual’s contribution has a small effect on the outcome. We show that group augmentation theory helps explain one of the most striking examples of collective action in nonhuman primates, territorial boundary patrolling by male chimpanzees: Males bear the short-term costs of patrolling even when they have little to gain immediately because patrolling enhances group size, increasing the male’s chances of future reproduction.
Abstract
How can collective action evolve when individuals benefit from cooperation regardless of whether they pay its participation costs? According to one influential perspective, collective action problems are common, especially when groups are large, but may be solved when individuals who have more to gain from the collective good or can produce it at low costs provide it to others as a byproduct. Several results from a 20-y study of one of the most striking examples of collective action in nonhuman animals, territorial boundary patrolling by male chimpanzees, are consistent with these ideas. Individuals were more likely to patrol when (i) they had more to gain because they had many offspring in the group; (ii) they incurred relatively low costs because of their high dominance rank and superior physical condition; and (iii) the group size was relatively small. However, several other findings were better explained by group augmentation theory, which proposes that individuals should bear the short-term costs of collective action even when they have little to gain immediately if such action leads to increases in group size and long-term increases in reproductive success. In support of this theory, (i) individual patrolling effort was higher and less variable than participation in intergroup aggression in other primate species; (ii) males often patrolled when they had no offspring or maternal relatives in the group; and (iii) the aggregate patrolling effort of the group did not decrease with group size. We propose that group augmentation theory deserves more consideration in research on collective action.
Footnotes
- ↵1To whom correspondence should be addressed. Email: kevin.langergraber{at}asu.edu.
Author contributions: K.E.L., D.P.W., and J.C.M. designed research; K.E.L., D.P.W., and J.C.M. performed research; L.V. contributed new reagents/analytic tools; K.E.L. and J.C.M. analyzed data; and K.E.L., D.P.W., L.V., and J.C.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Data used in this paper are available at datadryad.org (doi:10.5061/dryad.kk33f).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1701582114/-/DCSupplemental.
- aSchool of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287;
- bInstitute of Human Origins, Arizona State University, Tempe, AZ 85287;
- cDepartment of Anthropology, Yale University, New Haven, CT 06511;
- dPrimatology Department, Max Planck Institute for Evolutionary Anthropology, Leipzig, 04103, Germany;
- eDepartment of Anthropology, University of Michigan, Ann Arbor, MI 48109
Edited by Dorothy L. Cheney, University of Pennsylvania, Philadelphia, PA, and approved May 18, 2017 (received for review January 28, 2017)
Significance
Many animals, especially humans, carry out activities collectively because the benefits of doing so exceed those that can be achieved individually. But how can collective action evolve when individuals receive the benefits of cooperation regardless of whether they pay the costs of participation? Collective action may be especially difficult to achieve when groups are large, because each individual’s contribution has a small effect on the outcome. We show that group augmentation theory helps explain one of the most striking examples of collective action in nonhuman primates, territorial boundary patrolling by male chimpanzees: Males bear the short-term costs of patrolling even when they have little to gain immediately because patrolling enhances group size, increasing the male’s chances of future reproduction.
Abstract
How can collective action evolve when individuals benefit from cooperation regardless of whether they pay its participation costs? According to one influential perspective, collective action problems are common, especially when groups are large, but may be solved when individuals who have more to gain from the collective good or can produce it at low costs provide it to others as a byproduct. Several results from a 20-y study of one of the most striking examples of collective action in nonhuman animals, territorial boundary patrolling by male chimpanzees, are consistent with these ideas. Individuals were more likely to patrol when (i) they had more to gain because they had many offspring in the group; (ii) they incurred relatively low costs because of their high dominance rank and superior physical condition; and (iii) the group size was relatively small. However, several other findings were better explained by group augmentation theory, which proposes that individuals should bear the short-term costs of collective action even when they have little to gain immediately if such action leads to increases in group size and long-term increases in reproductive success. In support of this theory, (i) individual patrolling effort was higher and less variable than participation in intergroup aggression in other primate species; (ii) males often patrolled when they had no offspring or maternal relatives in the group; and (iii) the aggregate patrolling effort of the group did not decrease with group size. We propose that group augmentation theory deserves more consideration in research on collective action.
- chimpanzees
- Pan troglodytes
- collective action
- cooperation
- territoriality
Footnotes
- ↵1To whom correspondence should be addressed. Email: kevin.langergraber{at}asu.edu.
Author contributions: K.E.L., D.P.W., and J.C.M. designed research; K.E.L., D.P.W., and J.C.M. performed research; L.V. contributed new reagents/analytic tools; K.E.L. and J.C.M. analyzed data; and K.E.L., D.P.W., L.V., and J.C.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: Data used in this paper are available at datadryad.org (doi:10.5061/dryad.kk33f).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1701582114/-/DCSupplemental.
Chimpanzee patrolling
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