Optimal skew theory

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The optimal skew theory (possible translation: theory of “optimal skewness”) deals with the reproductive success of individuals in (social) animal species living together in groups. It looks at the factors that determine the formation and breakup of groups and group size in relation to the individual rate of reproduction. The theory goes back to the American biologist Sandra L. Vehrencamp. a. by Hudson R. Reeve, Laurent Keller, Peter Nonacs, Michael A. Cant.

Problem

In species whose individuals form social associations, the reproductive success is not evenly distributed between the group members. Usually there is a group hierarchy, with dominant individuals with high reproduction and subordinate individuals with less. The existence of the group then poses an optimization problem from the perspective of its members. Subordinate individuals could also leave the group in order to potentially produce more offspring elsewhere. Dominants could drive the subordinates out of the group and thus increase their own high proportion of the whole. Under what conditions can a group exist at all? In different species all possible relationships, from groups of low skewness, in which almost all individuals reproduce, to those of high skewness in which one or a few dominant individuals monopolize reproduction and their subordinates miss out, are realized. What determines which model prevails?

When introducing the theory, reproductive success in the group relative to individuals was considered an important factor. If groups are generally more successful, it is also better for dominants to sacrifice part of their reproductive possibilities in favor of group cohesion in order to keep them together. They can follow a “bribery” strategy by granting the subordinates advantages in order to persuade them to stay (so-called concession models), or “despotic” using coercion in order to increase their costs when leaving the group to drive (so-called restraint models) .; but even for them the exertion of coercion is not free and costs money.

A group can also be held together by subordinate individuals initially foregoing reproductive opportunities in the hope of becoming dominant at some point later. Under these conditions, groups can even remain in which the dominants do not share at all. The prerequisite is that the costs of leaving the group are high, for example because you can “inherit” a valuable property, such as a territory or a nest. The relationships to the related theories of the conflict between parents and offspring that are revealed thereby also exist in the mathematical version of the respective models.

As always when modeling social behavior, the influence of kinship between individuals also plays a role. If individuals abstain from procreation in favor of relatives, preferably graded according to Hamilton's rule according to the degree of kinship, the “inclusive fitness ” increases (see under relatives selection ). Depending on the model used, the optimal skew theory also predicts cooperation between non-relatives. Ecological factors play another important role , particularly the level of resources. If resources are scarce and costly to extract, the benefit of cooperation is usually greater.

Models

A large number of competing mathematical models have been proposed in the optimal skew theory for modeling the relationships. These models can be classified into different categories.

  • Transaction models are ultimate models that deal with the reasons for the development of behavior. Depending on the relationship between the reproductive success in the group and the individual alone, they predict a certain range of values ​​in which the reproductive success is higher for both dominant and subordinate group members when they coexist in the group. The decisive factor is not the absolute height, but only the ratio.
  • Compromise models are more interested in the, proximate, methods and strategies with which group members seek to maximize their respective reproductive success. You are looking for evolutionary stable strategies for the allocation of resources to reproduction and aggressiveness depending on the dominance status. According to these models, for example, the cohesion of a group depends in a predictable way on the resource level in the habitat.
  • Synthetic models try to combine both approaches in one model.

criticism

After receiving a great deal of initial attention, the optimal skew theory fell into a crisis from the late 1980s. It could be shown that many predictions of the theory depend on the exact structure of the mathematical models used. For example, early versions always assumed a linear relationship between resource allocation in intraspecific aggressive behavior and reproductive success. If the relationship is not linear, the results may be completely different. Due to the large number of models, the predictive value also decreased. Results that had been published as a refutation of the theory could be reinterpreted by changing the model assumptions at once to confirm them. The proponents of the theory, however, hold on to its value and try to develop new models that should refute the criticism.

Individual evidence

  1. ^ L. Keller & HK Reeve (1994): Partitioning of reproduction in animal societies. Trends in Ecology and Evolution 9: 98-102.
  2. ↑ Sandra L. Vehrenkamp (1983): Optimal degree of skew in Cooperative Societies. Integrative and Comparative Biology, Volume 23 Issue 2: 327-335. doi : 10.1093 / icb / 23.2.327
  3. ↑ PM Buston, HK Reeve, MA Cant, SL Vehrencamp, ST Emlen (2007): Reproductive skew and the evolution of group dissolution tactics: a synthesis of concession and restraint models. Animal Behavior 74: 1643-1654. doi : 10.1016 / j.anbehav.2007.03.003
  4. ^ Hanna Kokko and Rufus A. Johnstone (1999): Social queuing in animal societies: a dynamic model of reproductive skew. Proceedings of the Royal Society London Series B: 266, 571-578.
  5. ^ RL Trivers (1974): Parent – ​​offspring conflict. American Zoologist 14: 249-264.
  6. ↑ a b Michael A. Cant (2006): A tale of two theories: parent – ​​offspring conflict and reproductive skew. Animal Behavior 71: 255-263. doi : 10.1016 / j.anbehav.2005.03.040
  7. ↑ Barbora Trubenová & Reinmar Hager (2012): Reproductive Skew Theory. ELS citable reviews in the life sciences. doi : 10.1002 / 9780470015902.a0023661
  8. ↑ a b Peter Nonacs & Reinmar Hager (2011): The past, present and future of reproductive skew theory and experiments. Biological Reviews 86: 271-298. doi : 10.1111 / j.1469-185X.2010.00144.x