Behavior in which animals form groups and cooperate or compete is called social behavior. Understanding the evolutionary logic of social behavior, including territories, dominance hierarchies, altruistic behavior, and kin selection, is central to behavioral ecology.
Basic Knowledge
Territory: Behavior in which an individual or group exclusively uses and defends a particular area. Territories are established to secure resources such as food, mates, and nesting sites. Displays such as calls, scent marking, and visual displays are used to defend territories, avoiding direct fighting. A territory is advantageous when defense cost < resource benefit.
Dominance hierarchy: Individuals within a group have stable rank relationships. Higher-ranking individuals have priority access to resources. Examples include chimpanzee societies and the “pecking order” of chickens. A dominance hierarchy reduces direct competition among individuals and lowers the cost to the group.
Altruistic behavior and kin selection theory (Hamilton): Behavior that lowers one's own fitness while increasing the fitness of another individual. Inclusive fitness = direct fitness (one's own reproduction) + indirect fitness (contribution to the reproduction of relatives). Altruistic behavior can evolve when relatedness (r) × the beneficiary's benefit (B) > cost (C) (Hamilton's rule: rB > C). Examples include sentinel behavior in meerkats and reproductive abandonment by worker honeybees.
Reciprocal altruism (Trivers): Altruistic behavior can occur even between unrelated individuals when future returns are expected. Conditions: long life spans, individual recognition, and repeated encounters. It is analyzed using the “prisoner's dilemma” model. Example: food sharing among vampire bats.
Using Hamilton's kin selection theory, explain why worker bees can evolve behavior that helps the queen bee without reproducing themselves.
Answer: Hymenopterans have haplodiploid sex determination, so the relatedness r between worker bees and queen bees (between sisters) is very high at 0.75. This is higher than the relatedness r = 0.5 to one's own offspring, so helping the queen increase the number of sisters can produce greater inclusive fitness (rB) than reproducing oneself. Because Hamilton's rule (rB > C) is satisfied, altruistic reproductive abandonment can evolve through natural selection.
- Territory = defending space to secure resources through calls and scent marking
- Dominance hierarchy = stable rank relationships that reduce direct competition
- Inclusive fitness = direct fitness + indirect fitness
- Hamilton's rule: altruistic behavior evolves when rB > C
- Relatedness: parent–offspring and siblings = 0.5; hymenopteran sisters = 0.75
- Reciprocal altruism = altruistic behavior based on expected future returns
- Cost–benefit analysis of territories determines whether they are defended
Points to Note
① Accurate understanding of relatedness is important when calculating inclusive fitness. ② Reciprocal altruism does not require a kin relationship, but individual recognition and repeated interactions are necessary. ③ Territorial defense is less likely to develop when resources are scarce and dispersed or when they are excessively abundant.
Practice
- State the adaptive significance of forming territories and explain why direct fighting can be avoided.
- Using Hamilton's rule (rB > C), explain the conditions under which altruistic behavior toward siblings can evolve.
- State the three conditions required for reciprocal altruism to occur.