Sociobiology: Optimal Group Size, Unit of Selection, and Group Living

Lecture Content

15.2 Optimal Group Size

• Definition: Optimal group size is the number of individuals in a group that maximizes benefits while minimizing costs.

• Benefits of Larger Groups:

  • Better protection against predators.

  • More efficient hunting and foraging.

  • Greater reproductive opportunities.

• Costs of Larger Groups:

  • Increased competition for food and mates.

  • Higher risk of disease transmission.

  • Greater visibility to predators.

• Balance:

  • Too small → insufficient protection and cooperation.

  • Too large → competition and conflict.

• Examples:

  • Wolf packs (8–12 members) optimize hunting success.

  • Bird flocks balance predator avoidance with food competition.

15.3 Unit of Selection and Social Behaviour

• Definition: In sociobiology, the unit of selection refers to the level at which natural selection acts — individual, kin, or group.

• Levels of Selection:

  1. Individual Selection

     • Traits that increase personal survival and reproduction.

     • Example: Aggressive behaviour to secure food.

  2. Kin Selection

     • Helping relatives increases survival of shared genes.

     • Example: Alarm calls in prairie dogs.

  3. Group Selection

     • Traits that benefit the group, even if costly to individuals.

     • Example: Cooperative defence in meerkats.

• Social Behaviour:

  • Altruism, cooperation, dominance hierarchies, and communication are shaped by selection pressures.

  • Example: Worker bees sacrifice reproduction for colony survival.

15.4 Advantages and Disadvantages of Group Living

Advantages

1. Protection from Predators

   • Safety in numbers reduces risk.

   • Example: Fish schools confuse predators.

2. Cooperative Hunting and Foraging

   • Groups increase efficiency.

   • Example: Lions hunting together.

3. Reproductive Opportunities

   • Larger groups provide more mates.

   • Example: Lekking in birds.

4. Care of Offspring

   • Shared parenting improves survival.

   • Example: Meerkats guarding pups.

5. Information Sharing

   • Communication about food and danger.

   • Example: Honeybee waggle dance.

Disadvantages

1. Competition

   • More individuals → less food and mates.

2. Disease Spread

   • Close contact increases infections.

3. Predator Attraction

   • Large groups are more visible.

4. Conflict

   • Dominance hierarchies may cause aggression.

5. Resource Depletion

   • Overuse of local food and water.

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Summary / Key Takeaways

• Optimal group size balances cooperation benefits with competition costs.

• Natural selection acts at multiple levels: individual, kin, and group.

• Group living provides protection, cooperation, and reproduction benefits but also increases competition, disease, and conflict.

• Sociobiology explains how social behaviours evolve to maximize survival and reproduction.