ECOL 429 Lecture 16-17

Why Are Animals Social?

Animals are social because group living provides significant evolutionary advantages in survival, reproduction, and efficiency in navigating their environments.

Predator Avoidance:

1. Improved Vigilance:

   • The "many-eyes" hypothesis suggests larger groups detect predators more effectively because individuals share vigilance duties.

   • Example: In ostrich groups, individuals spend less time scanning for predators while overall group vigilance increases, enhancing collective safety (Bertram, 1980)

2. Dilution Effect:

   • The risk of being attacked decreases as group size increases because predators can only capture a fraction of the group.

   • Example: Biting flies attack larger horse groups more often, but each horse in a larger group receives fewer bites on average.

3. Predator Confusion:

   • Large groups make it harder for predators to focus on a single target.

   • Example: Bass hunting minnows struggle to capture prey as group size increases because they become overwhelmed by multiple targets (Landeau & Terborgh).

4. Communal Defense:

   • Group members can mob predators to protect the group.

   • Example: Black-headed gulls mob crows to reduce predation success, while dense guillemot colonies benefit from group defense against gulls.

Foraging Efficiency:

1. Information Sharing:

   • Groups act as information centers, helping members locate resources more efficiently.

   • Example: Hyenas are more successful at hunting zebras in packs compared to solitary hunting (Kruuk, 1972).

2. Cooperative Hunting:

   • Individuals in groups can adopt specific roles that improve hunting success.

   • Example: Lion hunts are more successful when individuals adhere to their roles (Stander, 1992).

Reproductive Advantages:

• Being part of a group increases opportunities to find mates.

• Example: Social structures in primates, such as baboons, facilitate reproductive interactions within groups.

Environmental Adaptations:

• Group living aids in conserving heat and water.

• Example: Huddling reduces heat loss in small mammals like mice (Canals et al.).

Benefits and Costs of Being Social

Benefits:

1. Protection from Predators:
   
   

   • Dilution Effect: Larger groups dilute individual risk.

   • Example: Mayflies time their emergence synchronously to overwhelm predators, reducing individual predation risk.

   • Predator Confusion: Groups confuse predators during attacks, reducing capture success.

   • Example: Schools of fish and flocks of birds use rapid, synchronized movements to confuse predators like cichlids or hawks.

2. Foraging and Resource Benefits:
   
   

   • Groups increase the chances of locating and capturing food.

   • Example: Packs of hyenas hunting zebras are significantly more successful compared to individuals hunting alone.

3. Thermal Efficiency:
   
   

   • Example: Mice in groups conserve up to 65% more energy through huddling.

Costs:

1. Increased Predator Attention:
   
   

   • Larger groups attract more predators.

   • Example: Falcon attack rates on swallow colonies increase with colony size (Lindstrom, 1989).

2. Increased Competition:
   
   

   • For Food: Larger groups experience more competition for limited resources.

   • Example: Guppy shoals face increased competition for food, limiting individual intake.

   • For Mates: Dominance hierarchies often skew reproductive success.

   • Example: In banded mongooses, dominant pairs monopolize reproduction.

3. Disease Spread:
   
   

   • Proximity increases the risk of parasite and pathogen transmission.

   • Example: Larger groups of mammals like primates show higher parasite prevalence (Patterson & Ruckstuhl, 2013).

Factors Affecting Group Size

Optimal Group Size:

• Groups are shaped by a balance of benefits and costs. The optimal size is the one where individual fitness is maximized.

• Example: Lions experience the highest per capita food intake in groups of two but often form larger groups due to other advantages (Caraco & Wolf, 1975).

Deviations from Optimal:

1. Kin Selection:

   • Groups may grow beyond optimal size if relatives join, as their success contributes to indirect fitness.

   • Example: Family groups in meerkats often allow related individuals to stay even when resources are limited.

2. Unregulated Entry:

   • Some groups cannot prevent additional individuals from joining.

   • Example: Flocks of birds or schools of fish may grow larger than optimal because entry barriers are weak.

Extrinsic and Intrinsic Factors Affecting Group Types

Extrinsic Factors:

1. Predation Pressure:

   • Higher predation risk encourages larger groups for safety.

   • Example: European minnows form larger shoals in the presence of pike predators (Orpwood et al., 2008).

2. Habitat Complexity:

   • Structured environments (e.g., dense vegetation) can reduce the need for large groups.

   • Example: Minnows form smaller shoals in complex habitats because natural cover provides safety.

3. Resource Distribution:

   • Patchy resources lead to temporary aggregations.

   • Example: Wildebeest herds concentrate around seasonal waterholes.

Intrinsic Factors:

1. Individual Roles and Experience:

   • Experienced individuals often lead group movements.

   • Example: Older African buffalo vote on group direction based on their knowledge of resource locations.

2. Positioning Within Groups:

   • Costs and benefits vary by position.

   • Example: Peripheral fish in shoals face higher predation risk, while central individuals are safer (Jakobsen & Johnsen, 1988).

3. Hunger or Motivation:

   • Hunger can affect leadership and group dynamics.

Example: Hungrier fish often lead groups to new foraging grounds.