Aggression

Animals engage in aggressive behaviour when competing for:

Food – Aggression can ensure access to limited resources.
Territory – Defending an area can provide long-term survival benefits.
Mates – Sexual selection drives aggressive competition for reproductive success.

Researchers ranked salmon based on aggression and recorded feeding success.
Findings:
- The two most aggressive fish obtained 50% of the available food.
- Non-aggressive fish struggled to access food in a restricted space.
Conclusion:
- Aggression benefits dominant individuals by ensuring food access.
- Low-ranking individuals must adopt alternative strategies (e.g., sneaky foraging).

Males fight for dominance to control harems of females.
Aggression is linked to size:
- Males are 5–6 times larger than females.
- Only the largest, most dominant males get mating opportunities.
Fighting is risky, so seals rely on vocalisations (index signals) to assess rivals before engaging in combat.

Males defend burrows to attract mates.
Signals are used to reduce fighting, but conflicts occur over territory.
Bluffing can sometimes work, but true dominance is often determined by physical combat.

Fighting is costly and dangerous.
Selection acts on individuals, NOT species → Individuals only fight when the benefits outweigh the risks.
Fatal fights are rare but occur when reproductive opportunities are extremely limited (e.g., male fig wasps, bowling doily spiders).

John Maynard Smith (1973) introduced evolutionary game theory to explain aggression.
Game theory models aggressive encounters as a strategic interaction between individuals.

Two strategies:
- Hawk: Always fights aggressively and risks injury.
- Dove: Displays to avoid conflict and never fights.
Possible interactions:

Encounter	Outcome
Hawk vs. Hawk	One wins, one is injured (-100 penalty).
Hawk vs. Dove	Hawk wins (+50), Dove backs down (0).
Dove vs. Dove	They display and share the resource (+15 each).

Implications:
- A population of only doves is unstable—a single hawk mutant would dominate.
- A population of only hawks is also unstable—injury costs are too high.
- Stable equilibrium occurs when there is a mix of hawks and doves (~7/12 hawks, 5/12 doves).

New rule: Individuals act as:
- Hawks if defending their own resource (territory, mate, food).
- Doves if intruding on another’s resource.
This strategy is an ESS → More stable than pure hawk or dove strategies.

Male cichlids follow a predictable fight sequence:
1. Size-up phase: Side-by-side displays.
2. Escalation: Tail beating, biting, mouth wrestling.
3. Resolution: Loser changes colour and retreats.
Findings:
- Closely matched opponents escalate to more dangerous phases.
- Larger size differences lead to quick resolution.

Territory owners usually win disputes.
Possible reasons:
1. Owners are better fighters.
2. Owners value the resource more than the intruder.
3. Arbitrary ownership rules dictate the outcome.

Experiment:
- Researchers removed male crabs from burrows and reintroduced them.
- Findings:
  - Original owners won more often than intruders.
  - Larger individuals had an advantage, regardless of ownership.
  - Fights lasted longer when the intruder was larger.

Experiment:
- Researchers removed territorial birds and allowed new residents to take over.
- Findings:
  - The longer a newcomer stayed, the more likely it was to defend its new territory.
  - Ownership was not absolute—resource value influenced fighting motivation.

Animals fight harder when the value of a resource is high.
Example: Narwhals
- Males have scars from tusk battles, indicating serious fights for mating rights.
- Some fights result in injury or death, rare in most species.

Hermit crabs fight over shells → a critical resource for survival.
Stages of a hermit crab contest:
1. Attacker assesses defender’s shell.
2. Attacker wraps its shell against the defender’s shell.
3. If successful, the defender is pulled out and the attacker takes the shell.
Experiment:
- Researchers rubber-coated shells to weaken the wrapping force.
- Findings:
  - Weakened attackers fought longer but won less often.
  - Shell wrapping is an aggressive signal, not a negotiation.