Why might the same animal attack a rival in one situation but ignore it in another?

An animal may attack a rival in one situation but ignore it in another because aggression is not an inflexible, automatic response to a specific stimulus. Instead, an individual's decision to fight is a dynamic process influenced by several internal and environmental factors.1

Internal and Physiological Factors

An animal's tendency to attack is modulated by its internal state, including the activity of specific structures in the nervous system and the levels of circulating hormones. For instance, the limbic system in vertebrates plays a key role in emotional behavior, where areas like the amygdala can promote aggression while the septum may inhibit it. Additionally, serotonin generally acts to inhibit aggression in vertebrates, and its levels can fluctuate based on an animal's social status or past fighting experience.123

Cost-Benefit Analysis

From an evolutionary perspective, animals often engage in a cost-benefit analysis to maximize their fitness. Fighting carries significant costs, such as:4
* Energy expenditure: Intense physical conflict depletes energy reserves.4
* Risk of injury: Fiercer fights increase the likelihood of harm.4
* Opportunity costs: Time spent fighting may detract from other vital activities, such as caring for offspring or foraging.4

Consequently, an animal may choose to ignore a rival if the potential costs of a fight—such as the risk of injury or the energy required—outweigh the potential benefits of winning, such as access to food, shelter, or mates. This is particularly common in established dominance hierarchies, where subordinate individuals may avoid conflict to survive, effectively "making the best of a bad job".4