Chapter 15: Aggression
•Aggression is especially common during mating season
•Conspecific aggression (between members of the same species) can be over females,
food, territories and safety of their families
•Aggression = agnostic behaviour.
•A predator/prey interaction is not considered aggression
•Dominance hierarchies: measured in group-living species- rank individuals based on
results of aggression
•Individuals in groups often cooperate with each other in order to compete, often
aggressively, with individuals in other groups
Fight of Flight:
•Surge of adrenaline and norepinephrine produces a quick increase in blood sugar. BG and
O2 are delivered to parts of brain, muscles and heart. Digestive and reproductive systems
•Ultimate perspective: costs and benefits of aggression? When benefits of victory
outweigh cost of fighting, natural selection will favour aggression.
•Proximate perspective: immediate causation and aggression? Endocrinal underpinnings
of such behaviours. Dominants usually have increased androgen levels (i.e. testosterone),
which also increase after a fight is won. Subordinates tend to have higher glucocorticoid
stress hormones such as cortisol or cirticosterone, which also increase once the fight is
•Fish often change colour when signaling that they are communicating subordination
when they start losing a fight
•Atlantic Salmon: aggression is most often associated with territorial defense. Circling,
biting and charging during fight. Losers have increased cortisol levels and swim close to
surface to avoid another fight. Dominance hierarchy: dominant males have dark vertical
eye-bands and subordinates develop darker body colour.
•Serotonin in animal aggression: in mammals, low serotonin levels are linked with high
aggression but lower social status. In fish, serotonin is higher in subordinates which leads
to decreased fighting ability.
oArctic Charr: Phase 1: constructed four groups each with four fish. Determined
the dominance hierarchy in each group. Phase 2: four top-ranked males in one
group, four second-ranked males in next, etc. and allowed new hierarchies to
form. Correlation between serotonin and social status: fish that were subs in
phase 2 shower higher serotonergic activity.
oCrustaceans: increased serotonergic functions leads to increased aggression &
high social status. After losing a
fight, avoid fighting for days, but
can be made more aggressive with
serotonin injections. If fluoxetine
(Prozac/serotonin inhibitor) is
injected into lobster at same time
as serotonin, the effect disappears.
Winner and Loser Effects