Lecture 7: The Evolution of SOCIAL Behaviour (Kin/NON-kin)
Why do some animals live in groups while others do not?
It comes down to benefits versus costs
Benefits: dealing with the physical environment, information, protection, territory
Costs: competition for food & mates, energy and time of dominance interactions, disease,
risk of brood parasitism, conspicuousness
competition for MATES
Female Aggression Over Matings in Red Deer: they all want to mate with the
same male; especially the oestrous female (alpha male)
competition over NESTING SITES;
best nesting sites hard to come by!
You share but you want your offspring to have the resources = so dump the other
female’s eggs (ovicide)
Female removing eggs has less laid eggs but has more eggs incubated
Dominance interactions: ENERGETIC COSTS
Takes time + energy to live in a group
A “poor start to life” affects ability to dominate in male Green Swordtails
PG; poor food in beginning
GG; good food all the time
GG tends to show more aggressive behaviour than PG; who show more defensive
Greater population means greater risk of infection
In the beginning, as brood size increases then LRS increases but later LRS
decreases b/c of increased parasite infections.
BUT...being social has its benefits!
Dealing/protection with the physical environment; e.g. huddle together and warm each
other can make difference of couple degrees. It helps them survive! Information Centres; reap the same benefits from individual who HAS spent time +
energy on gaining info.
Protection from predators; spend less time being vigilant with increased group size!
e.g. emit sounds + stand erect when predator is around!
QUESTION: Why does a California Ground Squirrel spend less of their time
being vigilant with increased group size?
Hypothesis 1: Dilution effect (safety in numbers; other individuals offer
protection by simply existing)
Hypothesis 2: Increased detection (“many eyes”; others offer protection by being
Hypothesis 2 is correct since the amount of vigilance is the same regardless of
distance between you and your neighbour. Individuals pay attention
Cooperative Hunting; obtain food you can’t obtain ALONE. It’s observed when
benefits>costs! e.g. small mammals
1. dealing with the ph