Week 10: Sociality; CH13 p457 – 470
The Evolution of Social Behaviour
The Costs and Benefits of Social Life
Social behaviour does not necessarily mean a better life (fitness and sexual reproductive
success), since more solitary species have on average left more descendants than social species. One
example is establishing social hierarchy and betas submitting to alphas in order to stay in the group.
(table on pg 458 on cost-benefits of social living). Reproduction interference also increases the price of
sociality, competing for females or brood parasites by other members. Ex. Acorn woodpecker, forms
social groups where females lay eggs in same nest. Those that lay eggs first often have their eggs
thrown out by other female members of the social group. Laying them together, finally, they incubate
the eggs and resolve the destroying of eggs.
Another 2 costs of sociality: 1) competition for food, 2) increased vulnerability to parasites and
pathogens. The 2 prediction is supported by the finding that the degree of sociality among bees is
linked to the ability of species to combat staph bacteria. Washed cuticle of social and solitary bees, and
compared the solutions. More protective chemicals were found in the social bees (300x more effective
in destroying staph bacteria) compared to solitary. If assumed that theses defensive compounds are
costly to produce then it adds another cost to sociality. Ex. Honey bees raise the temperature to kill
fungus pathogens but waste time and energy doing so. Ex. Some termites affected by fungal invaders
can grow immunity to it by being exposed to a member or group that is already immune. Ex. Cliff
swallows. They leaves nest side by side in the thousands. Exp. Introduced insecticide to some and
exposed others to blood-sucking swallow bugs. Parasitized birds were much smaller and less likely to
survive than those with the insecticide.
Advantages must outweigh the cost in order for sociality to stay. This is shown in cliff swallows who can
follow others to lead them to good feeding sites, or the male emperor penguins who huddle shoulder to
shoulder to produce thermal heat during harsh winters. Whereas others, such as lioness group to fend
off their same species in the case of infanticidal males. The biggest benefit to sociality is the improved
protection against predators (dilution effect, or teaming up “strength in numbers”). Large maomao fish
will group together to chase away egg eating predators, whereas smaller groups will not do this as often
(also bluegill sunfish).
Closely realted species that nest alone should suffer less predation (ex. Pumpkinseed sunfish), has
powerful biting jaws and can repel egg-eating enemies on its own.
The Evolution of Helpful Behaviour.
Social interactions can vary amongst individuals. Mutualism – two individuals help one another ex.
Lioness trapping an antelope together. When both parties enjoy large reproductive gains from their
interaction, their mutualism, or cooperation, generally requires no special evolutionary explanation. Ex.
Two or three male lions join together to take over a pride, share females equally and force other males to submit due to their cooperation. Ex. Yearling male lazuli buntings (have brownish plumage) have a
mutualism with the brightly coloured dominant yearling males. Bright coloured yearlings drive other
bright or intermediate coloured yearlings away from good territory but allow dull coloured yearlings to
stay. One hypothesis is that they do this since they can mate with these birds females, and they found