Living in a group

• Costs of group living

• Benefits of group living– Predation avoidance– Resource acquisition

• Optimal group size

Costs of group living

• Competition for resources

• Increased risk of parasitism or disease

• Increased opportunities for reproductive interference or suppression

Competition for food in fieldfares

Nestlings die primarily from starvation

Ectoparasitism in cliff swallows

Nests treated with insecticideproduce much larger chicks

Reproductive interference

Brood parasitism,Extra-pair copulations

Reproductive suppression

Predation avoidance benefits

• Reduce encounter rate with predator– Protected sites– Selfish herd

• Reduce success of predator– Vigilance– Dilution– Confusion, predator-predator interference– Mobbing, cooperative defense

Protected sites

The selfish herd

Predator dilution

Vigilance

More eyes and ears

Predator confusion

Predator confusion

As school size of prey increases, capture success decreases

Predator interference

“Schreckstoff”

Schreckstoff attracts other pike, which increases handling time

Cooperative defense: mobbing

Resource access benefits

• Passive attraction to limited resource• Active attraction due to joint benefits

– Reduce path overlap– Information transfer

• information center• producers-scroungers• acquire public information

– Group foraging– Communal hunting

Passive attraction

Butterflies at a salt deposit

Information transfer?

Information transfer in osprey

Information transfer in evening bats

Leaders “produce”, followers “scrounge”

Two pigeons produced, 14 scrounged

The producer-scrounger game

Spice finch were taughtto open lids and wereproducers. Other birdshad to wait to scrounge

Public information

• Forager joins group to acquire information about food availability and/or predator risk

• By observing foraging success of others in the group, a naïve forager can estimate the quality of a food patch

• Starlings left empty patches sooner when foraging with a partner than when foraging alone.

Public information

Reduce path overlap

Proposed for sparrow flocks in Mojave desert. Never been tested.

Group foraging

Cooperative hunting

Permits capture of prey largerthan possible by singletons

Optimal group size

Group hunting in wild dogs

Larger packs kill larger prey, have higher capture success, and travel shorter distances in a hunt, but must share kills with more members of the group

Optimal size of wild dog foraging groups

Lion foraging success