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 insecticide produce 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
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 taught to open lids and were producers. Other birds had 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 larger than 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