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Lecture 10

ANTHROP 3HI3 Lecture Notes - Lecture 10: Railways Act 1921, Horse-Fly, Coral Reef Fish

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Priscilla Medeiros

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Chapter 6 Livig i groups
- Why do individuals form groups (despite potential costs of increased
competition for resources and infection by pathogens)?
- Benefits:
o Protection from weather (ex; huddling for warmth)
o Aerodynamic advantages in locomotion
o Reducing predation
o Improving foraging success
- Is there an optimal group size to maximize individual fitness?
- How can grouping be stable despite conflicts of interest?
Summary of some anti-predator and foraging benefits of grouping
How grouping may benefit individuals
Dilute risk of attack (swamping predators;
selfish herding)
Predator confusion
Communal defense
Improved vigilance for predators
Better food finding (information centers)
Better food capture (group hunting)
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Diluting the risk of attack
o Predation and food influence the costs and benefits of group living
o Williams and Hamilton proposed that individuals associate with
others as a form of cover-seeking to reduce their personal risk of
o If an individual is alone, there is a greater risk of attack when
encountering a predator; if an individual is in a group of N individuals,
it has a 1/N chance of being a victim (ex; in a group of 100, an
individual has a 1/100th chance of being attacked)
o This dilution advantage will favor grouping as long as attack rate does
not increase proportionally with group size (so there need to be a
markedly less or equal amount of attacks when comparing groups and
o Grouping can dilute an individual preys risk of being attacked… even
when attack rate per group increases with group size
o Usually, predator attack rates increase with group size because larger
groups are more conspicuous
But there is a net dilution advantage
o EXAMPLE: Water skaters
Figure 6.2 on page 149
Prey are insects called water skaters that sit on water surface
Predators are small fish that snap insects from below (so
vigilance wouldnt really increase with group size)
Attack rate lower b/c of dilution and similar for groups of
different sizes
o EXAMPLE: Horses in Camargue marshes in France
Wild horses attacked by blood sucking flies (Tabanidae), which
remove blood and transmit bacterial and viral diseases
In the weeks when these flies are most active, the horses
aggregate into larger groups
When group size was varied experimentally, more flies were
attracted to larger groups of horses, but attack rate PER HORSE
was still lower in a larger group
o EXAMPLE: Winter aggregation of monarch butterflies in Mexico
Millions of butterflies aggregate in trees
Monarchs are sometimes attacked by birds in these roots
Experimenters counted how many butterflies remained in the
colonies after predation, and found that ALTHOUGH LARGER
PER INDIVIDUAL was LOWER in a larger colony
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So the advantage of dilution outweighed the cost of greater
o Synchrony in time can also dilute predator attacks per individual prey
figure 6.3 on page 150
o Synchrony works because it swamps the predators
o EXAMPLE: Mayflies in North America
Emergence of these flies occurs over two weeks in May-June,
where the larvae transform into winged adults at the water
surface, just before sunrise every day
The adults immediately mate, lay eggs, and then die within an
The predators are beetles, dragonflies, bats, and birds
So individual mayflies are safest from predation when a lot of
them emerge at the same time (in synchrony)
Synchronous emergence has PRIMARILY EVOLVED TO
However, parthenogenetic mayflies (asexual reproduction)
have similar synchronous emergence so this suggests that
reducing predation through predator swamping is a major
selective pressure favoring synchrony
o Reducing the domain of danger the domain of danger, or zone of
danger, is the area in which the animal is susceptible. If a frog is
between two other frogs, its domain of danger is much smaller than if
the next closest frog is 10 feet away.
o Individuals in the middle of a group may be safer than those at the
edge… but position choice may reflect a trade-off between predation
and foraging
o Individuals in the middle of a group may be safer than those at the
edge selfish herd effect may explain continuous movement of
groups in an attempt to gain the safest positions
o EXAMPLE: Cape fur seals in South Africa
Seals preyed on by great white sharks, which detected seals
silhouettes and then attacked from below, resulting in the
whole body of the shark breaching the water as it grabbed the
Experiment: made groups of decoy seals and attached them to
rafts, varied the distance between decoys on the raft (this was
their domain of danger)
Results: an individual decoys risk of shark attack increased
with an increase in its domain of danger
Distinguishes selfish herd effect from other factors that reduce
attacks on prey, such as improved prey vigilance or predator
o EXAMPLE: Lab experiments with dace and minnows
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