Biology Reference
In-Depth Information
Clutton-Brock, 2009c). The subordinates can be either individuals who have not
dispersed from their natal group, or immigrants who have joined a group. In birds, the
subordinates are often termed 'helpers at the nest'. Cooperative behaviours in vertebrates
include feeding and protecting the young or other members of a group. Similar forms of
help occur in some insect species, reaching their pinnacle in the eusocial insects (e.g.
ants, bees, wasps and termites) where the subordinates become sterile workers and give
up all chances of breeding independently.
Cooperation can be found in organisms that are not usually thought of from a social
perspective, such as bacteria. The most common form of cooperation in microorganisms
is the production of factors that are released from cells, and which provide a benefit to
the local group of cells, such as acquiring nutrients for growth, making them analogous
to what economists call 'public goods' (Fig. 11.7). This can even lead to bacteria living
in cooperative 'slime cities', such as dental plaque and the scum around sink plugholes.
Cooperation also occurs between species. In such mutualisms, the most common
form of cooperation is for one or both partners to provide a service or resource to the
other. For example: in cleaner fish mutualisms, the cleaners remove parasites from their
parasites; or in the legume-rhizobia interaction, the rhizobia bacteria provide nitrogen
to their legumous host plants, whilst the plant provides carbon to the bacteria.
In cooperative
breeders, the
subordinates in
the group help
rear the offspring
of the dominant
individuals
Bacteria
cooperate by
producing public
goods
Free riding and the problem
of cooperation
The problem of
cooperation is
why should an
individual carry
out a behaviour
that benefits
another
individual?
The problem of cooperation is that cooperative behaviour can be exploited by free riders,
which gain the benefits of others cooperating whilst avoiding the cost of cooperating
themselves. This is famously illustrated by the Prisoner's dilemma model, which was
originally developed to help us think about human behaviour, but provides a useful
model to illustrate the problems of achieving cooperation in animal societies (Axelrod &
Hamilton, 1981).
Imagine that two individuals are imprisoned and accused of having performed some
crime together. The two prisoners are held separately and attempts are made to induce
each one to implicate the other. If neither one does, both are set free. This is the
cooperative strategy. In order to tempt one or both to confess (defect), each is told that
a confession implicating the other will result in their release and a small reward. If both
confess, each one is imprisoned. But if one individual implicates the other, and not vice
versa, then the implicated partner receives a harsher sentence than if each had
implicated the other. The pay-off matrix for this game is given in Table 12.1 with some
illustrative numerical values. From a biological perspective, these values would represent
the gain in fitness from the interaction (e.g. number of offspring gained).
Imagine player A finds another individual B who always cooperates. If A cooperates
too it gets a reward of three, whereas if it defects it gets five. Therefore, if B cooperates,
it pays A to defect. Now imagine player A discovers that B always defects. If
A cooperates it gains nothing (the sucker's pay-off) whereas if it defects it gets one.
Therefore, if B defects, it pays A to defect. The conclusion is that irrespective of the other
player's choice, it pays to defect even though with both players defecting they get less
(one) than they would have got if they had both cooperated (three). Hence the dilemma!
In the Prisoner's
dilemma game
both individuals
would benefit
from mutual
cooperation but
both are tempted
to cheat
