Biology Reference
In-Depth Information
24 hours, before returning them to their natal group. When removal was carried out
in the breeding season, hence preventing helping behaviour, the returning helpers
were subjected to extreme harassment by the dominant male in the form of prolonged
chases and pecking (9/14 cases). In contrast, when removal was carried out in the
non-breeding season, when helping was not being performed, this never resulted in
aggression towards the returned individual (0/12 cases). However, whilst these results
are suggestive of a role for punishment, it has not also been shown that individuals
adjust their behaviour in response to being punished by increasing their levels of
helping.
An example where the consequences of punishment have been elucidated is provided
by the work of Redouan Bshary and colleagues on the cleaner fish
Labroides dimidiatus
.
This species lives on coral reefs where it removes and eats ectoparasites from its 'clients',
which refrain from consuming this potential prey while it performs the service.
Although parasite removal and food acquisition are clearly beneficial to the client and
cleaner, respectively, there is a conflict because the cleaners would prefer to eat the
tissue or mucus of their hosts, which is costly to the host. Field observations suggested
that when the cleaner does this, by taking a bite of its client, the client fish respond by
aggressively chasing the cleaner fish and/or fleeing away (Bshary & Grutter, 2002).
This punishment also led to a change in behaviour of the cleaner, with it making them
less likely to take a bite of their client in future interactions. Bshary & Grutter (2005)
then tested this role of punishment experimentally, by using Plexiglas plates to simulate
the feeding opportunities offered by clients. They allowed cleaners to feed on mashed
prawn or fish flakes from Plexiglas plates. The cleaners showed a strong preference for
prawn over fish flakes, so Bshary & Grutter tested how the fish changed their feeding
behaviour if they were punished for feeding on prawns, by removing the plate (to
simulate fleeing) or by chasing the cleaner with the plate. They found that in response
to both removal of or chasing by the plate, the cleaner fish adjusted their feeding
behaviour and were more likely to feed on the food type that did not lead to this
behaviour - fish flakes (Fig. 12.9).
Individuals who
are removed and
prevented from
helping are
punished upon
their return
Client fish punish
cleaners who feed
on them, rather
than their
parasites, by
chasing them or
fleeing away
Cleaner fish are
more likely to
feed on parasites,
and less likely to
feed on their
clients, after they
have been
punished
Soybeans sanction non-cooperative bacteria
Another way to enforce cooperation is to terminate interactions with relatively
non-cooperative individuals, so that cooperation is favoured to avoid such terminations.
Toby Kiers and colleagues tested for the possibility of such 'sanctions' in the interaction
between soybeans and the rhizobia bacteria that colonise their roots. Legume plants,
such as beans and peas, contain nodules within their roots which house rhizobia
bacteria that fix atmospheric nitrogen and then provide it to the plants which use it for
growth and synthesis. Nitrogen fixation is energetically costly to the bacteria, reducing
the resources that could be allocated to their own growth and reproduction, begging the
question of why they carry out this cooperative behaviour. Kiers
et al
. (2003) forced
the rhizobia to cheat (not cooperate) by replacing air (approximately 80% nitrogen,
20% oxygen) with a gas mixture (approximately 80% argon, 20% oxygen) that contains
only traces of nitrogen. In this case, the lack of atmospheric nitrogen meant that the
rhizobia could not cooperatively fix nitrogen. Kiers
et al
. repeated this experiment,
forcing cheating at the level of the whole plant, one-half of the root system and at the
