Biology Reference
In-Depth Information
one-way mirror in an experiment with minnows, so that some of the shoal could not
observe the predator themselves but they could observe the reaction of other minnows
who were threatened by the stalk of a model pike
Esox lucius
. Although the naïve
minnows couldn't see the pike, they nevertheless hid in response to the alarm behaviour
of the other fish. Similarly, Treherne and Foster (1981) showed that the approach of a
model predator caused anti-predator movements to spread across flotillas of water
skaters,
Halobates robustus
, as individuals bumped into their neighbours. They coined
this wave of alarm the 'Trafalgar Effect', after the Battle of Trafalgar when signals were
transmitted across chains of ships, allowing Admiral Nelson to know of the
approaching enemy even when they were over the horizon and he could not yet detect
them himself. So there's good evidence that individuals can benefit from the alarm of
others in the group.
Evidence that
individuals
respond to the
alarm of others
Cheating
Imagine an ostrich on its own. If it spent all its time with its head down, foraging, then
it would never starve to death, but it would eventually be killed by a lion. If it spent all
its time with its head up, scanning, it would always detect the predator but it would
eventually starve to death. So scanning involves a trade-off and, in theory, there will be
some optimum pattern of foraging and scanning that maximizes the individual's overall
chance of survival through avoiding both starvation and predation.
Now imagine a large group in which everyone occasionally scans and occasionally
feeds. Any individual that cheated, by reducing its scanning, would gain an advantage;
it could now devote more time to feeding and its selfish behaviour would have little, if
any adverse effect on overall group vigilance, so it could still benefit from any alarm
raised by its vigilant companions. The problem, of course, is that if everyone cheated
there would be little vigilance and everyone
would be vulnerable to predation.
Therefore, vigilance in groups clearly involves a
game, in which the best strategy for any one
individual depends on what everyone else is doing.
Just as with the analogous competitive games we
discussed in the last chapter (e.g. dung fly waiting
times, producers and scroungers), we need to
consider what will be the stable solution, or
evolutionarily stable strategy (ESS). This is the
vigilance strategy (pattern of looking up and
down) which, when adopted by the majority of the
population, cannot be bettered by any alternative
vigilance strategy. John McNamara and Alasdair
Houston (1992) calculated the theoretical ESS
vigilance strategy for individuals in groups of
different sizes. The precise pattern varied depending
on various parameters, such as relative risks of
starvation and predation, but the general result
was for vigilance to decline with increasing group
size, just as we observed in the ostriches (Fig. 6.9).
In theory, in
scanning groups it
may pay
individuals to
cheat
An ESS for
vigilance
Group size
Fig. 6.9
The evolutionarily stable
vigilance (ESS) for individuals in
groups of different sizes (see text
for explanation). From McNamara
and Houston (1992). With
permission from Elsevier.
