Living in Groups - An Introduction to Behavioural Ecology - page 157

(a) (b)

Fig. 6.10 A
cheetah (a) is less
likely to attack the
more vigilant
Thomson's
gazelles (b) in a
group. Photos
© Oliver Krüger.

In theory, the temptation to cheat will be reduced if individuals who spot the predator
gain an extra advantage over their non-vigilant companions. Observations suggest that
vigilant individuals do indeed gain a personal advantage, and that this accrues through
two effects. Firstly, predators may be less likely to target vigilant individuals. Claire
FitzGibbon (1989) observed cheetahs, Acinonyx jubatus , hunting pairs of Thomson's
gazelles, Gazella thomsoni , on the Seregeti plains, Tanzania (Fig. 6.10). She measured
the vigilance of each gazelle during the predator's stalk, and found that the cheetah
targeted the least vigilant gazelle in 14 out of 16 cases. Furthermore, gazelles that were
successfully caught by the cheetah had been less vigilant during the stalk than those
that escaped, because the latter detected the predator sooner.
Secondly, studies of small birds at feeders have shown that individuals who are more
vigilant at the initiation of an alarm flight depart more quickly to safety than non-
vigilant individuals (Elgar et al ., 1986). Lima (1994) demonstrated this experimentally
by rolling a ball silently down a ramp towards a single target bird in a flock of sparrows
and juncos. The sides of the ramp were high enough so only the target bird could see the
ball. Lima found that the target individual fled to cover first, followed by individuals who
had been vigilant during the target's departure, and finally by individuals who had not
been vigilant. Therefore, vigilant individuals gain a double advantage; they detect the
predator sooner and detect alarmed companions sooner, too.

Cheating may be
reduced if more
vigilant
individuals are
safer from attack

Sentinels
In some animal groups, for example meerkats, Suricata suricatta , and various species of
babblers, individuals often act as sentinels, watching for predators from prominent
look-out perches while the rest of the group forages on the ground below. When the
sentinel spots an approaching predator (a raptor, snake or carnivorous mammals) it
gives an alarm and everyone rushes to safety (Fig. 6.11).
At first sight sentinels seem to be behaving altruistically, risking their own welfare for
the good of the group (Chapter 11). However Peter Bednekoff (1997) showed, in a
theoretical model, that sentinel behaviour could arise through selfish individual actions.
His model's key assumptions are that individuals are only likely to go on sentinel duty
when they are satiated, and that acting as a sentinel may be beneficial to the sentinel
itself because it can detect a predator sooner. The outcome can then be a series of
change-overs in the group, as sentinels become hungry and are replaced by satiated

Sentinels warn
others …

… but sentinel
behaviour could
be best for selfish
individuals