Biology Reference
In-Depth Information
to safety (a burrow for a meerkat, a bush for a babbler). Finally, even solitary individuals
become sentinels when they are satiated, supporting the idea that sentinel behaviour
benefits individuals. Alarm calling by the sentinel is a case of mutualism (Chapter 12);
both caller and responders gain because individuals benefit from group living.
One aspect of sentinel behaviour is particularly fascinating; sentinels give quiet
vocalizations while on guard. Wolfgang Wickler (1985) suggested this might function
as a 'watchman's song', announcing to the rest of the group that they were safe because
someone was on guard duty. Playback experiments to both meerkats (Manser, 1999)
and pied babblers (
Turdoides bicolor
) (Hollen
et al
., 2008) revealed that foragers decreased
their own vigilance when they heard sentinel calls. In the babblers, this also led to a
marked increase in foraging success (Fig. 6.12). Sentinels may experience little cost
from these quiet vocalizations and may gain a benefit if improved foraging by others
leads to better coordination of sentinel activities in the group.
The 'watchman's
song'
How grouping can improve foraging
Better food finding
The comparative studies described in Chapter 2 revealed that species which feed on large
ephemeral clumps of food, such as seeds or fruits, often live in groups. For these animals,
the limiting stage in feeding is the problem of finding a good site; once the patch has
been found there is usually plenty of food, at least for a short while. Peter Ward and
Amotz Zahavi (1973) developed the idea that communal roosts and nesting colonies of
birds may act as 'information centres', in which individuals find out about the location
of good feeding sites by following others. The idea is that unsuccessful birds return to the
colony or roost and wait for the chance to follow others who have had more success on
their last feeding trip. Unsuccessful birds might recognize successful ones by, for example,
the speed with which they fly out from the colony on their next trip.
The phrase 'information centre' has the connotation of mutual cooperation in the
transfer of information, as for example in a honeybee or ant colony. As we shall see in
Chapter 13, there are special reasons to expect cooperation in social hymenopteran
colonies because individuals interact with close relatives. However, bird roosts or nesting
colonies often involve large anonymous, assemblages of unrelated individuals. So the
key question to ask is: what would informers gain from telling others where the food is?
Richner and Heeb (1996) suggested that the informers may simply gain the advantages
of group foraging (e.g. better protection from predators), and if this is the main benefit,
rather than information sharing, then a better term would be 'the recruitment centre'
hypothesis. Indeed, if predation pressure favours grouping, then a consequence may be
that successful individuals could be followed to food sources without any active signalling
on their part. In this case the group would simply be an 'eavesdropping centre'.
Nevertheless, game theory models show that the opportunity to share foraging
information can, in theory, also drive the evolution of communal roosts. If pooling
the independent search effort of many individuals is the most effective way of locating
rare food bonanzas (e.g. animal carcasses for carrion eaters such as crows and
ravens), and if there is little cost to foraging in a group (because the food patch is
large or only temporarily available), then the strategy 'search independently and
recruit others from the colony or roost' can be an ESS (Mesterton-Gibbons & Dugatkin,
Colonies and
roosts may act as
information
centres
