Biology Reference
In-Depth Information
their domain of danger, and this might explain the continuous movements in swarms,
flocks and shoals as individuals try to gain the safest positions in the group. He called
this the 'selfish herd' effect (Fig. 6.4a).
Alta De Vos and Justin O'Riain (2010) tested Hamilton's assumption that an
individual's risk of attack was related to its domain of danger. They studied Cape fur
seals,
Arctocephalus pusillus
, in False Bay, by the Cape Peninsula of South Africa. The
seals suffered heavy predation from great white sharks,
Carcharodon carcharias
, which
detected seals as silhouettes on the surface and then attacked at tremendous speed from
below, resulting in the whole body of the shark breaching the water as it grabbed the
prey. De Vos and O'Riain made groups of decoy seals from styrofoam boards, and fixed
these to a raft using reed poles attached to the dorsal surface of each decoy. By varying
the distance between decoys on the raft, they could present prey with different domains
of danger. The rafts were towed behind a boat near the seal colony and attracted the
sharks. As assumed in Hamilton's model, an individual decoy's risk of shark attack
increased with an increase in its domain of danger (Figs 6.4b, 6.4c). This is a particularly
neat test of Hamilton's model because it distinguishes the selfish herd effect from other
factors that may reduce attacks on groups of prey, such as improved prey vigilance or
predator confusion (see next section).
Jens Krause (1993a) tested Hamilton's prediction, that alarmed individuals would seek
safety amongst companions, in laboratory experiments with dace
Leuciscus leuciscus
and
minnows
Phoxinus phoxinus
. These cyprinid fish live in shoals and if they detect chemicals
from the damaged skin of a companion wounded by a predator, they form tighter shoals.
A shoal of fourteen dace was habituated to this chemical stimulus by repeated exposure
and then single naïve minnows were added to the shoal. Before the chemical was added
to the water there were no differences in the positioning of the minnows and dace in the
shoal. However, after the introduction of the chemical, the naïve minnows moved closer
to the other fish and positioned themselves so that they were surrounded by near
neighbours on all sides, just as predicted by the selfish herd model.
An individual's position in a group is likely to reflect a trade-off between foraging
benefits and predation risk. Experiments with fish, for example, show that hungry
individuals tend to occupy positions at the front of the shoal, where they will be the first
to encounter food (Krause, 1993b). However, front positions are also more vulnerable
to predation, so satiated fish tend to seek more central positions (Bumann
et al
., 1997).
Reducing the
domain of danger
Individuals in
the middle of a
group may be
safer than those
at the edge …
… but position
choice may reflect
a trade-off
between
predation and
foraging
Predator confusion
Individuals in groups may also be safer from attack because the predator has difficulty
in focusing on one target as different individuals in the group continually move across
its line of sight. We can experience this confusion effect ourselves when we are thrown
a group of balls; it is much more difficult to catch one of the group than when the balls
are thrown one at a time.
Neill and Cullen (1974) tested the hunting success of four aquatic predators in laboratory
experiments in aquaria, where the prey were small fish. Squid, cuttlefish and pike are
ambush predators; they have a slow, stalking approach and then make a rapid strike after
a short chase. The perch is a chasing predator; it dashes after prey, often with a long pursuit.
For all four predators the success of an attack declined with increasing prey shoal size
(Fig. 6.5) and Neill and Cullen suggest that this is mainly because of increasing predator
confusion. This was most evident with the perch, where shoals of prey disrupted attacks
Grouping may
confuse predator
attacks
