Biology Reference
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or provided many of the large items were
twigs rather than real prey. Experiments
showed that the tits did indeed mainly
ignore large prey under these conditions
(Fig. 4.8). The conclusion is that even
slight concealment, enough to impose
only a second or two extra discrimination
time, can still be of selective advantage
to a prey. To gain an advantage the
prey only has to be sufficiently better
concealed to make another prey item
more profitable for the predator. The
results of this experiment support the
idea that crude counter-adaptations can
indeed provide a starting point for an
evolutionary arms race.
80
76%
60
40
20
14%
A
B
Fig. 4.8
Great tits foraging for artificial
cryptic prey. The cost of distinguishing
cryptic profitable prey may cause the
predator to specialize on other more
conspicuous prey. In treatment A, twigs
were four times as common as the large
prey that resembled them; in B the large
prey were four times as common as the
twigs. The abundance of conspicuous
small prey was constant in A and B.
According to an optimal foraging model
(similar to those discussed in Chapter 3),
it pays the predator to specialize on
conspicuous prey in treatment A and
on cryptic prey in B. From Erichsen
et al
.
(1980).
Enhancing
camouflage
Some animals can vary their crypsis to
match the local background by changes
in skin pigmentation (cephalopods:
Hanlon, 2007; fish: Kelman
et al
., 2006;
chameleons: Stuart-Fox
et al
., 2008;
spiders: Théry & Casas, 2009). However,
crypsis does not only involve background
matching; other tricks also help to
reduce detection.
Disruptive colouration
Many cryptic moths, and other prey, have bold contrasting patterns on the periphery
which seem to help break up the body outline (Fig. 4.9a). Innes Cuthill, Martin Stevens
and colleagues tested the effectiveness of disruptive colouration by pinning artificial
'moths' to oak trees in a wood. The moths consisted of a dead mealworm for the body
and triangular paper wings made from digital images of oak bark. The results showed
that these cryptic moths survived attack from insectivorous birds much better than
control moths which had uniform brown or black wings. However, disruptive patterns
enhanced survival still further; when bold patterns were positioned on the wing edge
(where they broke up the body outline) the moths survived significantly better than
when the same bold patterns were placed inside the wings, with no overlap of the edges
(Fig. 4.9b). Further experiments showed that disruptive patterns worked best if they
matched bold colours on the background, but breaking up the body outline reduced
predation even if the disruptive colours were conspicuous (Stevens
et al
., 2006).
An experiment to
test for disruptive
colouration
