Biology Reference
In-Depth Information
experiments with artificial moths (mealworm bodies plus
triangular paper wings) have shown that what makes a wing
spot most effective in reducing predation is not eye mimicry
itself, but rather high contrast and conspicuousness. Thus,
stimuli with circles survived no better than those marked with
other conspicuous shapes (such as bars) and circular spots
more like real eyes (e.g. pale surrounds, dark centres) were no
more effective than others (e.g. dark surrounds, pale centres).
Therefore, eyespots seem to work simply by providing a
conspicuous, novel stimulus that halts the predator's attack
(Stevens
et al
., 2007, 2008).
200
160
120
80
40
0
Does even slight concealment confer an
advantage?
The experiments with the blue jays and moths show that
crypsis and polymorphisms can indeed evolve as prey defences
to thwart predators. But what was the starting point for the
evolution of such marvellous camouflage? Could even a small
increase in search time, caused by slight concealment, still
bring a selective advantage?
Jon Erichsen
et al
. (1980) tested this by an experiment in
which a great tit in a cage was presented with a 'cafeteria' of
prey moving past on a conveyor belt, like the one in Fig. 3.5a.
As with the jays, the clever design of this experiment is that it
enables the observer to control precisely both the order in
which the predator encounters prey and the rate of encounter.
Three items came past the great tit on the belt.
0
10
20
30
40
50
Number of generations
Fig. 4.7
In this experiment, blue jays
hunted for digital images of moths on a
computer screen (with rewards as in
Fig. 4.4). The founding population had equal
numbers (80 each) of three morphs, one
(dark blue line) being more cryptic. The moth
population 'evolved' over 50 generations
(see text) to a stable distribution of the three
morphs, with the more cryptic form
becoming the most abundant. From Bond
and Kamil (1998). Reprinted with permission
from the Nature Publishing Group.
Great tits hunting
for concealed
prey
(a)
Inedible twigs
. In fact, these were opaque pieces of drinking straw containing brown
string.
(b)
Large cryptic prey
. These were also opaque pieces of straw but had a mealworm inside.
(c)
Small conspicuous prey
. These were clear pieces of drinking straw with half a
mealworm inside, clearly visible.
The large prey were worth more energy per unit handling time than the small prey.
However, the problem of selecting large prey is that of distinguishing them from the
inedible twigs; the opaque straw which came past had to be picked up and examined to
see whether it contained a mealworm or just inedible string. The experimental design
therefore mimics the problem faced by a predator searching for a profitable, but
concealed prey item.
Now for the important point: the discrimination time for an inedible twig was only 3-4
seconds. Therefore, the tit, given time, could easily tell a twig from a large prey.
Nevertheless, in theory the tit would maximize its rate of energy intake by ignoring the
large prey altogether, provided conspicuous prey were encountered sufficiently frequently,
Even slight
concealment may
bring an
advantage
