Biology Reference
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C. retecta only
C. relicta only
non-runs; both species
1
2
3
4
5
6
7
8
Position in sequence
Fig. 4.5 The mean percentage of correct responses by jays when moths are presented
in sequences of the same species (runs of either Catocala retecta or Catocala relicta ) or
in a sequence containing both species in random order. The jays improved their
performance when runs of the same species were presented but not when the two
species were presented in a mixed sequence. From Pietrewicz and Kamil (1979).
Reprinted with permission from AAAS.
idea by presenting the jay with different sequences of slides. When, for example, runs
were presented of 'all morph A' or 'all morph B', the jay quickly got its eye in and
improved its success at pecking at the moth as the trial proceeded. When, however,
morphs A and B were presented in random order, the jay did not improve its detection
success with successive slides (Fig. 4.5). This shows that encountering polymorphic
prey does indeed seem to prevent effective search image formation by the predator.
Polymorphic cryptic colouration occurs in many other prey species, including
grasshoppers, homoptera, mantids and bivalve molluscs. However, it is particularly
common in moth species that rest during the day on tree trunks and vegetation.
About  half of the North American Catocala moth species are  polymorphic and some
species have as many as nine different morphs. In theory, these polymorphisms could be
maintained if predators focused on common prey types, so that individuals of rarer
forms of prey were more likely to be overlooked. This is known as 'apostatic selection'
(Fig. 4.6).
Alan Bond and Alan Kamil (1998) tested this idea using the same experimental
design with the blue jays, but this time the jays hunted for digital images of Catocala
moths projected onto a computer screen, with a patterned background (like a tree
trunk) against which the moths were hard to detect. The experiments began with a
founding population of equal numbers of three morphs of moth, one of which was
more cryptic. At the end of each day, detected moths were considered as killed and were
removed from the population. The population was then regenerated back to the initial
size, maintaining the relative abundance of surviving morphs. Over thirty days
('generations') the abundance of the most cryptic morph increased but it then stabilized
Apostatic
selection: rarer
prey types at an
advantage
Evolution of prey
polymorphisms
 
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