Biology Reference
In-Depth Information
(a)
Image not available in this digital edition.
Fig. 4.4
(a) A blue jay in the testing apparatus. The slides are back projected on a
screen in front of the bird. The advance key (see text) is to the left. A mealworm is
delivered through the circular red hole if the jay makes a correct response. Photo by
Alan Kamil. (b) The jays were more likely to detect
Catocala
moths on a conspicuous
background. A jay pecking indiscriminately at all slides gets a low score on the detection
index. From Pietrewicz and Kamil (1981).
mealworm for pecking at the slide, and the next slide was then shown after a short interval.
If there was no moth in the picture then the jay could peck a second smaller 'advance' key
to produce the next slide almost immediately. If the jay made either of two errors, namely
pecking at the slide when no moth was present or pecking the advance key when a moth
was present, it was 'punished' in the form of a delay to the next slide presentation.
This experimental procedure is ingenious for two reasons. Firstly, the predator is faced
with a perceptual problem only; there is no complication caused by other factors which
might influence predation, such as prey taste, activity or escape efficiency. Secondly,
because the predator is stationary and the prey are, in effect, moving past in front of it (in
the form of a sequence of slides), it is easy to control the frequency and order in which the
predator encounters prey. This would be more or less impossible if the predator was moving
about the cage searching for real moths. It was found that the jay made many more mistakes
if the moth was presented on a cryptic background than if presented on a conspicuous
background (Fig. 4.4b). This provides direct support for the hypothesis about crypsis.
Experimental test
of crypsis: jays
hunt for moth
pictures on a
screen
Polymorphic prey
prevents search
image use by
predators
Polymorphic cryptic colouration
In many species of underwing moths the forewings are polymorphic, that is there are
different colour forms coexisting within the same population. One hypothesis for this is
that when a predator discovers a moth it may form a 'search image' for that particular
colour pattern and concentrate on looking for another which looks the same (Box 4.1).
If all the population were of exactly the same colour then all would be at risk, but if
there was a polymorphism then a predator which had a search image for one morph
may be more likely to miss the other morphs. Pietrewicz and Kamil were able to test this
