Biology Reference
In-Depth Information
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Height of drop (m)
Height of drop (m)
Fig. 2.16 Dropping of whelks by crows. (a) When whelks are dropped, experimentally, from different heights it
is found that fewer drops are needed to break the shell when it is dropped from a greater height. (b) Calculation
of the total ascending flight needed to break a shell (number of drops × height of each drop). This is minimized
at the height most commonly used by the crows (arrow). From Zach (1979).
dropping shells from different heights allowed Zach to calculate the total vertical flight
needed to break an average shell from different dropping heights (Fig. 2.16b). The
observed average dropping height chosen by the crows (5.2 m) is indeed close to that
predicted to minimize the total vertical flight per whelk. However, the crow would have
to undertake almost the same total upward flight even if each drop was made from a
height somewhat greater than 5.2 m (this is indicated by the very shallow U-shaped
curve of Fig. 2.16b) because slightly fewer drops would be needed. Zach suggests that
there may be an additional penalty for dropping from too great a height: the whelk may
bounce away and be lost from view or may break into so many fragments that the pieces
are too small to retrieve.
The story of crows and whelks shows how calculations of costs and benefits can be
used to produce a quantitative prediction. The crow seems to be programmed to choose
a dropping height that comes close to minimizing the total vertical flight per whelk.
Other currencies, such as maximizing net rate of energy gain, predict much greater
drop heights (Plowright et al ., 1989).
Summary
Throughout this topic we will be using three methods to test hypotheses about
behavioural adaptations: comparison between individuals within a species,
experimental analysis of costs and benefits to individuals and comparison among
species (which, in effect, examines the outcome of 'experiments' done by natural
selection over evolutionary time).
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