Biology Reference
In-Depth Information
vulnerability made only a qualitative prediction. The idea would be consistent with
observations of gulls removing eggshells one, two, three or perhaps even four hours
after the chick has hatched, so that it is hard to test whether the hypothesis is right or
wrong. One way of trying to make a hypothesis more easily testable is to try to generate
quantitative predictions. If one could predict that the parent gull should remove its
eggshell after 73.5 minutes then one would have produced a very testable model indeed.
This is an approach which has been developed by using
optimality models
to study
adaptations. An optimality model seeks to predict which particular trade-off between
costs and benefits will give the maximum net benefit to the individual.
Thinking back to the gulls, if one could measure exactly how much the survival of
the brood is reduced by the conspicuous broken eggshell next to the nest, and exactly
how the risk of cannibalism by neighbours changes with time since the chick hatched,
one could start to calculate the optimum time for the parent to delay removal of the
shell. In this case the optimum might well be defined as the time that maximizes total
reproductive success for the season. But the currency of an optimality model does not
have to be survival or production of young. The overall success of an individual at
passing on its genes may depend on finding enough food, choosing a good place to nest,
attracting many mates and so on. In solving any of these problems an animal makes
decisions, and the decisions can be analysed in terms of an optimal trade-off between
appropriate costs and benefits. For a foraging animal, for example, currencies might be
energy and time. This will form the main theme of the next chapter. We close this
chapter with a simple example.
Crows and whelks
On the west coast of Canada, as in many coastal areas, crows feed on molluscs. They
hunt for whelks at low tide, and having found one they carry it to a nearby rock, hover
and drop it from the air to smash the shell on the rock and expose the meat inside. Reto
Zach (1979) observed the behaviour of north-western crows in detail and noted that
they take only the largest whelks and on average drop the shell from a height of about
5 m. Zach carried out experiments in which he dropped whelks of different sizes from
various heights. This, together with data on the energetic costs of flying and searching,
gave him the information to carry out calculations of the costs and benefits associated
with foraging. The benefit obtained by the crow and the cost paid could both be
measured in calories, and Zach's calculations revealed that only the largest whelks
(which contain the most calories and break open most readily) give enough energy for
the crow to make a net profit while foraging. As predicted from these calculations, the
crows ignored all but the very largest whelks even when different sizes were laid out in
a dish on the beach.
Usually the crow has to drop each whelk twice or more in order to break it open. Since
ascending flight is very costly, Zach thought that the crow might have chosen the
dropping height which would minimize the total expenditure of energy in upward
flight. If each drop is made from close to the ground, a very large number of drops is
required to break open the shell, while at greater and greater heights the shell becomes
more and more likely to break open on the first drop (Fig. 2.16a). The experiment of
Crows minimize
ascending flight
to break the
whelk
