Biology Reference
In-Depth Information
states that the pressure of a steadily moving fluid decreases when its velocity increases.
The velocity of air above the crater is greater than above the mound and the crater has
very still air inside because of its steep edges. The pressure drop between the inside and
outside of the crater is therefore higher than in the case of the dome, so the Bernoulli
effect causes air to be sucked out of the crater end of the tunnel. Vogel
et al
. (1973)
demonstrated, by means of laboratory experiments with miniature model tunnels and
by dropping smoke bombs down real tunnels in the field, that the mound system is so
effective that it causes the air in the tunnel to change once every ten minutes, even in a
very light breeze. The rate of air exchange is related to wind speed but is unaffected by
wind direction since the mounds are symmetrical. This second feature is important
because wind direction is unpredictable in the prairie dog's natural habitat.
The Prairie dog burrow example illustrates that a functional question 'what are
mounds for?' led to a detailed understanding of a mechanism question 'how do prairie
dogs get enough fresh air?'. Examples of mechanistic questions improving our
understanding of a functional question include: how the mechanism which worker
ants use to count the number of times their queen has mated can constrain their ability
to produce adaptive sex ratios (Fig. 13.12), or how parasitoid wasps determine the
number of other females that have laid eggs on a patch (Fig. 10.5).
The optimality models we have encountered in this topic bring together mechanisms
(in the form of constraints) and functions (the currency) in building up an explanation
of behaviour.
Prairie dog
burrows are
designed to
generate air flow
A final comment
What we have described as behavioural ecology in this topic is the present-day equivalent
of natural history. It stands in a lineage which gradually developed from detailed
descriptions of animal behaviour by naturalists such as Gilbert White and Henri Fabre
to experimental studies of natural history by Tinbergen and others. We have emphasized
the idea of making testable predictions about adaptation. To illustrate how this approach
has developed from studies of natural history, let us construct a hypothetical lineage of
studies of mating behaviour in dung flies.
A few hundred years ago naturalists would have been satisfied to discover that when
two dung flies were seen together, one riding on the other's back, the top one was a male
and the one underneath was a female and that the two were mating. A hundred years
ago Darwin realized that males in general compete for females. A description of the
natural history of dung fly mating at this stage would have included reference to the
fact that males are larger than females and that this may be a result of sexual selection.
Forty years ago, at the start of the 'behavioural ecology revolution', an evolutionary
biologist stressed the idea that males ride on the backs of females not only to inject
sperm but also for some time after copulation while the female lays her eggs. By guarding
the female in this way the male guarantees that his sperm are not displaced by those of
another male. A year or two later, an explanation was developed as to why it is that the
male copulates for 40 minutes and not 10, 20 or 60. In the last twenty years or so,
research has shown not only how the optimal copulation time relates to the sizes of the
mating male and female but also suggests how these optima relate to the transfer of
From natural
history to
quantitative
models
