Biology Reference
In-Depth Information
David Harper (1982) performed a similar experiment to test how a flock of mallard
ducks
Anas platyrhynchos
divided up between two foraging sites. Two people stood on
either side of a pond. Each person threw pieces of bread into the water, with one
throwing in bread at twice the rate of the other. Just as with the sticklebacks, the relative
numbers at each site matched the ratio of rates at which bread was thrown in, with
twice as many ducks at the site where bread was being thrown in twice as fast (Fig. 5.2b).
The stable distribution of sticklebacks and ducks could be achieved in two ways. For
example, if one habitat was twice as profitable as another then stability could come about
by: (i) competitor numbers adjusting so that twice as many individuals go to the good
habitat as to the poor one; (ii) all individuals visiting both habitats, but each spending
twice as much time in the good habitat as the poor one. These two experiments tested the
ideal free distribution by examining the
numerical prediction
(number of predators at each
site); as predicted the ratio of competitors numbers match the ratio of food input rates.
Two other predictions could also have been tested: the
equal intake prediction
, that is the
intake rate should be the same at both sites; and the
prey risk prediction
, that is prey
mortality should have been the same at both sites (Kacelnik
et al
., 1992).
These examples are of a 'continuous input' system, in which prey density does not
change with time because prey arrive at a constant rate and are eaten as soon as they
arrive. Whilst this may be a realistic representation of some natural foraging
environments, such as streams with insects drifting past waiting predators, more often
prey (or other resources) are likely to be gradually depleted. The predictions of the ideal
free model are then more complicated (Kacelnik
et al
., 1992).
Fish and ducks
settle in a stable
distribution
between feeding
patches
Competing for mates: dung flies
Female dung flies,
Scatophaga stercoraria
, come to fresh cowpats in order to lay their
eggs. Swarms of males are waiting for them on and around the dung (Fig. 5.3a), and
whenever a female arrives the first male to encounter her copulates with her and then
guards her while she lays her eggs (Chapter 3). Females prefer to lay in fresh dung and
as the pat gets older, and a crust forms over it (thus making it less suitable for egg laying),
fewer females arrive. The male's problem is: what is the optimum time to spend waiting
for females at each cowpat?
Just like the sticklebacks and ducks, the best decision for one individual depends on
what other competitors are doing. For example, if most males wait for short times then
a male who stayed a little longer would have high mating success because he could
claim all the late arriving females. If, on the other hand, most males were staying a long
time then it would pay our male to move quickly to a new pat to claim the early arriving
females there. This competitive situation is analogous to the one faced by the sticklebacks
and ducks, except that now we have frequency dependent pay-offs for different times
rather than at different places. In theory, we would again expect the outcome of
competition to be an ideal free, or stable, distribution. This is where the relative numbers
of males at a pat matches the expected relative numbers of arriving females, so no
waiting times are either over-or under-exploited compared to the rest.
What do male dung flies do? Geoff Parker (1970b) counted males on cowpats and
found that numbers declined exponentially with time (Fig. 5.3b). He then calculated the
expected male mating success at different waiting times, given this observed temporal
distribution of males. He found that expected mating success was indeed equal across the
different times (Fig. 5.3c). Therefore, male dung flies achieve the predicted stable
A competitive
game for male
dung flies: how
long to wait for a
female?
Males adopt
evolutionarily
stable waiting
times
