Biology Reference
In-Depth Information
(3)
Phylogenies are used to identify independent evolutionary transitions and the likely
order of trait changes during evolution.
(4)
The data are used wherever possible to discriminate between alternative hypotheses.
The comparative approach is particularly useful for looking at broad trends in evolution
and the general relationship between social organization and ecology. It generates
hypotheses that can be used as predictions for other groups of animals. It can also be
used to test hypotheses which are not amenable to experimentation, such as the effect of
polygamy on sexual dimorphism. Furthermore, it is impressive in the way it shows how
diet, predation, social behaviour and body size, for example, can all be interrelated.
However, we need a different approach to understand in detail the economics of why
individuals adopt particular strategies in relation to their ecology. Can we actually
measure food distribution and predation risk and then come up with precise predictions
as to how an individual will behave? Can we explain why a monkey goes round in a
group of 20 rather than in one of 15 or 25, why its home range is 10 ha rather than
8 or 12 ha, and why it spends one hour in a patch of fruit before moving on? Indeed we
can attempt to answer precise questions like these using optimality theory and an
experimental approach.
Comparative
method useful
for testing
hypotheses not
amenable to
experimentation
Experimental studies of adaptation
We now turn to a different, and complementary, way of looking at how selection moulds
behaviour. Instead of broad scale comparisons between species, the emphasis will be on
the behaviour of individuals of the same species and analysing their behaviour in terms
of
costs
and
benefits
.
The idea of trying to measure costs and benefits grew out of Niko Tinbergen's
experimental approach to studying the adaptive advantage of behaviour. For example,
Tinbergen observed that in a colony of black-headed gulls nesting on sand dunes in north-
western England, incubating parents always pick up the broken eggshell after a chick has
hatched and carry it away from the nest (Fig. 2.15a). Although carrying the shell takes
only a few minutes each year it is crucial for the survival of the young. The eggs and
young of the black-headed gull are well camouflaged against the grass, sand and twigs
around the nest. The inside of the broken shell, however, is white and highly conspicuous.
Tinbergen carried out an experiment to test the hypothesis that the conspicuous white
broken shell reduces the camouflage of the nest. He painted hens' eggs to resemble cryptic
gull eggs and laid them out at regular intervals in the gull colony. Next to some he placed
a broken shell. The results confirmed his prediction that the cryptic eggs were much more
likely to be discovered and eaten by predators, such as crows, if they were close to a broken
shell (Fig. 2.15b). So it is easy to see why the parent benefits by removing the conspicuous
empty shell soon after the chick has hatched: the camouflage of the brood is preserved
and the likelihood of the parent perpetuating its genes is increased.
But there is more to the story than this. The parent does not remove the eggshell
immediately; it stays with the newly hatched chick for an hour or more and then goes
off with the shell. To explain the delay in removing the shell we have to introduce the
idea of a trade-off between costs and benefits. If the parent flies off with the shell at
Costs and benefits
of eggshell
removal in gulls
A trade-off
involving timing
