Biology Reference
In-Depth Information
The final possibility is that males and females have a shared interest in males honestly
signalling their quality to females. Perhaps males and females form monogamous pairs
for life (peafowl do not, but this a hypothetical example), and pairs that are matched in
quality are able to raise more offspring. Possibly because they are genetically matched
their offspring are better at avoiding parasites. In this case, the number of eyespots is a
reliable signal of quality because while it could be faked, the sender and receiver both do
better if the sender signals honestly to the receiver. The honesty is therefore ensured
because the sender and receiver have a
common (or coincident) interest
in honest signalling.
To summarize, there are three possible reasons to ensure signal honesty: index, handicap
and common interest. With an index, dishonesty is not possible. With a handicap or when
there is a common interest, dishonesty is possible but unprofitable. The unprofitability
arises either through the cost of signalling (handicaps) or through other means (common
interest). Put simply, indices can't be faked, handicaps are too costly to be faked and there
is nothing to be gained from faking a signal when there is common interest.
Whilst it is very useful to distinguish between these different ways in which signals can
be reliable, it is unfortunately not always easy to disentangle them empirically. For
example, the Hamilton-Zuk hypothesis for sexual displays described on page 196 could
work by either an index or a handicap mechanism, with both predicting that sexually
attracted traits would be more exaggerated in species where parasite pressure is greater.
In the following sections, examples of these different kinds of signals, and the ways in
which their reliability is maintained, are discussed.
Signals can also
be reliable if the
sender and
receiver have a
common interest
There are three
ways to explain
reliable signals
- indices,
handicaps and
common
interest…
… - but they can
be hard to
distinguish
between
Indices
As discussed in Chapters 5 and 7, individuals frequently have to compete with others for
scarce resources such as food, territories or mates. When this happens, individuals often
use sequences of various displays, in which they assess each other's fighting ability, after
which the individual with the lower fighting ability backs down, rather than engage in
a serious fight (Box 14.1). We shall find that the cues used to signal and assess fighting
ability are often reliable because they cannot be faked, and are therefore indices.
Consider how red deer stags,
Cervus elaphus,
compete for females in the autumn rut.
A male's reproductive success depends on fighting ability; the strongest stags are able to
command the largest harems and enjoy the most copulations. Although fighting brings
great potential benefits, it also entails serious costs. Almost all males suffer some slight
injuries and between 20 and 30% of stags will become permanently injured sometime
during their lives, through broken legs or being blinded by an antler point, for example.
Fighting costs are minimized in competing stags by assessment of each other's fighting
potential, which avoids heavily unmatched contests (Clutton-Brock & Albon, 1979).
In the first stage of the display, the harem holder and challenger roar at each other
(Fig. 14.2a). Roaring is a good signal of fighting ability because it provides information
about the size of a male and to roar well a stag has to be in good physical condition. If the
defender can out-roar the intruder, the intruder retreats. If the challenger out-roars the
defender, or matches him, then the contest moves to the second stage, where both stags
engage in a parallel walk (Fig. 14.2b). This presumably enables them to assess each other
more closely. Many fights end at this stage, but if the contestants are still equally matched
then a serious fight ensues where they interlock antlers and push against each other (Fig.
14.2c). Body weight and skilful footwork are important determinants of victory, but there is a
Assessment is a
key feature of
many contests …
… which helps
prevent escalation
to costly fights
Red dear assess
fighting ability
with roaring and
parallel walks …
… only escalating
to physical
conflict when
equally matched
