Biology Reference
In-Depth Information
Firstly, observations on foraging Drongos showed that almost all of the suspected
alarm calls were made either in response to the presence of a predator (raptors, owls,
foxes and mongooses) or when an individual being followed was handling a food
item. The calls were not made when directly attacking for food or in non-alarm
contexts. Secondly, the calls made by drongos involved a mixture of drongo-specific
calls and what appeared to be the mimicked alarm calls of other species, especially
glossy starlings. A structural analysis of the drongo-specific calls showed that the
calls made when a predator was approaching (true alarms) did not differ from the
calls made when there was no predator (false alarms). Furthermore, the mimicked
glossy starling alarm calls made by drongos when there was no predator (false
alarms) did not differ in structure from the calls made by glossy starlings in response
to predators (true alarms). Taken together, these first two results suggest that
drongos sometimes deceptively signal that a predator is approaching. Thirdly, when
recorded drongo calls were played to meerkats, the foragers were more likely to scan
for predators and abandon their food in response to alarm calls than other types of
call that drongos make, but showed no difference in their response to true or false
alarms (Fig. 14.19). This suggests that meerkats are deceived by the false alarm calls
made by drongos.
Overall, these results show that drongos dishonestly signal the approach of a
predator, and that meerkats are deceived by these calls, making them abandon their
food to the drongos. Drongos obtain approximately 10% of their total food intake
through the use of these alarm calls. This impressively high proportion is comparable
to a human who could obtain one out of every 10 meals free of charge, by walking
into a restaurant and emptying it with a dishonest shout of 'fire'. It is hard to imagine
that anyone could get very far with such a strategy, raising the analogous question of
why should meerkats continue to respond to drongo calls? One factor is that not all
alarm calls are false - approximately half were true alarms, in which the drongos
also fled for cover and an approaching predator could usually be observed. It remains
an open question whether drongos are producing an evolutionarily stable strategy
(ESS) proportion of false calls, above which the reduced response to their alarm calls
would outweigh any benefit of making more false calls. Another factor is that the
cost of ignoring a potentially true alarm call is relatively large, as being eaten is far
worse than losing a food item. However, it would be wrong to think that meerkats are
powerless in this game of deceit, as observations have shown that they are less likely
to flee when drongos make a false alarm call in the absence of a predator. This
suggests that whether or not a forager flees depends partly on whether a drongo
makes an alarm call as well as upon whether the forager or one of their group
members can see a predator.
Fork-tailed
drongos make
false alarm calls
when there is no
predator …
… which make
meerkats
abandon food
and flee for cover
Dishonest Weapon Displays in Mantis Shrimp
Our above examples of dishonest signalling involve deception between species,
where one species is deceiving another. Another possibility is that dishonest
signalling can occur within species, such that some individuals signal honestly, but
others exploit this and signal dishonestly or 'bluff '. Mantis shrimps are fearsome
