A Belding's ground squirrel spots a hawk flying overhead. It sits upright and whistle loudly to warn the other ground squirrels - many of whom may be offspring or relatives of the signaller. When we think about alarm calls, this example the sort of thing we usually have in mind. But not all alarm calls are intended for members of the same species. Some are actually directed to the predator itself!
In their 1997 book The Handicap Principle Amotz and Avishag Zahavi describe such a case, based on their field observations of Arabian Babblers, a highly social species of birds native to Israel, Jordon, and parts of the Arabian peninsula.
When a member of a feeding babbler flock spies a predator, it lets out a
loud barking alarm cry. The other birds in the flock may initially
dive for cover, but often the entire flock joins the initial signaller
atop the tree, all loudly issuing alarm calls. As Zahavi and Zahavi
point out, this behavior is not easily explained as a warning to other
babblers. If the calls were intended for the others in the flock, why
would the birds continue calling long after the entire flock has
joined in? And why would the babblers employ such a loud alarm
which may attract the further attention from predators, when a much softer
call would seemingly be sufficient to alert the other members of the flock?
Zahavi and Zahavi conjecture that the answer to these questions is
that the alarm calls are actually directed to the predator, rather
than to conspecifics. By barking loudly, the babblers inform the
predator - say, a raptor - that it has been seen, and that any
attack will likely be unsuccessful. The babblers benefit from signalling because they convince a potential predator to move on; the raptor benefits from heeding the signal because it can then move on to search elsewhere for unsuspecting prey instead of wasting time and energy in a futile effort to catch prey that are already aware of its presence.
This explanation raises an additional question, however. If alarm calls actually deter the raptor's attack, why would a babbler not gain from calling indiscriminately, even before sighting a predator, on the
off-chance that one is present? Zahavi and Zahavi offer an answer to
this puzzle as well, and the answer is again grounded in the cost of the signal:
"A babbler who would cheat by going to the top
of the canopy and barking before it saw a predator would expose itself
to raptors it might not have noticed. That risk helps ensure that if a
babbler goes to the top of the tree and declares it has seen a raptor,
it has indeed seen one"
Zahavi and Zahavi (1997)
These babbler signals are signals of awareness; the potential prey deter pursuit by informing a
predator that it has been detected, and thereby persuading the predator to
look for an easier catch elsewhere. Alternatively, prey may be able to deter pursuit by convincing a predator that they are
particularly skilled at escape. A common example is provided by the spectacular stotting displays - repeated high, stationary leaps - that gazelles conduct when they see approaching predators. The leaping displays are thought to demonstrate the athletic prowess of the stotting prey, and persuade predators that any attempts at pursuit will be futile. A gazelle in good condition can afford to perform such a display and still have the remaining strength to escape handily if pursued; a gazelle in poor condition would either be unable to perform the display at all, or it would be ill advised to waste what strength it did have on stationary leaping. Predators, for their part, benefit from heeding these signals because they are able to concentrate their efforts on those prey that they are likely to catch.
In these examples of prey-to-predator signalling, we see common interest between predators and some subset of the prey. In this case, the common interest lies between the predator and those prey that are hard to catch. The hard-to-catch prey benefit by distinguishing themselves and thereby deterring predation; the predators benefit by drawing this distinction and instead pursuing quarry that are less likely to escape. Those other prey do not benefit from being thus distinguished, but the cost of the signal is so great they are unable to pose as the harder to catch individuals.
Nur and Hasson (1984), Vega-Redondo
and Hasson (1993), and Yachi (1995) developed mathematical models of how prey can signal escape ability to predators by stotting displays or similar. Bergstrom and Lachmann (2001) explore a model of signalling awareness, based up Zahavi's babbler scenario.