Costs and benefits of non-predator eavesdropping in mammal-bird  (Page 3/6)

There are also hypotheses that in many species alarm calling is even more selfish; these hypotheses include mobbing recruitment, selfish herd, and predator confusion (Wheeler 2008). In the mobbing recruitment hypothesis, the alarm call attracts the caller’s conspecifics, and together they “mob” the predator, attacking it and driving it away. In the selfish herd hypothesis, the alarm call causes the group to “bunch up,” packing together more densely and thus making it harder for the predator to pick off any one animal. In the predator confusion hypothesis, all the conspecifics respond to the call with flurries of movement or a cacophony of sound, confusing the predator and hopefully driving it off. In all of these examples, it is clear to see how the individual benefits.

No matter what the conspecific reason is for alarming, eavesdroppers can still benefit. Any of these calls, no matter what their intent is, signifies the presence of a predator. In some mixed-species flocks of birds, heterospecifics together mob a predator following an alarm call (Johnson et al 2003); most of the time, however, there is no interaction between the signaler and eavesdropper. The other exception to this rule is in heterospecific partnerships, but we will discuss that later.

Animals that use alarm calls often use variation within the types of calls to demonstrate different things. For instance, in yellow-bellied marmots, the type of call given changes depending on how much danger the threat presents (Blumstein&Armitage 1997). In great gerbils, the type of call given varies according to how close the predator is to the caller, or the nearness of the danger (Randall&Rogovin 2002). Some animals, such as the Diana monkey, even have different alarm calls for different predators (Rainey et al 2004a). Animals can also distinguish about when to listen to alarm calls. Squirrels discriminate between callers, paying more attention to neighbors and paying less attention to squirrels who alarm multiple times without cause (Hare 1998). Goshawks respond to goshawk alarm calls more often during certain times, such as nesting, than at others (Kennedy&Stahlecker 1993).

Increased vigilance following a heterospecific’s alarm call

Remember our chipmunk-titmouse example from our introduction? Studies have proven that chipmunks do, in fact, show heightened anti-predator behavior when they hear a tufted titmouse’s alarm call ( [link] ) (Schmidt, et al. 2008). In their study, Schmidt, et al. quantified the effect that birds’ alarm calls have on eavesdropping mammals (2008). To do this, they set up several different foraging areas, including food for the chipmunks, and played different sound samples. The samples included wood thrush song, a common sound in the area; the call of a broad-winged hawk, a predator of chipmunks; and three different titmouse calls: a “contact call” that they give under low-threat situations; a mobbing call that they give when an avian predator is perched nearby; and a “seet” call that they give when a predator is flying low or otherwise posing a major threat. They then observed and recorded how many seeds were left at each site, and determined that the more seeds there were left, the less the chipmunks in that area foraged, presumably because of their evaluation of the risk. If there were more seeds left, then the chipmunk spent more of its time watching for predators or fleeing than it did actually foraging. (Schmidt, et al. 2008)