Costs and benefits of non-predator eavesdropping in mammal-bird  (Page 4/6)

[link] shows the results of their experiment, with the comparative amount of seed left in the areas with each type of call played. Sure enough, the chipmunks responded to titmouse alarm calls by foraging less and engaging in anti-predator behavior more. The chipmunks responded to the “mobbing” call and the “seet” call, each of which signified the presence of a predator. The chipmunks did not respond substantially to the control call, with the songs of the wood thrush, nor did they respond to the non-alarm calls of the titmouse. (Schmidt, et al. 2008)

Interestingly, the researchers’ hypothesis regarding the hawk call was proven incorrect. They had assumed that the chipmunk would respond to this clear evidence of a predator’s presence with anti-predator measures, but instead the chipmunk had no clear response. Schmidt, et al. hypothesized that an attacking predator would not call, betraying its position, so a chipmunk hearing a hawk call could assume that the hawk presented no immediate danger for them. (Schmidt, et al. 2008)

Distinguishing between different types of alarm calls in eavesdropping

The last study included data about how the chipmunk has to respond to all of the titmouse’s alarm calls, since the titmouse does not have specific calls for specific predators. But this does not mean that no animal gives specific information about a predator. The Diana monkey, for example, gives a different type of alarm call depending on what predator is attacking ( [link] ) (Rainey, et al. 2004). The Diana monkeys then respond with different behaviors depending on what type of predator is attacking. For example, in Hauser and Wrangham’s study, it was reported that when an alarm call for a viper is given, primates look towards the ground; when an alarm call for an aerial predator is given, the primates look up (1990). The yellow casqued hornbill takes advantage of the differences in calls for specific predators in its eavesdropping, since it only shares one common predator with the Diana monkey. (Rainey, et al. 2004)

Crowned eagles attack both hornbills and Diana monkeys, but leopards attack the Diana monkey and not the hornbill. Rainey, et al. conducted their experiment to determine if hornbills, and by extension other eavesdroppers, can distinguish between different types of alarm calls. By using experimentation similar to that described in Schmidt’s study, they discovered that hornbills can indeed tell the difference between the different alarm calls; they respond with increased anti-predator behavior only when the alarm call identifying their common predator is given (Rainey, et al. 2004).