Costs and benefits of non-predator eavesdropping in mammal-bird  (Page 5/6)

Partnership in eavesdropping: giving alarms for another’s predator

The hornbill also shares a communication relationship with the dwarf mongoose, with an important difference that sets it apart from the previously-mentioned relationships ( [link] ) (Anne, et al. 1983). The hornbill and the dwarf mongoose share a mutualistic relationship, hunting together in their home region of Kenya. It is one of the closest heterospecific partnerships found in free-living vertebrates (Anne, et al. 1983). Hornbills will often congregate around the termite mounds where the dwarf mongooses sleep, waiting for them to wake up so the group can begin hunting. Likewise, if mongooses wake up and go outside and there are not many hornbills waiting, the mongooses will wait for more to arrive. The hornbills also sometimes wake up the mongooses, so they can begin hunting earlier.

The two species share the same types of prey and many of the same predators. They use these similarities to form a valuable partnership. Hornbills stand sentry duty, to warn the mongooses of approaching predators, while the mongooses hunt, flushing prey out in the open where the hornbills can also dine on them. Anne, et al. (1983) used observation to determine that dwarf mongooses use fewer of their own sentries when hornbills are present in the hunting groups, meaning that the dwarf mongooses trust their partners to warn them of danger. Likewise, the hornbills call out a warning even for raptor species that do not prey on them, but that do prey on the mongooses. This information implies that the heterospecific alarm calls between hornbills and dwarf mongooses are unusually deliberate; the hornbills seem to be warning the mongooses, implying a direct communication rather then eavesdropping.

[link] displays the results of Anne et al.’s study, with the numbers of times that mongooses flee when different calls are given, depending upon the presence of hornbills. When an alarm call was given, the mongooses fled more when there were no birds, implying that the mongooses were more skittish, devoting less time to foraging when their hornbill partners were not present. Both in the presence of alarm calls and not, the majority of mongooses only fled the area following the fleeing of the hornbills; clearly, the mongoose cues its defensive strategy off of the hornbill’s.

What are the costs and benefits of this partnership? The hornbill suffers the usual cost of being more visible to predators, as well as sacrificing time and effort alarming for something that doesn’t threaten them or their conspecifics.

Benefits and opportunity costs

Let us now examine the costs and benefits to the players in these interactions. The cost to the titmouses, the Diana monkeys, and the hornbills for alarming at the presence of predators is an increased chance of being spotted, the same cost it would have without the chipmunks’ eavesdropping. Their benefit is also not affected by the presence of the chipmunks, the hornbills, or the mongooses; namely, increased survival for themselves and their relatives and conspecifics, making the impact on the senders of the signal negligible. The cost to the receivers in all this interaction is that they had to learn to develop this behavior, as well as to recognize which of the calls are alarm calls, which are not, and sometimes which are relevant to mutual predators. Since all the eavesdroppers haven't a limited mental capacity, the effort of learning these skills takes the place of some other behavior that it would be able to learn (Schmidt et al. 2008). While this cost may not seem like very much, the benefit of knowing when a certain predator is approaching must outweigh the opportunity cost of potentially using that learning ability to better understand how to acquire mates or feed offspring and therefore is certainly not trivial. The benefit, however, is the ability to forage more safely and more efficiently and being able to rely on others’ alarm calls rather than spending great amounts of time looking around for threats. Instead, the eavesdroppers can concentrate on their foraging, so long as they keeps an ear out for alarm calls. Because the eavesdroppers benefit in the first two examples while the senders of the signals incur no harm, these interactions could be characterized as commensalism.