Hornbill birds, sharing the same habitat, are also predated by eagles but not by leopards and therefore respond only to eagle-specific Diana monkey alarm calls despite the similarity between both types
of calls (Rainey et al., 2004; Fig. 2a). In addition, Diana monkeys are sensitive to the semantic content of the alarm call of Campbell monkeys, which also provides information about the nature of the threat (Zuberbühler, 2000). Inadvertent information provided by heterospecific individuals detecting a predator threat may also be used to learn to identify an unknown animal as a threat. Woodfrog tadpoles can learn about the danger associated with salamanders by experiencing the anti-predator behaviour (decrease of activity) towards salamander chemical
cues of knowledgeable heterospecific tadpoles (of boreal chorus frogs) in mixed-species assemblages (Ferrari & Chivers, STI571 ic50 2008). Impressive though they may seem, many if not most of these ‘interpretations’ of heterospecific alarm cues have simple mechanistic explanations. In some cases, closely related species may simply respond to heterospecific calls that have similar acoustic properties to their own calls (de Kort & ten Cate, 2001; Fallow, Gardner & Magrath, 2011). A study on pipistrelle bats located in England and Northern Ireland found that three sympatric species selleck products all responded to each other’s distress calls – yet when one species of the bats was exposed to the distress calls of geographically isolated bats, endemic to Madagascar, there was also a significant
response. Analysis of the distress calls revealed apparent acoustic similarities in call structure between the different bat species (Russ, 2004). It is also likely that in the previous example with tadpoles, both tadpole species (boreal chorus see more frog and woodfrog) share a similar anti-predator behaviour or an alarm pheromone, thus explaining the direct association between the salamander cue and the natural unconditioned stimulus of the anti-predator behaviour. Even where such cross-species similarities in alarm calls do not exist, responses to heterospecific signals can often be explained by basic forms of classical conditioning, where an unconditioned stimulus (predator appearance) is reliably predicted by an arbitrary conditioned stimulus (e.g. the alarm call of another animal). If a sympatric species’ alarm call consistently predicts the presence of a generalist predator, then an association can be made between the alarm calls and a direct or indirect experience with that predator (Rainey et al., 2004; Fig. 2a). In free-living golden-mantled ground squirrels, it was found that a neutral sound, unrelated to any sympatric species, can be associated with the appearance of a predator (Shriner, 1999). This results in the (previously) neutral sound inducing an anti-predator response in the squirrels.
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