Data from: Heterospecific eavesdropping in ant-following birds of the Neotropics is a learned behaviour

Animals eavesdrop on other species to obtain information about their environments. Heterospecific eavesdropping can yield tangible fitness benefits by providing valuable information about food resources and predator presence. The ability to eavesdrop may therefore be under strong selection, although extensive research on alarm-calling in avian mixed-species flocks has found only limited evidence that close association with another species could select for innate signal recognition. Nevertheless, very little is known about the evolution of eavesdropping behaviour and the mechanism of heterospecific signal recognition, particularly in other ecological contexts, such as foraging. To understand whether heterospecific eavesdropping was an innate or learned behaviour in a foraging context, we studied heterospecific signal recognition in ant-following birds of the Neotropics, which eavesdrop on vocalizations of obligate ant-following species to locate and recruit to swarms of the army ant Eciton burchellii, a profitable food resource. We used a playback experiment to compare recruitment of ant-following birds to vocalizations of two obligate species at a mainland site (where both species are present) and a nearby island site (where one species remains whereas the other went extinct approx. 40 years ago). We found that ant-following birds recruited strongly to playbacks of the obligate species present at both island and mainland sites, but the island birds did not recruit to playbacks of the absent obligate species. Our results strongly suggest that (i) ant-following birds learn to recognize heterospecific vocalizations from ecological experience and (ii) island birds no longer recognize the locally extinct obligate species after eight generations of absence from the island. Although learning appears to be the mechanism of heterospecific signal recognition in ant-following birds, more experimental tests are needed to fully understand the evolution of eavesdropping behaviour.

动物会通过偷听其他物种来获取自身所处环境的相关信息。异种偷听（heterospecific eavesdropping）可通过提供食物资源与捕食者存在的关键信息，为个体带来切实的适合度收益（fitness benefits）。因此，偷听能力可能受到强烈的自然选择压力；不过针对鸟类混合种群（avian mixed-species flocks）警戒鸣叫（alarm-calling）的大量研究仅发现有限证据，表明与另一物种的紧密关联能够筛选出先天信号识别（innate signal recognition）能力。尽管如此，学界对偷听行为的演化以及异种信号识别（heterospecific signal recognition）的机制仍知之甚少，尤其是在觅食（foraging）这类其他生态场景中。为探明觅食场景下的异种偷听行为究竟属于先天本能还是后天习得，我们以新热带区的蚁伴鸟类（ant-following birds）为研究对象：这类鸟类通过偷听专性蚁伴物种（obligate ant-following species）的鸣叫声，来定位并招募同类聚集到布氏游蚁（Eciton burchellii）的蚁群——这是一种优质的食物资源。我们通过回放实验（playback experiment），比较了两个研究地点的蚁伴鸟类对两种专性蚁伴物种鸣叫声的招募反应：一处是两种专性蚁伴物种均存续的大陆样地，另一处是其中一种物种留存、另一种约40年前灭绝的邻近岛屿样地。研究结果显示，蚁伴鸟类会强烈响应两种样地均存在的专性蚁伴物种的鸣叫声回放，但岛屿种群并不会对已消失的专性蚁伴物种的鸣叫声回放产生招募反应。我们的结果有力表明：其一，蚁伴鸟类通过生态经验学习识别异种鸣叫声；其二，在某专性蚁伴物种从岛屿消失8代之后，岛屿种群已无法识别该物种。尽管后天学习似乎是蚁伴鸟类异种信号识别的机制，但仍需开展更多实验验证，才能全面阐明偷听行为的演化历程。