Estimating on the fly: the approximate number system in rufous hummingbirds (Selasphorus rufus)

AbstractWhen presented with resources that differ in quantity, many animals use a numerosity system to discriminate between them. One taxonomically widespread system is the approximate number system. This is a numerosity system that allows the rapid evaluation of the number of objects in a group and which is regulated by Weber’s Law. Here we investigated whether wild, free-living rufous hummingbirds (Selasphorus rufus) possess an approximate number system. The hummingbirds were presented with two experiments. In the first we investigated whether hummingbirds spontaneously chose an array containing more flowers than an alternate array. In the second we asked whether the hummingbirds could learn to use numerosity as a cue to which of two arrays contained the better reward. The birds did not spontaneously prefer an array containing more flowers. After minimal training, however, they learned to choose the more numerous array and could differentiate between arrays of five and seven flowers. These data support the presence of an approximate number system in the rufous hummingbird. It seems plausible that having such a system would enable much more efficient foraging in this species.
MethodsWe trained marked, wild rufous hummingbirds to feed from artificial flowers. In the first experiment we presented birds with two arrays of artificial flowers, one more numerous than the other (e.g., 7 flowers vs 1 flower). We presented birds with eleven different combination of arrays: 1vs2, 1vs3, 1vs4, 1vs5, 1vs6, 1vs7, 2vs7, 3vs7, 4vs7, 5vs, 6vs7and recorded which array the bird visited first. During the second experiment, the array with a smaller number of flowers had a lower quality reward (5% sucrose solution) than the array with more flowers (25% sucrose solution). After birds were presented with a single numerous array (7 flowers) that was highly reward and then with a single array (1 flower) that was rewarded poorly, the birds were presented with two arrays simultaneously for the eleven different array combinations. We recorded how many times it took birds to visit first the more numerous array three times in a row. The order of presentation of the different tests was alternated and counterbalanced on both experiments.

摘要：当面对数量存在差异的资源时，诸多动物会借助数量认知系统对其加以区分。分类学上广泛分布的一类数量系统为近似数量系统（approximate number system），该系统可快速评估群体内物体的数量，且遵循韦伯定律（Weber’s Law）。本研究旨在探究野生自由栖息的棕煌蜂鸟（Selasphorus rufus）是否拥有近似数量系统，共设置两项实验开展测试。第一项实验中，我们探究棕煌蜂鸟是否会自发选择花朵数量更多的阵列；第二项实验则验证其能否学会以数量作为线索，判断两组阵列中哪一组可获得更优质的奖励。实验结果显示，棕煌蜂鸟并未自发偏好花朵数量更多的阵列，但经过少量训练后，它们能够学会选择数量更多的阵列，并可区分包含5朵与7朵花的阵列。上述数据证实棕煌蜂鸟体内存在近似数量系统，该系统或可助力该物种提升觅食效率，此推论具备合理性。

方法：我们对佩戴标记的野生棕煌蜂鸟进行训练，使其可从人工花朵处取食。第一项实验中，我们为蜂鸟提供两组人工花朵阵列，其中一组花朵数量多于另一组（例如7朵花vs1朵花），共设置11种不同的阵列组合：1vs2、1vs3、1vs4、1vs5、1vs6、1vs7、2vs7、3vs7、4vs7、5vs7、6vs7，并记录蜂鸟首次造访的阵列。第二项实验中，花朵数量较少的阵列所提供的奖励质量更低（5%蔗糖溶液），而花朵数量较多的阵列则提供更优质的奖励（25%蔗糖溶液）。首先为蜂鸟依次呈现单次的高奖励数量阵列（7朵花）与单次的低奖励单朵花阵列（1朵花），随后再同时呈现11种不同组合的两组阵列，记录蜂鸟连续3次首次选择数量更多的阵列所需的尝试次数。两项实验中，不同测试的呈现顺序均进行了轮换与平衡处理。