Data from: Spatial discounting of food and social rewards in guppies (Poecilia reticulata).

In temporal discounting, animals trade off the time to obtain a reward against the quality of a reward, choosing between a smaller reward available sooner versus a larger reward available later. Similar discounting can apply over space, when animals choose between smaller and closer versus larger and more distant rewards. Most studies of temporal and spatial discounting in nonhuman animals use food as the reward, and it is not established whether animals trade off other preferred stimuli in similar ways. Here, we offered female guppies (Poecilia reticulata) a spatial discounting task in which we measured preferences for a larger reward as the distance to it increased relative to a closer but smaller reward. We tested whether the fish discounted reward types differently by offering subjects either food items or same-sex conspecifics as rewards. Before beginning the discounting tasks, we conducted validation tests to ensure that subjects equally valued the food and social stimuli in the quantities provided. In the discounting task, subjects switched their preferences from the larger to the smaller reward as the distance to the larger reward increased (spatial discounting), but the pattern and magnitude of discounting did not differ across the two reward types. These findings indicate that guppies show similar patterns of discounting food and social rewards in a spatial task. In an analysis of travel times, however, the fish swam faster to food rewards than to shoaling partners. This difference in travel times implies that fish temporally discounted social rewards less steeply than food rewards. Thus, reward type influences temporal discounting, suggesting a dissociation between temporal and spatial discounting. Our results illustrate how animals adjust choices and travel times depending on both the type of cost (time, distance) and benefits (food, social partners).

在时间贴现（temporal discounting）范式中，动物会在获得奖励的时长与奖励质量之间进行权衡，即在更早可得的小额度奖励与更晚可得的大额度奖励之间做出选择。类似的空间贴现效应也可发生在空间维度：动物会在距离更近的小额度奖励与距离更远的大额度奖励之间进行抉择。目前绝大多数针对非人类动物时间与空间贴现的研究均以食物作为奖励，但尚不明确动物是否会以类似方式权衡其他偏好性刺激。本研究以雌性孔雀鱼（Poecilia reticulata）为实验对象，设置空间贴现任务：我们测量了当大额度奖励的距离相较于更近但额度更小的奖励逐渐增加时，受试个体对大额度奖励的偏好变化。我们分别以食物或同性同类作为奖励，以此检验鱼类对不同类型奖励的贴现程度是否存在差异。在正式开展贴现任务前，我们进行了有效性验证实验，以确保受试个体对实验中提供的等量食物与社交刺激具有同等的偏好价值。在贴现任务中，随着大额度奖励的距离增加，受试个体的偏好会从大额度奖励转向小额度奖励（即空间贴现效应），但两种奖励类型下的贴现模式与幅度均无显著差异。上述结果表明，孔雀鱼在空间任务中对食物奖励与社交奖励的贴现模式相似。但在游动时长的分析中，鱼类前往食物奖励的游动速度显著快于前往集群同伴的速度。这种游动时长的差异意味着，鱼类对社交奖励的时间贴现程度弱于食物奖励。由此可见，奖励类型会影响时间贴现效应，这表明时间贴现与空间贴现之间存在分离现象。本研究结果揭示了动物如何根据成本类型（时长、距离）与收益类型（食物、社交同伴）调整其决策行为与移动时长。