Brain size does not predict learning strategies in a serial reversal learning test

Reversal learning assays are commonly used across a wide range of taxa to investigate associative learning and behavioural flexibility. In serial reversal learning, the reward contingency in a binary discrimination is reversed multiple times. Performance during serial reversal learning varies greatly at the interspecific level, as some animals adapt a rule-based strategy that enables them to switch quickly between reward contingencies. Enhanced learning ability and increased behavioural flexibility generated by a larger relative brain size has been proposed to be an important factor underlying this variation. Here we experimentally test this hypothesis at the intraspecific level. We use guppies (Poecilia reticulata) artificially selected for small and large relative brain size, with matching differences in neuron number, in a serial reversal learning assay. We tested 96 individuals over ten serial reversals and found that learning performance and memory were predicted by brain size, whereas differences in efficient learning strategies were not. We conclude that variation in brain size and neuron number is important for variation in learning performance and memory, but these differences are not great enough to cause the larger differences in efficient learning strategies observed at higher taxonomic levels.

反转学习实验（reversal learning assay）范式已被广泛应用于各类生物分类群，用于探究联想学习（associative learning）与行为灵活性（behavioural flexibility）。在系列反转学习（serial reversal learning）范式中，二元辨别任务（binary discrimination）内的奖赏偶联（reward contingency）关系会被多次反转。系列反转学习中的表现在种间水平（interspecific level）存在显著差异：部分动物会采用基于规则的策略（rule-based strategy），使其可在不同奖赏偶联间快速切换。有假说指出，更大的相对脑容量（relative brain size）所带来的学习能力提升与行为灵活性增强，是造成该种间差异的重要因素。本研究在种内水平（intraspecific level）上对该假说开展了实验验证。我们选取经过人工选育、具有不同相对脑容量且神经元数量对应存在差异的孔雀鱼（Poecilia reticulata），开展系列反转学习实验。我们对96只个体完成了10轮系列反转测试，结果发现脑容量可预测个体的学习表现与记忆能力，但无法解释高效学习策略的个体差异。本研究结论表明：脑容量与神经元数量的差异对学习表现和记忆能力的个体差异具有重要影响，但这类差异尚不足以造成在更高分类群水平上观察到的高效学习策略的显著差异。