Cognitive ageing is the general process when certain mental skills gradually deteriorate with age. Across species, there is a pattern of a slower brain structure degradation rate in large-brained species. Hence, having a larger brain might buffer the impact of cognitive ageing and positively affect survival at older age. However, few studies have investigated the link between relative brain size and cognitive ageing at the intraspecific level. In particular, experimental data on how brain size affects brain function also into higher age is largely missing. We used 288 female guppies (Poecilia reticulata), artificially selected for large and small relative brain size, to investigate variation in colour discrimination and behavioural flexibility, at 4–6, 12 and 24 months of age. These ages are particularly interesting since they cover the life span from sexual maturation until maximal life length under natural conditions. We found no evidence for a slower cognitive ageing rate in large-brained females in neither initial colour discrimination nor reversal learning. Behavioural flexibility was predicted by large relative brain size in the youngest group, but the effect of brain size disappeared with increasing age. This result suggests that cognitive ageing rate is faster in large-brained female guppies, potentially due to the faster ageing and shorter lifespan in the large-brained selection lines. It also means that cognition levels align across different brain sizes with older age. We conclude that there are cognitive consequences of ageing that vary with relative brain size in advanced learning abilities, whereas fundamental aspects of learning can be maintained throughout the ecologically relevant life span.

认知老化是某些心理技能随着年龄的增长而逐渐恶化的一般过程。在整个物种中，大脑物种中大脑结构的降解速率有所降低。因此，拥有更大的大脑可能会缓冲认知衰老的影响并积极影响老年人的生存。然而，很少有研究在种内水平研究相对大脑大小与认知衰老之间的联系。尤其是，关于大脑尺寸如何影响到高龄的大脑功能的实验数据也大量缺失。我们使用了288只雌性孔雀鱼（Poecilia reticulata）（针对相对大小的大脑）进行人工选择，以研究4-6、12和24个月大时的颜色歧视和行为灵活性的变化。这些年龄特别有趣，因为它们涵盖了从性成熟到自然条件下的最大寿命的寿命。我们没有证据表明，无论是最初的颜色识别还是逆向学习，大脑女性的认知衰老速度都较慢。行为灵活性是由年龄最小的相对较大的大脑预测的，但是随着年龄的增长，大脑的影响消失了。该结果表明，大脑雌性孔雀鱼的认知老化速度更快，这可能是由于大脑选择线的更快衰老和较短寿命所致。这也意味着随着年龄的增长，不同大脑大小的认知水平是一致的。我们得出的结论是，衰老的认知后果随高级学习能力的相对大脑大小而变化，而学习的基本方面可以在整个与生态相关的生命周期中得以维持。