Senescence of cognitive abilities in eastern mosquitofish, Gambusia holbrooki

It is widely assumed that cognitive function declines with age, but few studies have measured the cognitive performance of animals using standardized assays taken at the start and end of an individual's natural life span. We investigated how ageing affects cognition in male eastern mosquitofish, measuring five cognitive abilities: spatial, associative and reversal learning, initial impulsiveness and inhibitory control. We especially wanted to examine how senescence affects inhibitory control, impulsiveness and spatial learning because these traits predicted paternity under sexual selection. Each male was measured 6 and 24 weeks after he had matured. We found a significant decline in cognitive performance for two of the five measures (spatial and reversal learning). Crucially, however, there was no evidence for a cognitive trade-off: males that performed better than average when young were not worse than average when older. Instead, there was a significant positive relationship between a male's performance when young and old for both reversal learning and the composite measure of cognition. Our results demonstrate cognitive senescence in a wild vertebrate, reveal variation in cognitive decline and suggest that such declines do not necessarily impose strong early–late life trade-offs on cognition.