The link between relative brain size and cognitive ageing in female guppies (Poecilia reticulata) artificially selected for variation in brain size.

Annika Boussard,Niclas Kolm,Severine D Buechel,Alexander Kotrschal,Mirjam Amcoff

doi:10.1016/j.exger.2020.111218

Annika Boussard, Niclas Kolm + Show 3 more

Open Access

https://doi.org/10.1016/j.exger.2020.111218

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Abstract

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 24months 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.

Highlights

Cognitive ageing is the process of age-related decline of certain cognitive abilities, caused by degradation of brain structures (Hedden and Gabrieli, 2004; Kandel et al, 2013; Marner et al, 2003; Phillips et al, 2018; Ritchie et al, 2015; Scahill et al, 2003)
Our results show that cognitive ageing is a complex process, generated by a wide array of intrinsic and extrinsic factors that most likely cause extensive variation at all taxonomic levels in the affected neural networks
We conclude that the ‘brain reserve’ hypoth esis does not fully explain the relationship between relative brain size and cognitive ageing at the intraspecific level

Summary

Introduction

Cognitive ageing is the process of age-related decline of certain cognitive abilities, caused by degradation of brain structures (Hedden and Gabrieli, 2004; Kandel et al, 2013; Marner et al, 2003; Phillips et al, 2018; Ritchie et al, 2015; Scahill et al, 2003). Humans process information at a slower rate and show a severe decline in reasoning and working memory with increasing age (Ritchie et al, 2015), dogs progressively decline in associative learning and behavioural flexibility with increasing age (Tapp et al, 2003), and associative learning and memory declines with increasing age in nematodes (Zwoinska et al, 2013). The causes and consequences of many aspects of cognitive ageing are very well studied in humans and are focused on how to prevent and cure age-related cognitive diseases. We focus on the cognitive consequences of ageing in a laboratory bred guppy population with known differences in brain size

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