# Age-related changes and selective disappearance shape variation in bold-shy continuum in guppies

## TL;DR

Guppies become less bold as they age, and bolder individuals tend to live shorter lives, which helps explain why both bold and shy personalities persist in populations.

## Contribution

This study provides longitudinal evidence of age-related changes in boldness and its genetic and phenotypic links to survival in guppies.

## Key findings

- Boldness in guppies decreases with age.
- There is a negative phenotypic and genetic correlation between boldness and survival.
- Boldness levels differ between sexes, but genetic variance contributions do not.

## Abstract

The persistence of animal personalities within populations contradicts the optimality principle, assuming a single optimal trait value for all individuals. With the advance of research on the maintenance of animal personalities, 2 types of data have emerged as particularly needed: (i) longitudinal data, allowing to distinguish population-level and cohort-level variance in personality traits, and (ii) estimates of additive genetic variance in personality traits and their genetic correlations with life history traits. While longitudinal studies are beginning to emerge, most research to date has relied on behavioral measures at a single life-stage, limiting our understanding of the interplay between ages and genotypes. To address this gap, we employed a 3-generations pedigree design in a captive guppy (Poecilia reticulata) population, measuring boldness, a behavioral trait showing among-individual variance, at different stages of ontogeny and collecting long-term survival data. This design enabled us to investigate the genetic contribution to variance in boldness and survival and examine how it changes across the lifespan in the behavioral trait. Our results show that boldness decreased with age. Further, we found support for phenotypic covariance between boldness and survival, resulting in a negative association between the traits. Additive genetic effects contributed to both boldness and survival, and we found negative genetic correlation between those traits, in line with POLS scenario. Lastly, average boldness was higher in males than females, while the contribution of additive genetic variance did not differ across sexes. Our findings highlight the complex, dynamic interplay of age, genotype, and sex in shaping individual behavior.

Being bold can have advantages in many animals, such as helping them find mates. However, some individuals are shy. Our long-term study of guppies shows that bold individuals have shorter lifespans, suggesting that being bold comes with disadvantages. We also found that guppies become less bold as they age. These factors help explain why both bold and shy personalities continue to exist in populations.