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

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 (<i>Poecilia reticulata</i>) 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.