Male conspicuous coloration covaries with aggression but not with exploration in the short-lived annual fish Nothobranchius guentheri.

The pace-of-life syndrome (POLS) hypothesis posits that life-history trade-offs drive the integration of behavioral and morphological traits. Species with extreme life histories, where such trade-offs are pronounced, are prime candidates for detecting such integration. We tested this in captive males of the annual killifish Nothobranchius guentheri by assessing behavioral repeatability and estimating among-individual correlations for aggression, boldness, exploration, body size, and conspicuous coloration using Bayesian mixed models. Aggression and boldness were highly repeatable, whereas exploration was not. Body size showed a positive but non-significant correlation with aggression. Coloration traits were significantly intercorrelated and correlated with body size. Male coloration covaried positively with aggression: darker males with larger, redder opercular spots exhibited higher levels of ritualized aggression. In contrast, coloration showed no consistent association with boldness. The identified aggression-ornamentation module suggests a potential life-history trade-off: more aggressive and conspicuously colored males likely gain a competitive advantage but may pay a cost through increased risk of predation and injury. Our findings provide preliminary evidence for phenotypic integration consistent with the POLS hypothesis in a short-lived species. However, direct fitness measures are needed to confirm this relationship.