Neglected Patterns of Variation in Phenotypic Plasticity: Age- and Sex-Specific Antipredator Plasticity in a Cichlid Fish.

The ability of organisms to plastically respond to changing environments is well studied. However, variation in phenotypic plasticity during ontogeny is less well understood despite its relevance of being an important source of phenotypic variation in nature. Here, we comprehensively study ontogenetic variation in morphological antipredator plasticity across multiple traits in Pelvicachromis taeniatus, a western African cichlid fish with sexually dimorphic ornamentation. In a split-clutch design, fish were raised under different levels of perceived predation risk (conspecific alarm cues or distilled water). Morphological plasticity varied substantially across ontogeny: it was first observable at an early juvenile stage where alarm cue-exposed fish grew faster. Subsequently, significant plasticity was absent until the onset of sexual maturity. Here, alarm cue-exposed males were larger than control males, which led to deeper bodies, longer dorsal spines, larger caudal peduncles, and increased eye diameters. Sexual ornamentation emerged delayed in alarm cue-exposed males. In later adulthood, the plastic responses receded. Despite small effect sizes, these responses represent putative adaptive plasticity, as they are likely to reduce predation risk. In females, we did not observe any plasticity. In accordance with theory, these results suggest fine-tuned expression of plasticity that potentially increases defenses during vulnerable developmental stages and reproductive output.