What the Dying are Saying to the Living: Mixed Conspecific Alarm Cues Reverse Competitive Hierarchies by Reshaping Genotype-Specific Life Histories.

A central principle of chemical ecology theory posits that reliable information about predation risk, conveyed through conspecific alarm cues, enables adaptive phenotypic plasticity. While responses are known to be finely tuned to predator identity and diet, the consequences of conflicting information, specifically from mixed alarm cues of genetically distinct conspecifics, remain unexplored. We test the novel hypothesis that such mixed cues constitute "chemical interference," which disrupts adaptive decision-making by overriding canonical plastic responses. Using two obligate parthenogenetic Daphnia genotypes as a model system, we compared life-history responses to exposure to genotype-specific (single) versus mixed-genotype alarm cues and linked these responses to outcomes from long-term intraspecific competition experiments. While single cues triggered the canonical adaptive "life-history switch," characterized by accelerated growth and reproduction, mixed cues did not amplify this response. Instead, they induced a state of chronic stress, leading to severe, genotype-specific suppression of growth and reproduction and a phenotypic restructuring toward a conservative, slow-growth strategy. This phenotypic reprogramming was associated with altered competitive outcomes. The genotype (A5) whose mixed-cue phenotype was less constrained in its growth trajectory showed strong competitive performance, contributing to a shift in the competitive hierarchy compared to single-culture controls. In contrast, the other genotype (A1) suffered significant competitive suppression. Our findings demonstrate that the informational content of alarm cues (pure vs. mixed) acts as a critical environmental filter, inducing divergent, genotype-specific life-history strategies. This leads to a context-dependent reversal of competitive dominance, where each genotype's fitness is maximized under a specific informational scenario. This mechanism provides a novel, non-genetic explanation for how variation in chemical information flow can contribute to the maintenance of genetic and phenotypic diversity within populations facing uncertain risks.