Olfaction and reaction: The role of olfactory and hypothalamic investment in the antipredator responses to chemical alarm cues by northern redbelly dace

Neuroplasticity enables teleosts to promote or downregulate the growth of their brains regionally. To compensate for the effects of predation pressure, teleosts may alter their brain morphology and behavioral responses to mitigate its impact on individual fitness. High-predation environments often promote specific patterns of brain growth and produce bolder and more proactive populations. Owing to the expense of maintaining neural tissue, relative size indicates the regions most relied upon. In northern redbelly dace Chrosomus eos, as little as 2 weeks of elevated predation pressure, resulted in increased investment in their olfactory bulbs and optic tecta, while the imposition of captivity produced smaller, less symmetric hypothalami. Taken together, these results suggest that an individual could potentially become better able to detect a threat, and simultaneously less inclined to react to it, making the impact of either change in isolation is difficult to discern. Here, we compared interindividual variation in gross brain morphology, risk-taking tactics in a novel arena (shy–bold personality), and responding to olfactory cues (proactive/reactive stress-coping style). We hypothesized that olfactory investment would positively correlate with response intensity to predator cue concentration and respond across a wider range of cue concentrations, while hypothalamus size would correlate with shyness and reactivity. Exposure to heightened risk produced more bold/proactive individuals, with larger olfactory bulbs and smaller hypothalami. However, the direction of the correlation between hypothalamus size and behavior varied by treatment, and olfactory investment only corresponded with response intensity amongst proactive individuals. Our findings illustrate the potential pitfalls of relating gross brain morphology to complex behavior and suggest that stress-coping style is a relevant consideration in future studies.