From spikes and shells to toxic defences, many species have striking traits to help them avoid becoming someone's dinner. But predators can affect prey physiology in more subtle ways. In the lab, simply living alongside predators can inhibit brain cell proliferation. We don't really understand why this is and, until recently, it wasn't even clear how predators affect brain development in wild animals. Perhaps in nature, predators provide the stimulating environment that promotes brain cell proliferation, rather than inhibits it? To test this idea, Kent Dunlap and colleagues studied wild populations of the weakly electric fish Brachyhypopomus occidentalis. The team asked how predation density, near misses with hungry catfish and the stress of capture affect brain development.
Brachyhypopomus occidentalis lives in rivers in Panama alongside electro-receptive predatory catfish. To see how life in risky environments affects brain structure, Dunlap and colleagues measured cell proliferation in different brain regions in fish from populations with varying levels of predation. The team also assayed plasma cortisol levels to see whether this might help explain any differences in brain structure. Fish living alongside more predators had fewer proliferating cells in their forebrains. Clearly, living under the high threat of predation affects brain development.
To see whether a close encounter with a predator had more pronounced effects on brain development, the team compared cell proliferation and cortisol levels in intact fish and in fish with nibbled tails. They found that injured fish had lower levels of forebrain cell proliferation than intact fish.
The reduced proliferation in nibbled fish might suggest a trade-off between making new brain cells and repairing damaged tails. Alternatively, the stress of capture itself might cause these effects. To see how capture affects brain structure, the team assayed fish euthanized a few minutes after collection from the river and fish euthanized after a few hours of being held in a bucket. Even this brief window of stress reduced brain cell proliferation.
These results show that living alongside predators and the injury and stress caused by predation affect brain development. However, these effects were not driven by cortisol, which did not vary consistently between fish as a function of predation risk. If not cortisol, what mechanisms could induce these effects? The stress of living alongside predators may have direct physiological effects on brain cells or predators may restrict prey behaviour and keep them away from environmental stimuli that trigger brain development. Whatever mechanisms are responsible, it is unclear whether reduced cell proliferation is always a bad thing. Brain cell proliferation may improve how well fish learn and remember but this, in turn, might favour risky exploratory behaviours. If reduced cell proliferation makes fish more cautious, then this may improve fish survival despite reducing cognitive capacity. Dunlap and colleagues show us that we can address these exciting outstanding questions in the wild as well as in the lab to really understand how predation pressure affects behaviour and physiology in prey.