Sometimes stress can be good for a fish. When there are more predators around, killifish in Trinidad grow more brain cells than those that face no predators, and they do so even into adulthood.
“I was surprised to find this because in previous studies, we found that predators inhibit the production of brain cells,” says Kent Dunlap at Trinity College in Connecticut. It seems that killifish swim their own way.
Dunlap and his colleagues examined the brains of a type of wild caught killifish (Rivulus hartii) from three streams on the Caribbean island. In each stream, they gathered about eight adult fish from a location with a high number of predators and about eight from a location with little to no predation. They only used males because previous research on these fish showed that predation affects male but not female brains.
The researchers measured the size of the males’ brains as well as the density of newly grown cells. They found that fish from both spots in each stream had brains similar in size relative to their bodies, but those that had to fight off more predators had nearly double the amount of new brain cells. Dunlap says this may mean that instead of fairly static brains that respond to predators in a timid way, the new brain cells could allow for more responsive behaviour.
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To sort out whether this effect is genetic or purely a response to their environment, Dunlap and his team raised fish from each location and then dissected their brains. In the lab, even with an absence of predators, they saw that the increased brain cell growth persisted in fish descended from those that lived in high-predation areas.
“Over evolutionary time, predation has caused the populations to differ genetically, so there’s this intrinsic difference now that’s upheld,” says Dunlap. He adds that this pattern would likely show up in other animals that continue to grow brain cells into adulthood.
“We mammals and birds, once we reach sexual maturation our body and brain don’t grow very much,” he says. “But fish grow throughout their lifetime, as do many other non-birds and non-mammals.”
Journal reference: Royal Society Proceedings B, DOI: 10.1098/rspb.2019.1485
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