https://phys.org/news/2018-06-male-guppies-larger-brains-response.html > "Guppies offer an excellent model for evolutionary research because they have colonized multiple independent rivers in Trinidad where they are exposed to a variety of different conditions", said Dr. Adam Reddon, now at Liverpool John Moores University's School of Natural Sciences and Psychology. "We were particularly interested in finding out how the brains of these widely-distributed fish have evolved for dealing with the challenges of living under predation threat." > The researchers looked at whether there are differences in relative brain mass between wild guppies collected from high and low predation populations and found that, for their body size, males collected from high predation sites had on average 17% heavier brains compared to males from low predation sites in the same river. Female guppies, by contrast, did not show this pattern.