New research published in the journal Functional Ecology has come to an interesting conclusion about male guppies, a tropical fish found in most fresh and saltwater environments across the globe. Even those that have never had the chance to travel to the Tropics have most likely come across this multicoloured species as it inhabits the vast majority of home aquariums.
The findings suggest that male guppies exposed to high predation levels often develop larger brain sizes than those living in low-risk environments. Consequently, researchers have reason to believe that the development of larger brain sizes among the male guppy population acts as a mechanism allowing for better chances of survival under high predation conditions. In other words, smarter fish appear to live longer.
Until recently, most research on guppies was focused on the female brain. Indeed, female guppies naturally have larger brains than males and a team of researchers from Sweden and the United Kingdom sought to find out if differing brain sizes within the female population had the potential to influence mating choices. They found that, on average, female guppies with larger brains preferred to mate with the more colourful and thus arguably more attractive males. When it came to male guppies, however, it wasn’t as clear if the fish had anything to gain from developing larger brains.
The team began its research in northern Trinidad in the southern Caribbean. Guppies have colonised most of the aquatic environment on the island and the researchers decided to concentrate their observations on populations located in Aripo and Quare, two independent rivers. In each, they collected male guppies living above and below the waterfalls that exist along the rivers’ routes. A small amount of predators colonised the waters above the waterfall, therefore the local guppy population had evolved under predation-poor conditions into a predation-poor population.
On the other hand, below the waterfall, predators were found to be proliferating and the local guppy population had consequently adapted to this dangerous environment. Scientists noticed that in both environments, male guppies located above the waterfalls often displayed on average smaller brain sizes than males living under the waterfall. Those observations formed the conclusion that guppies from high-predation populations have heavier brains than those collected from predation-poor populations.
The next step was to find out if male guppies from high-predation population only had larger brains as result of long evolutionary processes or if they were capable of increasing their intellectual abilities over a life span as a direct response to exposure to risk during development. This question took the team away from the Tropics and back to the laboratory, taking a sample group of guppies originating from high predation groups with them from which they reared laboratory-grown descendants. These were divided into two test populations. The first group was exposed to the sight and smell of a predator living in an adjacent aquarium for five minutes at a time, five times a week, for the first 45 days of their lives. The other guppies served as a control group and lived in the vicinity of non-predatory fish. The results showed that males exposed to predator cues during development were found to have 21 per cent heavier brains than those of the control group, demonstrating that predation conditions influenced bodily characteristics amongst the guppy population.
‘Females have bigger brains than males in general and it could be that it pays females to invest in their brains regardless of the predator conditions,’ says Adam R. Reddon, a Ph.D lecturer in behavioural ecology at Liverpool John Moores and co-author of the study. ‘Females live longer and tend to have a slower, more careful life history, so perhaps females always need a larger brain. But it’s only useful for males when they face the extra challenge of predators.’
Male guppies are generally more colourful than females and thus more likely to be exposed to predation threats. In oder to cope with such risk, male guppies are naturally smaller than their female counterparts, allowing them to evade predators more efficiently. But when exposed to an unusually high number of predators, an increase in intellect could offer males advantages such as the ability to detect and react to danger earlier, ultimately increasing their chances of survival.
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