New research on guppies recently published in the journal Functional Ecology has found that larger brain sizes correlate to higher survival rates.

The familiar guppy is a popular choice for tropical aquariums the world over. It is a small, colourful, active fish that is found in most fresh and brackish water environments around the globe. Native to parts of the Caribbean and northern South America, populations elsewhere - and these are extensive -have been introduced, either accidentally, or deliberately for mosquito control.

Scientists studying these fish have found that male guppies which are exposed to high predation levels often develop larger brains compared to their counterparts living in lower-risk environments. This would suggest that the larger brain size is instrumental in ensuring better survival rates under conditions of high predation. Simply put, these smarter guppies live longer.

Much of the previous research on guppies was centred upon the female brain, which is physically larger than that of the male. This is because females not only attain a greater adult size, but they tend to live longer and have a more careful life history. Earlier investigations found that female guppies with larger brains preferred to mate with the most vigorous and colourful of males. However, at the time, it was unclear as whether male guppies had anything to gain from developing larger brains.

The recent research took place in northern Trinidad, a country to the south of the Caribbean that has had much of its waterways colonised by the native, ubiquitous guppy. Two independent rivers were selected and assessed: the Aripo and the Quare. Both river routes were punctuated by waterfalls, and it was found that in each case, the area above the waterfall had low numbers of predatory fish, and in the region below the falls, predatory fish were prolific. Male guppies were collected from the areas both above and below the waterfalls on both rivers.

It was found that the male guppies located in the predator-poor areas above the waterfalls had, on average, a smaller brain size compared to their counterparts living below the corresponding waterfall in the more dangerous predator-rich waters. However, it wasn't clear whether the guppies had developed larger brains as the result of a long and complex evolutionary process, or if they could increase brain size over a single lifespan as a direct result of exposure to a high risk environment.

Taking a sample of guppies from waters containing a high number of predators, the scientists then reared young from this group back in the laboratory. The offspring were divided into two separate groups, with the first being exposed to a predatory fish in an adjacent aquarium (which they could sense via sight and smell) for 5 minutes per session, 5 times per week, for the first 45 days of their lives. The second group of guppies were monitored as a control group and housed in an aquarium free from predators.

The results indicated that males which were exposed to predator cues during their development were found to have much larger brains compared to those of the control group. Astonishingly, under perceived conditions of threat, the brains of those particular male guppies had become 21% heavier in just 45 days, proving that predation conditions could indeed quickly influence bodily attributes amongst the guppy population.

With male guppies typically being much more colourful compared to the females, and thus more obvious to predators, they are naturally smaller than the females to help evade predation. However, it seems that when they are exposed to an unusually high number of predators, an increase in brain size and mental agility is likely to further their chances of survival.