There are many fish in the sea, but having a large school to choose from doesn't make mate choice decisions any easier. In fact, selecting a mate requires members of the choosing sex to process, integrate and recall a great deal of information related to mate quality across many potential mates. As this choice is directly related to fitness, it stands to reason that the cognitive ability of the choosing sex is under considerable selective pressure. Therefore, Alberto Corral-López, a PhD student in Niclas Kolm's laboratory at Stockholm University, Sweden, along with several colleagues, decided to test the hypothesis that smarter individuals choose better mates. They knew from previous work in the Kolm lab that female guppies with big brains have a higher cognitive ability than female guppies with small brains, so they staged a dating game to test whether or not the big-brained guppies made better mate choice decisions.
To begin the matchmaking experiments, Corral-López presented individual big- or small-brained female guppies with two males to choose from. One male was brightly coloured with a large tail, as these traits are known to represent a high-quality mate in guppies. The other male was dull-coloured and nondescript – a less optimal choice by far. Corral-López tracked how much time a female spent with each male and found that while small-brained females spent just as much time with the ugly male as the attractive male, the big-brained females preferred to swim with the beautiful beau. While this result supports a connection between female brain size and mate choice decisions, Corral-López wanted to rule out that the preferences he observed weren't caused by inherent differences in the female visual system.
So, for his next experiment, Corral-López tested the ability of females to distinguish and perceive colour. He tracked how well each female oriented herself during an optomotor response test, which involved moving alternating bands of colour (red and green to match the males) along the wall of the fish tank while recording her movements. He also varied the intensity of each colour band to create high- and low-contrast colour stimuli, similar to the attractive and dull male fish. Corral-López found that all females, irrespective of brain size, were better at following high-contrast colour bands compared with low-contrast bands, meaning that brain size does not influence colour perception. Furthermore, small- and big-brained females had similar gene expression profiles in their retinas for the opsins that are essential proteins for vision and colour perception. Therefore, he concluded that the big- and small-brained female guppies were equally capable of distinguishing colourful males from dull males. This means that small- and big-brained females gathered the same visual information from the two males that were presented to them; however, the females with large brains were better at using this information to choose the superior mate. Or, put another way, big-brained guppies are smart enough to judge a fish by its colours.