Corral-López A, Bloch NI, Kotrschal A, van der Bijl W, Buechel SD, Mank JE, and Niclas Kolm. Female brain size affects the assessment of male attractiveness during mate choice. Science Advances, 22 March 2017. doi: 10.1126/sciadv.1601990
The Pixar movie “Finding Dory” recounts the quest of a fish (Dory) in search of her parents from whom she was separated as a wee fingerling. To make matters more interesting, Dory experiences regular bouts of short-term memory loss, rendering her unable to hold together a chain of thought for more than a few seconds. Much to the audience’s delight, Dory’s cognitive quirk sets her off on a series of amusing adventures as she faces every situation as if for the first time, relying on her quick wits to beat her rapidly escaping memories to the punch.
While verisimilitude or scientific rigor might be too much to ask of a movie featuring talking fishes, the makers of Finding Dory may actually have been onto something in their portrayal of the consequences of short-term memory loss on fish-brained decision-making. Indeed, if the film has a thesis, it is that a fish faced with short-term memory-loss exhibits an utter lack of discrimination, whether it be through a disregard for danger, or in the collection of a motley crew of friends ranging from an octopus with seven tentacles to a near-sighted whale shark.
According to a recent study conducted by a team of English and Swedish scientists suggests, cognitive function, as defined by the ability to store and process information (i.e. memories), might have an important role to play in mate selection in a common species of freshwater fish, the guppy. The results of the study that links larger brain size with a greater degree of choosiness in female guppies are reported in the journal Science Advances.
The orthodoxy of the population genetics postulates that sexual selection by the choosier sex (often the female in the equation) is driven by an invariant set of traits termed “attractive”. However, anecdotally and even scientifically speaking, sub-optimal matches happen pretty much all the time. So, what leads to variability in mate choice? In so far as what goes into choosing a mate, the field of population genetics formulates a dichotomy between what it calls extrinsic and intrinsic traits of selection. Extrinsic traits are a result of social influence like Hollywood and fashion magazines that may define some norm of attractiveness, while intrinsic traits include things that are more nature than nurture, like cognitive ability and conditioning. If your memory serves you, and the last James Dean lookalike turned out to be a real jerk, then maybe move on to Brando impersonators.
Determined to demonstrate the role of cognitive function in mate choice, the team lead by Alberto Corral-López chose as a model organism the Trinidadian Guppy, which had previously been bred for lines of small-brained and large-brained fish. Prior work demonstrated that physiological performance in the fish did not differ between the small- and large- brained fish, suggesting that brain size could be used as a robust proxy for cognitive ability.
Corral-López and colleagues reared thirty-six apiece of small- and large-brained fish isolated at birth as to avoid any prior contact with the opposite sex. They also raised sixteen females with typical brain sizes derived from wild populations in an area of high predation where sexual selection would be expected to be at its peak.
The researchers next sought out males who could be objectively assigned as attractive or unattractive using information from previous studies that indicated that females preferred males with larger areas of orange coloration. To systematically characterize individual male attractiveness, the researchers photographed both sides of gently anesthetized male fish and used an imaging software to quantify the degree of orange-ness (i.e. attractiveness) of a given individual. Using a quantitative threshold for attractiveness, they then paired attractive and unattractive males of similar size to form eight attractive-unattractive pairs.
Individual females were then placed in special tanks and were presented with an attractive and unattractive male displayed on either side of the tank such that the fellows could not be viewed simultaneously without the female having to swim to one viewpoint or another of the tank. The researchers then filmed the fish over a fifteen-minute period, taking stock of how much time they spent gazing at one male or another, or in a neutral position out of eyeshot of either suitor.
These “dichotomous choice” experiments irrefutably demonstrated that large-brained and wild-derived females exhibited a strong preference for classically attractive males. Meanwhile small-brained females were fairer of mind, exhibiting no preference for attractive males. Despite their lack of preference, small-brained females displayed an equal sexual appetite to their big-brained and wild sisters, wiling away a comparable amount of time gazing at males.
To rule out the possibility that the difference in behavior between the small-brained fish and the rest were not simply due to difference in the perception of color (think “The Dress” of viral fame), the researchers conducted a sophisticated series of vision tests involving rotating display of orange bands known to elicit specific responses in fish such as swimming madly in circles, twirling, or swirling around to an abrupt halt. Comparing the three classes of females—small-brained, large-brained, and the wild-derived lineage—they observed no difference in the range of responses among the fish. To make doubly certain that perception of color was not affected by brain size they further examined the vision response at a molecular level by measuring the levels of opsins, light-sensitive proteins found in the retina responsible for the first step pathway that leads to vision, in the eyes of the three classes of females. Consistent with the vision tests, they found no difference in the level of opsins among the three groups of females.
Based on their experiments, Corral-López and colleagues conclude that the differences in mate choice between the small-brained, and large-brained and wild-derived females arises from differences in cognitive function. While all fish perceived color in the same way, the small-brained fish apparently lacked the ability to make any use of the information it conveyed on attractiveness. Although the experiments presented in the study are of a highly controlled and artificial nature, they open the door to understanding the role of cognition in more complex scenarios such as natural habitats. The authors note that the large variation in brain size observed in natural populations of guppies might owe to a cost-benefit between the energy required to produce a large brain, and the advantages of being able to remember danger in situations like predator avoidance. In an environment where the chances of getting eaten are higher, large-brained fish stand a better chance of survival, while in a situation where food is scarce, small-brained fish might help the cause of survival of the species by producing offspring who can grow on less. Hence a natural range in cognitive function and mate choice variability is maintained in nature. Remember Dory?