Sexual Selection: Big-brain vs. Small-brain

Gianna Amatucci, Nick Mulvey, Caitlin Welsh, & Cayleigh Shufelt (Stonehill College, BIO 323 Evolution, Fall 2019)

Predominantly residing in the tropics of South America, guppies are small and colorful freshwater fish. They are omnivorous animals, primarily consuming algae and brine shrimp. Unfortunately, guppies are preyed upon by a number of larger creatures, including birds, larger fish and mammals. While constantly having to avoid such predators, guppies are always in search of a suitable mate to spread their gene pools to future offspring. Alberto Corral-López and colleagues studied how predation pressure, in addition to cognitive ability and brain size, affected sexual behavior and sexual selection in guppies. The actions of both large-brained and small-brained female and male guppies were observed by Corral-López in order to study this phenomenon.

Domestic male guppy in an S-curve mating display. Image credit: “older guy 22feb08” by Alice Chaos is licensed under CC BY-NC-SA 2.0

The researchers utilized 45 large-brained and 45 small-brained female and male guppies to conduct their study. Both sexes were separated in the lab by Operational Sex Ratio (OSR), which is the ratio of sexually active males to females available for mating. Two treatments were used for both the female and male guppies: female-biased OSR with and without predator model, and male-biased OSR with and without predator model. When a population of any species is “biased,” it is an indication of the population’s predominant sex. A female-biased population would have more females than males, while a male-biased population would have more males than females. Once the researchers separated the guppies into the appropriate treatments, different strategies were used for observation depending on the sex of the guppies. For males, sigmoidal displays, sneak attempts, male sexual interferences, male aggressive behavior, and sexual behavior ratio were the chosen strategies. For females, number of glides, number of copulations, receptiveness towards males, female sexual interferences, and number of female aggressive behaviors were the chosen strategies. 

            Corral-López and his fellow researchers found significant results from their study. For male guppies, it was discovered that those with small brains were more aggressive in male-male interactions, and they courted more actively than large-brained males. However, although large-brained males courted less actively, they did not have a lower copulation rate than small-brained males. One possible question that this idea brings up is whether or not large-brained males are more attractive to females, but Corral-López will need to conduct more research to investigate this possibility. For female guppies, there was an increase in copulation when there was a higher competition among females than males. However, only large-brained females were willing to copulate under low predation rates. It was eventually discovered that females could modify their mating behavior based on the presence of predators when they had larger brains. Small brained females and males, on the other hand, seemed unable to split their attention between mating and avoiding predators. Overall, this evidence points to the idea that female brain size plays a role in maximizing reproductive output since only large-brained females knew to avoid copulation when the predation pressure was high. This data seems to point to the possibility of fitness differences between large-brained and small-brained females. According to Corral-López, however, “Little experimental data exists on how cognitive ability affects such fitness-associated aspects of behavior.” Future research will hopefully put all of this data into better perspective.


  1. Corral-Lopez, Alberto, Maksym Romensky, Alexander Kotrschal, Severine D. Buechel, and Niclas Kolm. 2019. “Brain size affects responsiveness in mating behavior to variation in predation pressure and sex-ratio.” Journal of Evolutionary Biology 10:1111. (
  2. A-Z Animals. 2019. “Guppy.” Retrieved Dec. 2, 2019 ( )
  3. (Alberto Corral-López’s Twitter)
  4. (Corral-López’s website)
  5. (Corral-López’s Google Scholar)
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  7. (Our Twitter account)

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