AP Biology class blog for discussing current research in Biology

Big Brains Come at a Cost?

Having a big brain can be great. Whipping your friends at trivial pursuit, acing every test you take, and flaunting your vast knowledge to the world. But what is the cost? Researchers recently reported in Current Biology on tests run on guppies

photo credits to iosonoadry

and discovered some evolutionary setbacks to large-brained guppies.

Through tests with large-brained and small-brained guppies, the scientists determined that a large brain can have adverse effects on gut size and reproductive output.

The reason behind this, and the reason why this is important for humans as well, is because of the amount of mass the brain has versus the amount of energy it requires. In humans, the brain accounts for only 2 percent of the total body mass but makes up 20 percent of the energy requirement of the body. As Niclas Kolm said  “It is a remarkably costly organ energetically.” The idea is that there is a tradeoff between the brain and other organs, and as the brain gets bigger and requires more energy, the other organs must get smaller.

The group’s research suggest that humans and primates, animals with large brains, have relatively small family size because of the tradeoff between brain size and reproduction ability.


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  1. dwil

    I never thought having a bigger brain would be considered a bad thing, but this is very interesting. I had no idea about that statistic you mentioned when you said the brain accounts for only 2 percent of the total body mass but makes up 20 percent of the energy requirement of the body.” I guess maybe that is why doing homework makes me so tired all the time. I also agree with the post when you make the point that one organs usage can cut of another’s, as I have heard many stories of when one sense is lost all the others improve.

    Here is another article on the subject:

  2. jk1234

    Hippopotami have a 1:2789 brain to body ratio compared to 1:40 in humans. They have a much smaller brain compared to their mass.

  3. evolucious

    Why are humans’ brains so large? Psychologist Robin Dunbar believes that we require more processing power to keep track of social relationships. He suggests that there is a direct correlation between size/complexity of one’s social group and a species’ average brain size. Other researches have even discovered a link between density of gray matter in the brain and number of Facebook friends! This article is pretty interesting; check it out:

  4. sayrest4

    Some animals, such as the Koala Bear, are quite the opposite of humans. They have adapted to have a very small brain. In fact, they are the only vertebrate to have a brain that does not fit the size of its skull; they require very little energy. This is why Koalas sleep for 80% of the day, and the rest of the time they just sit and eat.

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