Brain size and extinction risk in mammals: or the most difficult paper to publish (yet)

rejectedMy last entry was about a paper that I really enjoying doing. This entry is also about a paper I enjoyed doing (skip to the subtitle to read more about that) and how my joy was ruined by the publishing hurdles.

We submitted this paper for the first time nearly three years before it was finally accepted. There were several submissions to diverse journal and many rejections. In a particularly frustrating one the editor stated that our findings were “common knowledge”. Two years later that journal published a paper with similar, presumably commonly known, findings. Whether the decision came from lack of editorial consensus (we had different editors), change of perspective, or worse, from biases we may never know. We also had two separate instances of submitting up to three revised versions (addressing reviewer concerns) to a given journal only to have the paper finally rejected by the editors. This is not unheard of (rejection of revised versions), but it is difficult to avoid frustration when this means you spent a whole year addressing comments from one journal to then have to start over again. For this paper that happened twice!

Of course, at some point you question whether your paper is just not that good. Yet, I think this paper is GOOD. So, What went wrong? I think we started aiming too high (Nature). Nevertheless, at some point I suspect we were at the “right level” and still were getting rejections. It is possible we are not good at selling the story. The case above where our paper was rejected as “common knowledge” may be a good example of how we failed to transmit novelty. Note to self: there is room for improvement here. Finally, I think we simply had bad luck. The reviewing process is not infallible. At best one gets three separate reviewers for a given journal, which means there is plenty of room for “stochastic” processes to interfere. Someone having a bad day or disliking you personally (double-blind can help here). Of course, it is possible that the paper is just not that good and the reviewers in Evolution (where it is published) are the ones that got it wrong! Feel free to judge for yourself by reading the brief summary below and/or the paper (from the journal page, researchgate, and coming soon a publicly available repository copy).

Size matters: large brain size leads to higher risk of extinction

The big bad brainLarge brain size is often associated with being better able to cope with novel, changing environments due to an ability to innovate and being “smart”. We would then expect that species with relatively large brain size (for their body mass) could fare better in today’s changing world. As humans modified the environment, changing landscapes and even the climate, large brained species may be able to find “smart” solutions to cope with changes. However, that is not the case. My colleagues and I gathered data for 474 mammalian species which show that large brains have actually become a burden.


Our study links brain size (accounting for body size) with conservation status as described by the International Union for Conservation of Nature (IUCN) explicitly considering how brain size influences life-history and ecological traits such as gestation length and population density. We know that large brains are energetically costly to grow and maintain. For instance, in humans the brain represents about 2% of the total body mass but uses up to 20% of the energy. This means that species with relatively large brains tend to have low reproductive rates, gestation is longer and offspring are weaned later, which in turn increase extinction risk. Over evolutionary times, some species have presumably evolved big brains because these costs were outweighed by the benefits of being “smart”. What this study shows is that this balance has been altered and the species with the largest brain size, humans, is making it very difficult for other large brained species to persist.

This study was done in collaboration with Alejandro Gonzalez-Voyer, Carles Vilà and Eloy Revilla. The paper reflects their views, these ramblings may not.


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