Energetics and the evolution of human brain size

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The human brain stands out among mammals by being unusually large. The expensive-tissue hypothesis1 explains its evolution by proposing a trade-off between the size of the brain and that of the digestive tract, which is smaller than expected for a primate of our body size. Although this hypothesis is widely accepted, empirical support so far has been equivocal. Here we test it in a sample of 100 mammalian species, including 23 primates, by analysing brain size and organ mass data. We found that, controlling for fat-free body mass, brain size is not negatively correlated with the mass of the digestive tract or any other expensive organ, thus refuting the expensive-tissue hypothesis. Nonetheless, consistent with the existence of energy trade-offs with brain size, we find that the size of brains and adipose depots are negatively correlated in mammals, indicating that encephalization and fat storage are compensatory strategies to buffer against starvation. However, these two strategies can be combined if fat storage does not unduly hamper locomotor efficiency. We propose that human encephalization was made possible by a combination of stabilization of energy inputs and a redirection of energy from locomotion, growth and reproduction.

At a glance


  1. Correlations between the masses of visceral organs, brains and adipose depots in mammals.
    Figure 1: Correlations between the masses of visceral organs, brains and adipose depots in mammals.

    The analysis is based on a sample of 100 mammalian species and controls for phylogenetic relationships and fat-free body mass. Statistical details are listed in the Supplementary Information 3.1.

  2. Correlation between residual brain mass and residual adipose depots mass in wild-caught female mammals, controlling for fat-free body mass.
    Figure 2: Correlation between residual brain mass and residual adipose depots mass in wild-caught female mammals, controlling for fat-free body mass.

    Species-level values: N = 28 species, r2 = 0.258, P = 0.006. PGLS: λ = 1.00, β = −0.12, t value = −3.42, P = 0.002.

  3. The expensive-brain framework proposes complementary pathways for an adaptive increase in relative brain size.
    Figure 3: The expensive-brain framework19 proposes complementary pathways for an adaptive increase in relative brain size.

    First, brains can get larger when energy inputs are stabilized on a higher level (higher total metabolic turnover20) through an increase in mean dietary quality (for example, more animal fat and protein in early Homo4, 22, 24), energy subsidies from other individuals (for example, cooperative breeding, allomaternal care19, 21) or by reducing fluctuations in energy inputs (for example, cognitive solutions15, including culture). Second, at constant total energy intake, energy allocation to other functions may be reduced, such as locomotion (for example, efficient bipedalism27, 28) or production (for example, slower life history pace30).


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Author information


  1. Anthropological Institute and Museum, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland

    • Ana Navarrete,
    • Carel P. van Schaik &
    • Karin Isler


K.I. and C.P.v.S. designed the project. A.N. performed the pilot study and collected the data. A.N. and K.I. performed the analyses and all three authors wrote the manuscript.

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The authors declare no competing financial interests.

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Supplementary information

PDF files

  1. Supplementary Information (1.3M)

    This file contains Supplementary Text comprising Supplementary Data, Supplementary Methods, Supplementary Results and Discussion (see Content list for more details); Supplementary Figures 1-3 with legends; Supplementary Tables 1-11 and additional references.

Excel files

  1. Supplementary Data (Navarette_SupplData) (70K)

    This file displays the compiled dataset of organ mass and metabolic data for 100 mammal species.

Additional data