1. Testing the validity of the Expensive Tissue Hypothesis in mammals
Navarrete, C. van Schaik & K. Isler
Anthropological Institute & Museum
University of Zurich

2. Big brains are advantageous...
Most of the studies done to understand the reasons underlying the development of big brains have centered their efforts in finding out the benefits of encephalization. Therefore, the main branch of brain evolution research works on a one-sided perspective, with the only premise that big brains are developed because of the advantages that are associated to enhanced cognition. However, this perspective lacks of objectivity, as it does not include the disadvantages associated to big brains as well.
Enhanced cognition

3. Big brains are costly Muscle Others Liver Brain Body weight (kg)
BMR
Muscle
Others
Liver
Brain
Body weight (kg)
Homo sapiens
Holliday 1986

4. Metabolic requirements
Big brains are costly
Predictions
The degree to which brains will increase or decrease is governed by the degree to which animals can afford the energetic demands of their brains.
A more complete explanation of the taxonomic variation in relative brain size will be achieved only if we incorporate costs as well as benefits.
Metabolic requirements
Enhanced cognition

5. Expensive Brain framework
BRAIN SIZE
BMR
Allocation
Other organs
Expensive Tissue Hypothesis
Production
Van Schaik & Isler 2008

6. The Expensive Tissue Hypothesis (ETH)
BMR
Brain
Muscle
Others
Liver
Muscle
Others
Liver
Brain
One of the most important and famous contributions in this field is the Expensive Tissue Hypothesis of Aiello & Wheeler (1995).
Body weight (kg)
Aiello & Wheeler 1995

7. The Expensive Tissue Hypothesis
With Homo sapiens:
n = 18
r = -0.69
P < 0.001
------
Without Homo sapiens:
n = 17
r = -0.65
P = 0.005
Aiello & Wheeler 1995

8. The Expensive Tissue Hypothesis
r = -0.33
P = 0.16
IC:
N= 19
r = -0.38
P = 0.11
Aiello & Wheeler 1995
Isler et al. 2008

9. The ETH in mammals n = 36 r = -0.23 P = 0.18 IC: n = 35 r = -0.37
Pitts & Bullard 1968

10. Criticism to the ETH Best-fit lines vary with the taxonomic level
Quality of the data available
Sample size
Quality of the data available is not the best to perform the analyses, as reliable total body composition data is not available for many anthropoid primates, and most species averages are based on small sample sizes. Additionally, there is no dataset including brain mass and gut mass in single individuals.

11. The construction of a new dataset
Directly from dissections
Measurements of gut, other expensive organs (liver, kidneys, heart, spleen), skeletal muscle
Endocranial volume (ECV)
Frozen or in alcohol preserved individuals

12. Methods
Control for the different treatments (data from laboratory mice)
Correction factor BW
1.298
Kidneys
1.969
Spleen
2.382
Pancreas
2.054
Liver
1.734
Repr-Females
2.038
Repr-Males
1.766
Stomach
1.883
Gut
2.064
Heart
2.176
Lungs
1.437
Muscle
2.343
F2 = 12.15, P =

13. Methods Organs weights corrected for BW (without fat)
Regression analyses
Independent contrasts (Mesquite)

14. Dataset used for the presentation
Preliminary sample
Dataset used for the presentation
98 individuals
52 species
6 orders
BMR: 35 sps
N = 4
N = 19
N = 8
N = 2
N = 13
N = 6

15. Results - Mammals n = 52 r = -0.09 P = 0.54 IC: n = 51 r = 0.06

16. Results - BMR BMR included Model: Dependent variable: relative ECV
Independent variables: relative BMR, relative gut mass
n = 35
r = 0.013
P = 0.94
IC:
n = 34
r = 0.02
P = 0.91 IC
Estimate p
BMR
0.07
0.80
Gut Mass
-0.12
0.57

17. Results - Rodents Results - Rodents Correlations with relative ECV
Organs (relative weights)
Raw data IC
IC (BMR)*
Kidneys -
n.s.
Spleen
Liver
(-)
Stomach +
Gut
Heart
Lungs
BMR
Organs (relative weights)
Raw data IC
Kidneys -
n.s.
Spleen
Liver
(-)
Stomach +
Gut
Heart
Lungs
Organs (relative weights)
Raw data
Kidneys -
Spleen
n.s.
Liver
(-)
Stomach
Gut
Heart +
Lungs
Organs (relative weights)
Raw data IC
Kidneys -
n.s.
Spleen
Liver
(-)
Stomach +
Gut
Heart
Lungs
BMR
*Model:
Dependent variable: IC ECV
Independent variables: IC BMR, IC gut mass
N = 19

18. Results - Carnivora Results - Rodents Correlations with relative ECV
Organs (relative weights)
Raw data IC
IC (BMR)*
Kidneys
n.s.
Spleen
Liver
Stomach
Gut +
Heart -
Lungs
BMR
Organs (relative weights)
Raw data IC
Kidneys
n.s.
Spleen
Liver
Stomach
Gut +
Heart -
Lungs
Organs (relative weights)
Raw data IC
Kidneys
n.s.
Spleen
Liver
Stomach
Gut +
Heart -
Lungs
BMR
Organs (relative weights)
Raw data
Kidneys
n.s.
Spleen
Liver
Stomach
Gut +
Heart -
Lungs
*Model:
Dependent variable: IC ECV
Independent variables: IC BMR, IC gut mass
N = 13

19. Conclusions Conclusions Evidence supporting the ETH in mammals is weak
Gut mass does not correlate significantly with ECV
No correlation with other organs was found
Within primates, a negative correlation is found in catarrhines only when gut surface is used (BMR?)
In rodents, a negative trend between ECV and gut mass is found
In carnivores , a positive correlation between ECV and gut mass is found
More data is needed

20. Expectations for our dataset
Further in the future
Expected sample: 100 species
data of 97 species being processed at the moment

21. Thank you for your help! Acknowledgements Acknowledges Robert Martin
Jürg Wermuth
Carsten Schradin
Dr. Marcus Clauss
Mr. & Mrs. Clemens
Zoo Zürich
Knies Kinderzoo
Wildpark Langenberg
Synthesys grant
A.H. Schultz-Stiftung
Swiss National Science Foundation