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

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

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

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

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

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

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

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

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

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.

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

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 = 0.0002

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

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

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

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

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

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

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

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

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