A life history perspective on size, health and adult mortality in the Gambia Rebecca Sear London School of Economics

Life history theory and demography Life history is concerned with allocation of energy over the life course How do organisms solve problem of adaptively allocating energy between growth, body subsistence and repair, and reproduction? Demography a key component of LHT – both a constraint on, and an outcome of, life history decisions

Adult mortality and resource availability This study investigates relationship between adult mortality and resource availability Measures of resources availability –Height –BMI –Haemoglobin (Hb)

Height Height: measure of past investment in growth –Previous research: Human studies (height): –height usually found to be negatively correlated with mortality –but recent evidence suggests mortality higher in tall people for certain causes Animal studies (size): –caloric restriction increases longevity –costs of fast growth

BMI & Hb BMI & Hb: measures of current body condition –Mortality should be inversely related to both BMI and Hb but What is the shape of the relationship? Does this relationship differ between women and men? –Previous (human) studies suggest a non-linear relationship between BMI and mortality

Analysis Analysis of adult (21+ years) mortality by: –Height –BMI –Haemoglobin level Investigating both whether there is a relationship, and the shape of the relationship For both women and men

Data Data collected rural villages Demographic data collected continuously Anthropometric surveys conducted (at least) annually

Adult anthropometrics WomenMen Height (cm) Mean Range N (measurements)13,2909,680 BMI Mean Range % underweight1613 % overweight41 N (measurements)11,5989,674 Hb (g/dl) Mean Range % anaemic4833 N (measurements)10,9869,653

Height and age

BMI and age

Hb and age

Adult mortality

Methods Discrete-time event-history analysis on probability of adult death (21+ years) Separate models for women and men Anthropometric variables entered as time dependent Controlled for birth cohort

Results WomenMen VariableEstimate (SE) Constant 84.3 (37.0)*4.81 (4.64) Age-0.02 (0.03) 0.07 (0.01)* Height Height (0.47)* (0.001)* (0.01) BMI BMI (0.27)* 0.02 (0.01)* (0.39)* 0.02 (0.01)* Hb Hb*age (0.14)* 0.01 (0.002)* (0.03)* No. deaths No. survivors823683

Height and mortality

BMI and mortality

Hb and mortality

Conclusions I Height: –matters for female mortality – though relationship not linear –Why not for men? Measures of current body condition significantly correlated with mortality for both sexes –High BMI increases mortality (costs of fat storage?), but low BMI more important –Hb important for both sexes, especially women (maternal mortality?)

Conclusions II Clearly, there is variation in adult mortality by resource availability Though these relationships tend not to be linear Current health/body condition matters for both sexes, height only for women –growing tall has costs for women (or linked to greater reproductive effort of tall women?), but apparently not for men? Influence of genetic factors?