title: Fatness and fitness: exposing the logic of evolutionary explanations for obesity.
abstract: To explore the logic of evolutionary explanations of obesity we modelled food consumption in an animal that minimizes mortality (starvation plus predation) by switching between activities that differ in energy gain and predation. We show that if switching does not incur extra predation risk, the animal should have a single threshold level of reserves above which it performs the safe activity and below which it performs the dangerous activity. The value of the threshold is determined by the environmental conditions, implying that animals should have variable 'set points'. Selection pressure to prevent energy stores exceeding the optimal level is usually weak, suggesting that immediate rewards might easily overcome the controls against becoming overweight. The risk of starvation can have a strong influence on the strategy even when starvation is extremely uncommon, so the incidence of mortality during famine in human history may be unimportant for explanations for obesity. If there is an extra risk of switching between activities, the animal should have two distinct thresholds: one to initiate weight gain and one to initiate weight loss. Contrary to the dual intervention point model, these thresholds will be inter-dependent, such that altering the predation risk alters the location of both thresholds; a result that undermines the evolutionary basis of the drifty genes hypothesis. Our work implies that understanding the causes of obesity can benefit from a better understanding of how evolution shapes the mechanisms that control body weight.
authors.affiliation: Centre for Research in Animal Behaviour, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QG, UK School of Biological Sciences, Life Sciences Building, University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TQ, UK adhigginson@gmail.com.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Higginson
authors.affiliation: School of Mathematics, University of Bristol, University Walk, Bristol BS8 1TW, UK.
authors.forename: John M
authors.initials: JM
authors.lastname: McNamara
authors.affiliation: School of Biological Sciences, Life Sciences Building, University of Bristol, 24 Tyndall Avenue, Bristol BS8 1TQ, UK.
authors.forename: Alasdair I
authors.initials: AI
authors.lastname: Houston
mesh_terms: D000818:Animals; D005075:Biological Evolution; D003657:Decision Making; D004435:Eating; D005247:Feeding Behavior; D005523:Food Supply; D006801:Humans; D008962:Models, Theoretical; D009765:Obesity / Q000209:etiology*; D011235:Predatory Behavior; D013217:Starvation; D013269:Stochastic Processes
keywords: energy balance; intake regulation; obesity; thrifty genotype; thrifty phenotype