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Aim: Humans evolved to withstand harsh environments by adaptively decreasing their body size. Thus, adaptation to a hostile environment defers the infancy-childhood transition age (ICT), culminating in short stature. In natural-fertility human societies, this transition is associated with weaning from breastfeeding and the mother's new pregnancy. We therefore used the interbirth interval (IBI) as a surrogate for the ICT., Methods: We hypothesized that long IBI will be associated with smaller body size. The sample used is 22 subsistence-based societies of foragers, horticulturalists and pastorals from Africa, South America, Australia and Southeast Asia., Results: The IBI correlated negatively with the average adult bodyweight but not height. After correction for 'pubertal spurt takeoff' and 'weight at age 5', the IBI explains 81% of 'average adult weight' variability., Conclusions: This inter-population study confirms that body weight is adaptively smaller in hostile environments and suggests that the selected trait for this adaptation is the ICT age., (© 2011 The Author(s)/Acta Paediatrica © 2011 Foundation Acta Paediatrica.)

Life-history theory posits a fundamental trade-off between number and size of offspring that structures the variability in parental investment across and within species. We investigate this 'quantity-quality' trade-off across primates and present evidence that a similar trade-off is also found across natural-fertility human societies. Restating the classic Smith-Fretwell model in terms of allometric scaling of resource supply and offspring investment predicts an inverse scaling relation between birth rate and offspring size and a (-1/4) power scaling between birth rate and body size. We show that these theoretically predicted relationships, in particular the inverse scaling between number and size of offspring, tend to hold across increasingly finer scales of analyses (i.e. from mammals to primates to apes to humans). The advantage of this approach is that the quantity-quality trade-off in humans is placed into a general framework of parental investment that follows directly from first principles of energetic allocation.

Use of space by both humans and other mammals should reflect underlying physiological, ecological, and behavioral processes. In particular, the space used by an individual for its normal activities should reflect the interplay of three constraints: (i) metabolic resource demand, (ii) environmental resource supply, and (iii) social behaviors that determine the extent to which space is used exclusively or shared with other individuals. In wild mammals, there is an allometric scaling relation between the home range of an individual and its body size: Larger mammals require more space per individual, but this relation is additionally modified by productivity of the environment, trophic niche, sociality, and ability to defend a territory [Kelt DA, Van Vuren D (1999) Ecology 80: 337-340; Kelt DA, Van Vuren D (2001) Am Nat 157:637-645; Haskell JP, Ritchie ME, Olff H (2002) Nature 418:527-530; Damuth J (1987) Biol J Linn Soc 31:193-246; Damuth J (1981) Nature 290:699-700; and other previously published work]. In this paper we show how similar factors affect use of space by human hunter-gatherers, resulting in a nonlinear scaling relation between area used per individual and population size. The scaling exponent is less than one, so the area required by an average individual decreases with increasing population size, because social networks of material and information exchange introduce an economy of scale. [ABSTRACT FROM AUTHOR]