The Rationale of the Model
Data on monthly rainfall (R) for the period January 1871-December 2002 were obtained from the IRI/LDEO Climate Data Library (http://iridl.ldeo.columbia. edu), for the region of India designated “core monsoon,” located at 76°E, 22.50N. The typical pattern of variability is shown in Fig. 3.2, illustrating both annual variability and the 1900 ENSO event. We treated rainfall level as a proxy for food availability, an ecological cue especially relevant to the organism during development.
The primary outcome variable was birth weight (B), and our model assumed that this trait is subject to two opposing tensions. On the one hand, below a certain threshold, birth weight is inadequate for sustaining an infant. This favors increasing birth weight. On the other hand, above a certain upper threshold, fetal growth exceeds the mother’s capacity to extract energy from current or recent environments. In other words, we considered that the challenge for the offspring is to calibrate its fetal growth to ecological conditions in a way that avoids these extreme phenotypes.
Fig. 3.2 The pattern of rainfall in the Indian data set used for this model, showing raw monthly values and average annual values
Birth weight correlates strongly with adult size (Li et al. 2003; Sayer et al. 2004; Euser et al. 2005). While larger birth weight brings higher fitness returns in adult life (Wells 2007b), the trajectory of growth toward large adult size is inherently constrained by the availability of maternal resources during early life. This assumption is supported by evidence that human gestation length is constrained by energetic rather than biomechanical factors (Leutenegger 1982; Ellison 2008; Dunsworth et al. 2012).
We posed two key questions; how sensitive should birth weight be to environmental cues, and how can useful information be extracted by the offspring from the crude ecological record?