Introduction

If there are indeed many pathways to the adult endpoint during growth, what becomes of interest is the nature of those different pathways and, more significantly, the consequences of differences in pathways for early survival, for the timing of the onset of reproductive activity, and for subsequent lifespan. This chapter explores some of the strategies for growth observed in primates, and outlines potential consequences of these strategies. I do this in the context of an abundance of detailed literature on growth rates, growth constraints and models of rates of growth for humans (see for example Bogin, 1999) and non-human primates (e.g. Leigh, 1996). Models of growth can provide explanations for primate life history variation (Leigh, 1995; Dirks & Bowman, 2007) and, in addition, potentially elucidate the variation that exists at the level of the individual and population between rapid and late maturation (e.g. Wilson et al., 1983; Bercovitch & Berard, 1993; Setchell et al., 2001; Altmann & Alberts, 2005).

Physical maturation is obviously a consequence of growth, but the timing of physiological or hormonal reproductive onset, rather than simply attained size, determines much of the subsequent reproductive success of the individual. Early reproduction, combined with high survival, provides significant advantages in terms of lifetime reproductive success.

INTRODUCTION

Studies of sexual selection in primates or other animals tend to focus on outcomes – sexual dimorphism, differential mating and reproductive success for adult males and females. However, adult sex differences represent the end-points of complex and interrelated developmental processes, and arise from differences in behaviour and physiology between males and females. In most vertebrates, including primates, the sexes are nearly identical in size and shape during early development, and adult differences are thus the product of divergent growth strategies (Badyaev, 2002). Sex differences in growth and development arise as a result of the different roles played by the two sexes in reproduction and the corresponding determinants of reproductive success for males and females, which are intricately linked to social organisation and mating system (Kappeler & van Schaik, 2002). Evolution shapes processes throughout the lifecycle, and the mechanisms for partitioning resources among growth, reproduction and survival are, to a large part, established during development, while consequences may not be observed until the end of the lifespan. A developmental perspective is therefore fundamental to studies of the action of sexual selection (see also Pereira & Leigh, 2003).

For mammals in general, and primates in particular, past work on sexual selection and development has concentrated on the influence of growth on sexual dimorphism (e.g. ungulates: Jarman, 1983; Georgiadis, 1985; Clutton-Brock et al., 1992; seals: Trillmich, 1996; primates: Leigh, 1995; Pereira & Leigh, 2003) or growth and life-history traits such as rates of reproduction (e.g. Gordon, 1989; Pontier et al. , 1989; Lee & 2003).

Introduction

Why are the problems of growth interesting? Somatic growth relates to individual survival; growth failure or faltering is associated with an increased likelihood of mortality (rhesus: Small and Smith, 1986; humans: Skuse et al., 1995), and weight for age can be used as a proxy for morbidity or mortality risks (humans: Tomkins, 1994). Maternal mass and body composition can influence reproductive potential and efficiency in terms of fetal growth and lactation capacity (Martin, 1984; Lee and Bowman, 1995). Growth is also a variable with a time component and as such it links with a suite of life history traits within a species, and has been used to describe variation in the timing of events within a life history strategy (Harvey, Clutton-Brock and Martin, 1987; Ross, 1988; Lee, 1996). Growth thus raises two distinct questions in primate comparative biology. The first concerns the underlying selected mechanisms for attaining metabolic and reproductive efficiencies. The second concerns whether and how such growth strategies are achieved at the population or individual level.

Growth can be partitioned into separate periods, each with possibly independent rates and trajectories, as well as unique problems to be solved. These distinct periods may have repercussions on subsequent growth stages, as well as an influence on behaviour and reproductive ability (Bercovitch, 1987; Altmann, 1991). The three major periods are fetal growth, postnatal growth during the period of lactation, and finally growth between weaning and the attainment of a terminal mass or stature, often associated with the onset of reproduction among females (see Leigh, 1994a; Leigh and Shea, 1996).