Maternal Characteristics and Offspring Growth in Chacma Baboons: A Life History Perspective

Abstract

The pace of growth and development of offspring is a critical component of female reproductive success. Optimality models assume selection favoring maternal characteristics associated with a growth trajectory of maximum rate and minimum cost. Moreover, these attributes vary with social and ecological context. This chapter examines the interaction between maternal and offspring characteristics that affect immature growth in chacma baboons. The analyses evaluate offspring growth, measuring the probability of a weight measurement below the weight-for-age curve for the population. As maternal age increases beyond 10–14 years, offspring growth rate significantly decreases, controlling for maternal rank. Male offspring with younger mothers grow more slowly than others, but the trend is not significant. Maternal rank significantly affects female growth independent of maternal age variation. The female offspring of lower-ranking mothers grow more slowly than their higher-ranking peers. Maternal rank appears to have a weaker and opposing effect on male offspring growth. Male offspring of the lowest-ranking mothers seem to grow slightly faster than their higher-ranking counterparts. However, this effect disappears when maternal age is included in the model, suggesting minimal impacts of maternal rank on male growth. The effects of maternal rank on female growth are consistent and strong, controlling for variation in maternal age and variation in rainfall. These analyses indicate that the benefits of rank to reproductive success in terms of female offspring growth are most pronounced under conditions of environmental stress. These findings contribute to our understanding of the complex dynamics relating maternal characteristics of rank and age to the growth trajectories of male and female offspring. Identifying mediating variables that affect trajectories of growth and development allows us to synthesize the allocation rules that affect maternal decision making and the pattern of investment. The addition of environmental variation permits an evaluation of the varying intensity with which maternal rank affects reproductive success.