This study examined the ontogeny of body mass (i.e. "growth") of Otolemur garnettii and Galago senegalensis. Growth is a proximate causal mechanism for adult size variation and growth patterns themselves can be the target of selection with adult size being the end result. Therefore, growth patterns of species can be the result of adaptation to species-specific social system, ecology, and life-history. The goals of this study were to: (1) Assess whether interspecific body mass variation was due to differences in growth rate, growth duration, a combination of the two, or neither; (2) test the hypothesis that sexual size dimorphism is attained by differences in relative growth rate as predicted by sexual selection theory; and (3) test the hypothesis that frugivorous O. garnettii grow at a relatively lower rate than gummivorous Go. senegalensis as predicted by an ecological risk aversion hypothesis. Growth rates and durations of Otolemur garnettii and Galago senegalensis males and females were compared both interspecifically and intraspecifically. The hypotheses regarding the ontogeny of sexual size dimorphism and the risk aversion hypothesis were not supported. O. garnettii males and females grow at an absolutely higher rate and for a longer duration compared to Go. senegalensis males and females respectively. O. garnettii females grow at a relatively higher rate compared to Go. senegalensis females as well. This may relate to weaning habits. O. garnettii infants are weaned during the dry season when feeding competition would be presumably high making large mass at weaning advantageous. While the growth of females might be strongly influenced by natural selection and competition for resources following weaning, the growth of males may be more strongly influenced by sexual selection relating to contest competition for females. Sexual size dimorphism results from differences in growth duration in O. garnettii and from differences in both growth duration and growth rate in Go. senegalensis. The results of this study highlight the need for more data on the growth patterns, mating and social systems, feeding competition, and life history schedules for these and other galagids. Study of how and why growth patterns have diverged through evolution is important in discerning the evolutionary history of each species.