Causes and Consequences of Life-History Variation Among Strepsirhine Primates

Abstract

The aims of this study were to examine potential causes of variation in fundamental life-history traits and to illuminate their consequences for social behavior in the most primitive living primates. Specifically, I reexamined the claim that life histories of strepsirhine primates (lemurs and lorises) are metabolically constrained and tested the hypothesis that female social dominance, a behavioral idiosyncrasy of Malagasy strepsirhines, is the result of unusually high energetic maternal investment in reproduction. I collected data on body mass, brain mass, metabolic rate, and fetal and postnatal litter growth rates for 21 lemur and 13 loris species to examine the relationship between maternal investment and the other variables after controlling for allometric and phylogenetic effects. I found that evolutionary changes in fetal growth rates, relative litter mass, and postnatal growth rates were associated with evolutionary changes in maternal mass but that neither brain size nor metabolic rate were independently correlated with variables reflecting maternal investment. Evolutionary changes in all three variables reflecting maternal investment were positively correlated with one another, however, indicating that they are coadapted, only broadly constrained by body size, and unconstrained by brain size and metabolic rate. Lemurs and lorises were found to have similar postnatal litter growth rates, which reflect the bulk of maternal reproductive investment, indicating that the energetic costs of reproduction did not figure prominently in the evolution of female dominance. The suggested unusual energetic stress of reproducing lemur females is further refuted by qualitative compari- son with higher primates and other mammals. Thus, strepsirhine life histories are not narrowly constrained by nonadaptive forces and have no direct consequences for social relations between adult males and females. Principal life-history traits determine the size of an organism at birth; how fast and how long it will grow; at what age and size it will mature; the number, size, and sex of its offspring; and its reproductive and total life span (Stearns 1992). These traits are connected by numerous trade-offs, involving differential alloca- tion of energy to growth, maintenance, and reproduction. Because interactions among life-history traits result in phenotypic adaptations that determine individ- ual fitness through their direct effect on survival and reproduction, life histories are the key to understanding the action of natural selection (Gadgil and Bossert 1970; Stearns 1976, 1992). Two types of explanations have been proposed to account for the diversity of mammalian life histories. One is based on the observation that, across species, life histories tend to be highly correlated with body size, brain size, and metabolic