Preparation and activation: determinants of age at reproductive maturity in male baboons

Abstract

Age at maturity is a particularly important life history trait, but maturational data are rare for males in natural populations of mammals. Here we provide information on three maturational milestones and their social and demographic correlates among 43 wild male baboons, Papio cynocephalus, in a natural population in Amboseli National Park, Kenya. We examined (1) age at testicular enlargement, which signals puberty and the onset of subadulthood, (2) age at attainment of adult dominance rank, which we consider to be the beginning of adulthood, and (3) age at first sexual consortship, which is the best measure available for age at first reproduction in male baboons. Testicular enlargement (median age = 5.69 years) occurred earlier among sons of high ranking mothers, and was not influenced by rainfall or seasonality. Attainment of adult dominance rank (median age = 7.41 years) was also accelerated among sons of highranking mothers, and among males whose mothers had died while the males were juveniles. First sexual consortship (median age = 7.92 years) was not influenced directly by maternal characteristics, but attainment of adult dominance rank always preceded first consortship. The lag time between attainment of adult rank and first consortship (median = 2.5 months; range = 5-526 days), was predicted by the number of sexually cycling females in the group when the male attained rank, and by how high ranking the male became in his first months as an adult. We suggest that the age at which a male baboon is ready to begin reproducing is influenced by a relatively stable maternal characteristic that exerts its influence early in development, but the timing with which this potential is realized depends on activation by more proximate, often stochastic triggers such as female availability. This two-level organization of influences is likely to contribute to the variance both in age at first reproduction and in lifetime fitness. Differences in the relative magnitude of the two levels will lead to both intra-and interspecific variability in the opportunity for maternal selection and sexual selection.

Social correlates of testosterone and ornamentation in male mandrills

“…Adolescent male development was also related to maternal variables, with sons of higherranking mothers maturing faster. Similar findings have been reported for rhesus macaques (Bercovitch, 1993) and baboons (Alberts and Altmann, 1995b). Males born to heavier mothers also matured faster.…”

“…Similarly, in wild Amboseli baboons, high maternal rank can lead to a developmental advantage, but demography determines the age at which males attain adult rank and first consortship (Alberts and Altmann, 1995b). Male rank in both natal (aged 5-9 yr) and new troops (age 6-15 yr) is also independent of maternal rank in baboons at Gombe National Park, Tanzania (Packer et al, 2000).…”

Paternal effects on offspring fitness in a multimale primate society

“…The first two were age at menarche in females and age at testicular enlargement in males; these milestones represent the attainment of sexual maturity (puberty) for each sex (31,32,34). The third was age at first live birth in females, another important life history milestone.…”

“…Third, previous work on this population has identified several sources of variance in age at maturity in both sexes, but has left considerable variance unexplained (31)(32)(33).…”

Evolution of Paternal Investment

“…Direct investment involves providing offspring with nutrients during gestation or egg production and postnatally, as well as protecting them from predators (Clutton-Brock, 1991). For highly social species, direct investment can also involve assistance in establishing position in the social hierarchy and navigating social discourse (Alberts & Altmann, 1995;Buchan, Alberts, Silk, & Altmann, 2003). Separating the effects of direct from indirect parental investment is complicated by potential interactions between genetic and environmental influences on offspring.…”