Who Reaps the Rewards and Who Pays the Costs of Precocious Investment in Reproduction? Adolescent Reproduction in the Bolivian Amazon

Lisa S. McAllister, University of California, Santa Barbara
Geni Garcia, University of California, Santa Barbara
Aaron D. Blackwell, University of California, Santa Barbara
Michael D. Gurven, University of California, Santa Barbara

Age at first reproduction (AFR) negatively correlates with adult body size. It is generally assumed cessation of growth determines AFR. We investigate the AFR-growth relationship among the Tsimane, Bolivian forager-farmers (women’s mean AFR=18.25±2.60, n=1335, 13% reproduce before 16). We find: (1) faster growth before age 13 predicts earlier AFR (b*= -0.25, p=0.03) and menarche (b*= -0.24, p<0.01), and greater adult height (b*= 0.30, p<0.01); (2) early AFR predicts shorter adult height despite post-partum growth (b*= 0.27, p=0.05); (3) women with AFR =15 have more children, despite lower infant survivorship (adjusted total fertility rate by AFR: =15=9.67, 16-19=8.86, =20=7.04); and their first-born sons, but not daughters, are small-for-age (F(2,559)=5.23, p<0.005). Our results suggest rapid early growth enables early AFR and that early AFR limits growth rather than an earlier growth asymptote. Furthermore, in a preindustrial population, and likely during our evolutionary past, the benefits of early reproduction may outweigh the costs.