Silver spoons, reproduction, and growth catch-up in eastern Grey kangaroos

Early growth and environmental conditions can shape life-history trajectories. Long-lived iteroparous species with indeterminate growth face growth-reproduction trade-offs for most of their lives. Poor early conditions can delay primiparity and restrict growth, potentially compensated for by faster growth later in life, at the cost of reduced reproduction. We explored the variation in age at primiparity and early reproduction in eastern Grey kangaroos, based upon 13 yr of monitoring about 100 known-age females. We then examined associations between early reproduction, later reproduction, and lifetime growth. We used a modified von Bertalanffy growth function to model the indeterminate growth of females and to test the effects of early reproduction on lifetime growth. Favorable environmental conditions, large size, and condition as a subadult led to earlier reproduction and higher reproductive success at ages 3 to 5. As females aged, size and condition had diminishing effects on reproductive success. Females with greater early-adulthood reproduction had slightly higher reproduction later in life. We did not detect a growth cost of early reproduction. Large females in good condition favored early reproduction over growth, while those with poor early growth exhibited growth catch-up at the cost of reproduction both early and later in life. As reported for other long-lived iteroparous species with indeterminate growth, female kangaroos prioritize growth over reproduction for much of their lifespan. Eastern Gray kangaroos show heterogeneity in early growth and reproductive strategies. Early primiparity and reproduction are constrained by body condition, size, and environmental conditions when females are pre-reproductive subadults.

Reproduction is demanding, but for young female kangaroos, being larger helps. Kangaroos grow continuously for most of their lives, but energy constraints often force a choice between growth and reproduction. Taller, heavier young females that have experienced favorable environmental conditions tend to reproduce earlier and more frequently in early and late stages of life. In contrast, smaller females prioritize growth to catch up in size. Overall, early-life conditions influence reproductive success among female kangaroos.