Why do juvenile kangaroos die during drought? Answering this question is important because juvenile mortality typically drives whole population dynamics of large herbivores. To clarify reasons for the vulnerability of juveniles we investigated the ecophysiology of young red kangaroos (Macropus rufus), Australia’s largest marsupial herbivore. Compared with adults, juveniles had higher energy and water needs; these were related to requirements for growth and thermoregulation. Most importantly, juveniles could not maintain growth on poorer quality (high-fibre) forage due to an inability to expand gut-fill. Adults could adjust gut-fill to compensate, thereby increasing their survivability in dry conditions when easily digestible, low-fibre forage is scarce. We have provided a specific mechanism linking juvenile kangaroo mortalities with food quality, which is in turn driven by rainfall and hence climate change impacts.