Red kangaroos (Macropus rufus) are large (> 20 kg) herbivorous marsupials common to the
arid and semi-arid regions of inland Australia. The population dynamics of M. rufus is tightly
linked to environmental factors, which operate partly through the survival of juveniles. A
crucial period is the young-at-foot (YAF) stage when juveniles have permanently left the
mother?s pouch. YAF and weaned kangaroos have the highest drought-related mortalities of
any cohort and show notable differences from adults in their basic physiology. YAF and
weaned M. rufus, for example, had a resting metabolic rate (kJ kg-1
d-1) twice that of
mature females and 1.5 times that expected for an adult marsupial of equivalent body mass
(i.e., kJ kg-0.75
d-1). This greater energy turnover was largely explained by their metabolic
demands for growth; juveniles required 70 - 95% of the digestible energy intake (kJ d-1) of
mature, non-lactating females. Meeting these costs may not be a problem for juveniles when
high-quality, low-fibre forage is available, but they were constrained when only hard-to-digest,
high-fibre forage was available. YAF and weaned kangaroos, for example, were unable to
sustain growth on forages of more than 40 ? 50% fibre, fibre levels characteristic of forages in
arid regions during drought. Yet mature, non-lactating females were capable of maintaining
body mass on similar forage. Additionally, juvenile M. rufus required relatively more water
than adults for thermoregulation (by up to 2.5-fold), especially under hot conditions, and may
need to drink more frequently than adults. Thus, juveniles appear constrained to remain close to
water points, increasing their risk of predation and limiting their ability to find the high-quality
forage needed for their growth and survival.