The ecophysiology of survival in juvenile red kangaroos Macropus rufus: greater demands and higher costs.

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.