Ideal for kangaroos: Out of the pouch, but still living at home

Eastern grey kangaroos (Macropus giganteus) are gregarious animals. They live in so-called fission-fusion societies and usually forage in groups of three to four animals, possibly because this provides protection against predators. Rather than forming strong bonds with other female adults, kangaroo mothers prefer keeping close to their offspring. Females can produce a single offspring annually. After ten months, the young kangaroos permanently leave the safety of their mothers' pouches, but are only weaned by the age of 18 to 23 months. Some of these young kangaroos, referred to as young-at-foot, stay close to their mothers to suckle, even though she might already be caring for another kangaroo baby in her pouch.

To study aspects of parental care, King's research team observed 129 marked offspring, aged 10 to 21 months, and their mothers over six years in Wilsons Promontory National Park in Australia. The animals were tagged, and measured and weighed once a year.

Young kangaroos between the ages of 18 and 21 months who spent more time with their mothers were found to have six per cent longer limbs and weigh 19 per cent more by the age of two years than others. Those who spent more time alone with their mother between the ages of 10 to 13 months were most likely to survive until they were weaned. Prolonged nursing up to the age of 23 months provided more nutritional value to sons in terms of growth and development. The social bond between mothers and their daughters remained closer for longer than was the case with sons. Daughters kept close to their mothers even after being weaned.

King's team further established that the proportion of time that a young kangaroo spent alone with its mother closely correlated with suckling: both sons and daughters between 18 and 21 months of age whose mother had a new pouch young (and therefore must have been weaned) were less likely to be found alone with their mother than those whose mother did not have a new pouch young.

King notes that these sociability indices indicate different facets of the mother-offspring relationship in kangaroos.

"Because suckling juveniles follow their mothers, being in the same foraging group as the mother reflects their decision to stay with her as she moves among groups in this fission-fusion society," explains King. "In contrast, being alone with the mother is a reflection of the mother's decision to leave or join groups, with the offspring following."

According to King, the possible fitness consequences of mother-offspring behavioural associations could affect population dynamics. "These aspects are relevant to conservation when mothers may be harvested or killed by vehicles, as is the case for many large herbivores," she notes.