Having a social life comes at a cost for grey kangaroos, with 'mingling' mothers reducing the chances of survival for their offspring.
Lead author and recent University of Queensland School of Biological Sciences graduate Dr Wendy King said young kangaroos that spent more time with their mothers grew faster and gained more weight than other joeys of the same age, while those that spent more time alone with their mothers had higher survival.
"For kangaroo sons, spending time with their mother correlates closely with receiving milk, and so sons are likely to benefit nutritionally from this extra maternal care," Dr King said.
"Some daughters, however, associate closely with their mothers even if the mother no longer provides milk, and must benefit in other ways."
Dr King said grey kangaroo mothers did not defend their young from aggressive adult females but mothers could reduce harassment of their offspring just through their presence.
"Juveniles feeding alone with their mothers do not need to compete with others for food, and have more time to feed and grow more rapidly," Dr King said.
"However, mother kangaroos rarely interact with their young through activities such as playing and grooming, so it is unlikely juveniles benefit from behavioural development or parasite removal."
Co-author Associate Professor Anne Goldizen of UQ said the findings suggested there was a cost to being sociable for female kangaroos.
"That cost may relate to young more easily losing their mothers when in a large group," she said.
"This cost of being social raises the question of why kangaroos usually forage in groups."
The study of 129 kangaroo offspring aged 10 to 21 months, and their mothers, took place over six years at Wilsons Promontory National Park, Victoria, and involved collaboration between UQ, the University of Melbourne and Universite de Sherbrooke (Canada).
While previous studies have investigated the links between mother-offspring relationships and offspring growth and survival in mammals, none have done so in marsupials.
Dr King said the mother-offspring bond in macropods such as kangaroos appeared to be quite special.
The paper appears today in the journal Behavioral Ecology and Sociobiology (doi: 10.1007/s00265-017-2297-1).