Long-term consequences of mother-offspring associations in eastern grey kangaroos
Close behavioural association between mothers and offspring should enhance survival and growth of the young. Eastern grey kangaroos Macropus giganteus are gregarious and live in fission-fusion societies where adult females do not form strong bonds with other females but associate closely with their juvenile offspring. We aimed to determine whether the strength of these mother-offspring associations correlated with offspring size, survival and reproduction. We observed 129 marked offspring, aged 10 to 21 months, and their mothers in a high-density population at Wilsons Promontory National Park, Australia. We used half-weight indices to quantify mother-offspring associations and determined the proportion of time offspring spent with their mother, but isolated from other kangaroos, while foraging. We found strong cohort effects on size, mass, body condition, survival and reproduction. Mother-offspring sociability indices were not correlated with offspring body condition as 2-year-olds or reproduction as 3-year-olds. Juveniles that spent proportionally more time with their mothers at 18–21 months, however, were 6% larger and 19% heavier as 2-year-olds than those that did not associate closely with their mothers. In addition, juveniles that were often found alone with their mothers were more likely to survive than those that were more often found in larger groups. Stronger mother-offspring associations before weaning likely reflected nutritional maternal care in sons but non-nutritional care in daughters and had a beneficial effect on juvenile growth and survival.
The possible fitness consequences of mother-offspring behavioural associations can affect reproductive decisions by mothers. These fitness consequences affect population dynamics and are relevant to conservation when mothers may be harvested or killed by vehicles, as is the case for many large herbivores. We show that variability in these associations in kangaroos affects correlates of offspring fitness. Juveniles that spent proportionally more time with their mothers between 18 and 21 months of age were larger and heavier as 2-year-olds. In addition, juveniles that spent proportionally more time with their mothers but isolated from other kangaroos experienced improved survival. This study is among very few to examine the relationship between mother-offspring sociability and reproductive success in a non-primate mammal. Improved juvenile growth in sons appears to result from nutritional maternal care through prolonged nursing. Isolation of the mother with her young-at-foot occurs in all large macropod marsupials, and adaptive benefits of this behaviour should therefore occur in other species of macropods.
KeywordsHalf-weight index Juvenile growth Maternal care Macropus giganteus Survival
This research was funded by the Natural Sciences and Research Council of Canada and the Holsworth Wildlife Research Endowment. U. Gélin helped monitor the population in 2009 and C. Le Gall-Payne and R. Glass did so in 2012. S. P. Blomberg provided statistical advice. D. T. Blumstein, E. Z. Cameron, D. A. Fisher, M. J. Noad and two anonymous reviewers commented on previous versions of this manuscript.
Compliance with ethical standards
This study was funded by the Natural Sciences and Engineering Council of Canada and the Holsworth Wildlife Research Endowment. Captures were undertaken with ethics approval from the University of Melbourne (no. 0810628.1 and no. 0911512.1) and research permits from the Victorian Department of Sustainability and Environment (no. 1004582 and no. 1005558). Following captures, 9 of 241 pouch young were abandoned (4%); however, non-captured females also abandoned young occasionally. There was 1 death in 132 captures of sub-adults (1%; an apparent fox predation during recovery) and 1 death in 369 captures of adult females (0.3%). There were no observed capture-related injuries in the 5 years of study. Observations were conducted with animal ethics approval from the University of Queensland (no. SIB/206/09/(NF)). All applicable international, national and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. This article does not contain any studies with human participants by any of the authors.
Conflict of interest
The authors declare that they have no conflict of interest.
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