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.