Wild kangaroos become more social when caring for young and may maintain long-term affiliations with popular individuals

Kangaroos are an iconic group of Australian fauna. Despite considerable research on kangaroo behaviour, key gaps remain in our understanding of their social organization in the wild. In particular, it remains largely unknown whether kangaroos form long-term social bonds and what factors might prompt individuals to associate or dissociate from one another. Over 6 years, we monitored the social affiliations of individually identified eastern grey kangaroos, Macropus giganteus, in a large wild population. We investigated the short-term and long-term relationships of kangaroos and the extent those relationships varied with age, sex and reproductive state. We found evidence that long-term relationships among eastern grey kangaroos are possible, especially between adult females. Those individuals that were more sociable within years were also more likely to establish affiliations across years. Contrary to previous studies, we observed females actively associating with other mothers in the years in which they had young. These data suggest that the fission-fusion dynamics of eastern grey kangaroo social behaviour allow females to modulate their social position with conspecifics according to their current reproductive state. We highlight the adaptive implications of the formation of long-term bonds and the changes in social behaviour observed in females.

Methods
The study was based on identifying individual eastern grey kangaroos from high-resolution digital photographs. These were taken during a systematic annual survey of a wild population on private property near the locality of Wollar in the New South Wales Central Tablelands. This population ranged in size from 34 to 71 individuals. The photographic survey was conducted annually over six years (2015–2020) by the second author and at approximately the same time each year (see below for the exception). The survey was done in the Austral spring, except for 2016 when it took place in winter. Photos were taken using a 200mm lens on a Canon EOS 7D digital SLR from portable hides placed at least 12 hours prior to the first day of observation. The survey was conducted over two to three hours at dawn and again at dusk for three to four consecutive days.

This population of eastern grey kangaroos was particularly suited for this study because individuals rested during the day within a eucalypt woodland surrounding an open grassy valley. At dusk, kangaroos progressively moved out into the grassland of the valley from the woodland to graze and ultimately dispersed out into the surrounding areas around the valley after dark. At dawn, kangaroos would return to the valley to graze until mid-morning and then return to shelter in the eucalypt woodland. This meant hides could be strategically placed around the valley in order to survey virtually all individuals residing in the valley. Hides would be entered at least an hour before dawn and prior to kangaroos returning to the valley, and again at mid-afternoon before kangaroos had left diurnal rest spots inside the eucalypt woodland.

Multiple photos of individuals and their associates were taken to ensure kangaroos could be adequately identified in most instances. Each annual survey typically resulted in a photo library of 500 to 700 images and was considered to be an exhaustive survey of all individuals residing in the valley. In general, a photographic survey is an optimal choice for kangaroos because of its minimal invasiveness (e.g., Austin and Ramp, 2019). Similar survey designs have also been used to take representative ‘snapshots’ of population dynamics in eastern grey kangaroos (Toni et al., 2021) and other animals (Piefke et al., 2021).