Quantifying the interactions between koalas in a high-density population during the breeding period

A species' social structure influences its patterns of gene flow and disease transmission. Knowledge of social structure therefore is critical for understanding conservation challenges and informing management strategies. We examined the social structure of 33 free-ranging koalas (Phascolarctos cinereus) in a high-density population (6.7 koalas per hectare) during the early and peak phase of the breeding period (August-December) at Cape Otway, Victoria. We used GPS and proximity loggers to quantify koala space use and interactions, and direct observations to determine behaviours associated with interactions. Our proximity loggers recorded 661 interactions (64% male-female, 28% male-male and 8% female-female). A peak in male-male interactions at the onset of the breeding period suggested that males may physically compete for mates at this time. Male-female interactions increased from the onset of the breeding period, and were mostly of short duration (median duration 5.5 min). From field observations of interactions, and the absence of pouch young in December, many of these interactions may have been unsuccessful copulation attempts. Space use and social network analyses revealed that most males had strong links (total interaction duration >30 min) with only one or two females, despite their spatial overlap and interactions with multiple females. Body mass did not influence a male's mating success in terms of number of female mates. Our results contrast with those of a lower-density population in central eastern Queensland, suggesting that some aspects of koala social structure may be plastic relative to population density.