The Role of Bioacoustic Signals in Koala Sexual Selection: Insights from Seasonal Patterns of Associations Revealed with GPS-Proximity Units

Abstract

Despite being a charismatic and well-known species, the social system of the koala (Phascolarctos cinereus—the only extant member of the family Phascolarctidae) is poorly known and much of the koala’s sociality and mating behaviors remain un-quantified. We evaluated these using proximity logging-GPS enabled tracking collars on wild koalas and discuss their implications for the mating system of this species. The frequency and duration of male-female encounters increased during the breeding season, with male-male encounters quite uncommon, suggesting little direct mating competition. By comparison, female-female interactions were very common across both seasons. Body mass of males was not correlated with their interactions with females during the breeding season, although male size is associated with a variety of acoustic parameters indicating individuality. We hypothesise that vocal advertising reduces the likelihood of male-male encounters in the breeding season while increasing the rate of male-female encounters. We suggest that male mating-season bellows function to reduce physical confrontations with other males allowing them to space themselves apart, while, at the same time, attracting females. We conclude that indirect male-male competition, female mate choice, and possibly female competition, mediate sexual selection in koalas.

Subharmonics increase the auditory impact of female koala rejection calls

“…Hence, the intensity of sexual selection on male koala bellowing is likely to be strong . Interestingly, a recent study on free-ranging koalas revealed that close-range male-male interactions are not higher during the breeding season when compared to other times of the year (Ellis et al, 2015), which also suggests that male intra-sexual competition is largely mediated via signalling behaviour (as in many other animal species: Bradbury & Vehrencamp, 2011).…”

“…Hence, the intensity of sexual selection on male koala bellowing is likely to be strong (Ellis & Bercovitch, ). Interestingly, a recent study on free‐ranging koalas revealed that close‐range male–male interactions are not higher during the breeding season when compared to other times of the year (Ellis et al., ), which also suggests that male intra‐sexual competition is largely mediated via signalling behaviour (as in many other animal species: Bradbury & Vehrencamp, ). We therefore suggest that female rejection calls function to attract the attention of nearby males, in order to increase male intra‐sexual signalling behaviour, and maximise the chances of the female mating with a high‐quality male that can outcompete other rivals.…”

Passive acoustics and sound recognition provide new insights on status and resilience of an iconic endangered marsupial (koala Phascolarctos cinereus) to timber harvesting

“…In addition to occupancy, bellow rate is correlated with koala density [ 39 ]. Bellowing also serves both to avoid male-male interactions by functioning as a signal of body size [ 48 , 49 ] and to attract females [ 29 ], and thus an increase in bellow rate likely reflects an increase in breeding activity in a population [ 29 , 50 ].…”

When an iconic native animal is overabundant: The koala in southern Australia

“…Drought and heat waves also are limiting factors (Adams‐Hosking, Grantham, Rhodes, McAlpine, & Moss, ; Briscoe, Kearney, Taylor, & Wintle, ). Koalas are relatively solitary animals that primarily interact during the spring/summer breeding season (Ellis et al, ; Watchorn & Whisson, ), and generally occur at densities of less than one koala per hectare (Melzer, Carrick, Menkhorst, Lunney, & St. John, ).…”