The social organisation of the grey-headed flying-fox, Pteropus poliocephalus

Justin Welbergen
King's College
2005

A thesis submitted to the University of Cambridge
in candidacy for the degree of Doctor of Philosophy

Thesis summary

In animal societies conflicts arise because different individuals face different trade-offs concerning reproduction and survival. Social organisation reflects such conflicts within and between species. To better understand social organisation in terms of individual selective benefits and in terms of underlying social and ecological pressures, I studied the roosting, emergence and foraging ecology of the grey-headed flying-fox, P. poliocephalus in northern NSW, Australia.

In my study area this species of bat shared its day-roosts with the closely related black flying-fox, P. alecto. Some roosts contained tens of thousands of individuals amongst the foliage and branches of canopy trees. grey-headed flying-foxes roosted lower than black flying-foxes. Before the mating season, young and females roosted lower and more towards the periphery than males. The density of bats increased towards the centre of the roost where male grey-headed flying-foxes competed for ‘mating territories’ that were visited by females during the mating seasons. The oldest, largest males that lived closest to centre of the roost had the highest reproductive access.

At dusk the grey-headed flying-fox left the colony to forage for pollen, nectar and fruit. The exact timing of the start of the emergence reflected the outcome of a trade-off between the risk of predation and the need for foraging. The emergence timing of males also depended on their social status: bachelor males left before harem holders and the latter left only when the last female in their harem had left.

At the feeding sites males also maintained territories; however, mating outside the day-roost was extremely rare. During the peak of the mating season dominant males forewent much of their foraging opportunities and remained in the ‘day’-roost for most of the night to defend their mating territories against takeovers from rival males. Since mating territories did not comprise an essential resource for females, the mating system of the grey-headed flying-fox may best be described as a “lek” in which the location the male territory relative to the centre of the roost informs females about the quality of the male territory holder.

Besides the above ecological and social factors, proximate environmental extreme events also had important ultimate consequences for social organisation in Pteropus spp.. I found that temperature extremes have caused the death of tens of thousands of flying-foxes in the past. During one afternoon, extremely high temperatures in the study area decimated populations of the recently invaded black flying-fox but left populations of the native grey-headed flying-fox relatively less affected. I further found that mortality was sex and age-specific, which affected the social organisation at the roosting sites through the negative impacts on the effective breeding population and recruitment. Since temperature extremes are expected to increase in frequency and intensity in the future, mass die-offs are expected to become more frequent and widespread and this will undoubtedly have a negative impact on the sustainability of Pteropus populations.