Housing Bubble in Marsupial Tree Hollows

Extended families often live together under the same roof. Indeed, in many human societies even distant relatives (aunts, uncles, cousins) share the same dwelling. Less common (except for college students) is house sharing among unrelated individuals. Under what circumstances would there be a fitness advantage to living with unrelated individuals? Does the availability of housing change these patterns?

Hollow-bearing trees are a potentially limited resource for nocturnal marsupials such as
Leadbeater’s possum (Gymnobelidus leadbeateri) and the mountain brushtail possum (Trichosurus cunninghami) in Australian forests. Both species rest in tree hollows during the day and emerge to forage at night. Because tree hollows take many years to form and deforestation continues to reduce forest area, tree hollows are becoming prime real estate for arboreal possums (Figure 1).

brushtail_possum
Figure 1. A closely related common brushtail possum (Trichosurus vulpecula) (From Flickr/wollombi)

In some regions of southeastern Australia arboreal marsupial populations have declined as forests have been cleared. In certain tall montane forests, however, some possum populations have remained stable despite the continued loss of tree hollows. Mountain brushtail possums are known to share tree hollow dens and scientists predicted that den sharing would increase with competition for den sites (
Banks et al. 2011). But, do they share hollows only with kin or with strangers?

Banks and colleagues (2011) studied Leadbeater’s and mountain brushtail possums in tall mountain ash forests in Victoria, where wildfires and logging have reduced the number of potential tree hollows in this region. They predicted that when den sites were in limited supply, possums would share dens with kin and thereby gain
inclusive fitness. In contrast, when den sites were plentiful, the benefits of kin selection are negligible and selection may favor den sharing with unrelated individuals. For example, sharing a den with an unrelated stranger may reduce inbreeding or reduce the risk of pathogen transfer (susceptibility to pathogens is higher among genetically similar individuals).

The researchers fitted proximity data logging collars to 18 possums and tracked them to their daytime dens over the course of 48 days (for a total of 195 den sharing events). In forest patches where dens were in short supply, possums used fewer unique dens and were less likely to share those dens. When they did share dens, they were more likely to share with siblings. In contrast, when tree hollows were relatively abundant, possums preferred to den with unrelated individuals (Figure 2).

tree_hollows
Figure 2. The probability of daytime den sharing by sibling pairs (black lines) and non-sibling pairs (grey lines) of mountain brushtail possums versus the availability of hollow trees. (From Banks et al. 2011)

Thus it appears that natural and anthropogenic changes in the abundance or distribution of a key resource can alter the animal’s social behavior. This study illustrates the importance of considering social behavior in landscape ecology models.



References

Banks, S., Lindenmayer, D., McBurney, L., Blair, D., Knight, E., & Blyton, M. (2011). Kin selection in den sharing develops under limited availability of tree hollows for a forest marsupial Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.2657