Monogamous possums grin and bear it
The sex life of this male bobuck, a type of Australian mountain marsupial, depends on where he lives. If resources allow, he'll be polygamous (Image: Martin Cohen)
Why is it that some types of male possums remain true to their sweethearts while others roam the countryside mating with as many females as they can?
Because they have no choice, it appears.
University of Melbourne PhD student Jenny Martin says that the availability of nesting sites, food and other resources dictate how many sexual partners they have.
Martin will present her findings on mountain brushtail possums, or bobucks, at the Ecological Society of Australia's conference in Brisbane at the end of the month.
Bobucks are arboreal marsupials that sleep in tree hollows during the day and feed and socialise at night.
In recent years, humans have carved up much of the bobuck habitat. So Martin studied two neighbouring populations in the Strathbogie Ranges in northeast Victoria to see the effects on the bobucks' social and mating system.
She put radio collars on bobucks from a population inhabiting a small forest patch, and on another population living as little as 2 kilometres away in the thin strips of habitat flanking the road.
She monitored which trees the bobucks slept in during the day, how far their home ranges extended, and whether their home range overlapped with those of other bobucks in the area.
"In that way I could work out who was mating with who, who was socialising with who," Martin says.
She also took DNA samples to determine which males were fathering the young in each population.
"What I found was the populations were doing really different things," she says.
Sex by the roadside ...
As is typical of mammals, she found the roadside population was polygamous; males had large home ranges that overlapped those of two or three females and sired multiple young a year.
But the forest population was socially monogamous. Pair members shared den-trees, had strongly overlapping home ranges and 'hung out' together. In fact, the couples stayed together until one died.
Martin determined that the females in the roadside bobuck population basically had all the resources they required - trees to sleep in and silver wattle to feed on - close at hand, so had small home ranges. This allowed the males to cover a large area and mate with various females in that range.
... and in the forest
But the forest females had to travel long distances between their food supply and tree nests because the forest had been selectively logged in the past.
This meant they had large home ranges, making it physically difficult for males to 'service' more than one female at a time.
"Basically, they cut their losses ... and stay with one female to ensure they get to sire at least one lot of young," Martin says.
