Promiscuous possums embrace the married life

Jenny Martin believes her findings on possums have wider implications. Picture / Euan Ritchie

CANBERRA - Something strange is happening in Australia's gum forests.

Where once they lived a life of unashamed promiscuity, a community of mountain brushtail possums has now settled to strictly monogamous married life.

It is as though marsupial Calvinists have come calling.

Yet down the road the old life continues, with males rampantly mating with as many females as possible and no thoughts - on either side - of ever settling down.

Melbourne University zoologist Dr Jenny Martin found the moral divide by chance during PhD studies in northeast Victoria, comparing one site that had been selectively logged with another nearby that had never felt axe or chainsaw.

Her findings have cast new light on the impact of man not only on possums, but possibly other forest dwellers as well. They may also force new thinking on the development of conservation areas.

Martin spent five years radio-tracking the possums, a naturally promiscuous species.

In the untouched forest, they remained that way.

There were more females than males, and males ranged over larger areas, their territories overlapping those of two or three females.

Most sired multiple babies every year.

But in an area where large trees had been removed the entire social structure had changed.

The possums paired with one mate for life.

There were equal numbers of males and females, living in pairs with closely overlapping home ranges, sharing den trees and remaining close to each other during their roaming at night.

The pairings ended only on the death of one of the partners.

In contrast to their promiscuous neighbours, males sired only one baby a year. The reason, Martin said, lay in resources - the number of large, old trees with hollows suitable for sleeping, and food supplies.

When hollows and food - especially silver wattle - was plentiful, as in the unlogged forest, females could live close together without spending too much time travelling, allowing males easy access.

In the logged forest, females occupied and travelled through much larger areas, encouraging monogamous pairings.

Martin said both populations could be affected by human activity.

In the unlogged area, land clearing by farmers at the fringes of the trees encouraged the growth of silver wattle, which in turn could encourage larger numbers of promiscuous possums.

"So in some ways these guys have benefited completely by the way humans have influenced things, because you get more of their food source," Martin said.

Other effects may not be so good.

"I think there definitely are implications for lots of different levels - probably for disease, because it all comes down to how many different males and females are going to be mating with each other and how much connection in that sense there is going to be."

Martin said there may also be implications for monogamous populations with less genetic diversity in breeding. And the findings may be important for studies of other forest mammals, which are usually based on single, specific sites.

"Humans love to pigeonhole species but the data that's coming out now - not just from me but from other people as well - is showing that we just can't do that," Martin said.

"It think it does raise a lot of questions and it means we all have to be careful about the conclusions that we make."