When Alex Hams, land conservation manager at Bush Heritage Australia, gave a tour to visiting scientists and volunteers in mid-June, he got more than he bargained for. He opened the lid of a nesting box for pygmy possums in the Monjebup Nature Reserve, which his organization manages in southwestern Australia. There, among the leaves, he found not only a family of pygmy possums, but also a small, orange-eyed lizard known as a western spiny-tailed gecko.
“They climbed over each other and neither of them seemed to mind,” he said. “They were more concerned about the large human heads peering in through the top.”
He had never seen anything like it, nor had he asked anyone.
Mr Hams returned twice in the following two weeks and little had changed. The pygmy possum mother, her litter of babies, and the two to three-inch gecko didn’t just pass by. They were real roommates, sharing a crowded room no more than eight inches deep and the same length.
“They’re really small boxes,” says Mr. Hams, who shared the story of shared animal habitation on Facebook with Bush Heritage. “Dwarf possums are tiny creatures — you could keep an entire family on your hand.”
In the boxes, which mimic the kind of natural cavities on which so many native Australian mammals and birds depend for shelter, it’s a comfortable set up. “The pygmy possums use the eucalyptus leaves from nearby trees to make the nest,” Mr Hams said. “We provide the structure, they provide the interior.”
Scientists tried to explain what they saw.
Conrad Hoskin, a gecko expert at James Cook University in Australia, noted that the two animals would have no interest in eating each other: possums eat nectar and insects, and geckos eat insects and spiders. But the gecko may be getting something from its nest mates.
“The gecko will benefit from being in the warmth of those small mammals,” he said. “I suspect the gecko’s presence is neutral to the possum — a gentle and harmless creature among them.”
Euan Ritchie, a professor of wildlife ecology and conservation at Deakin University in Melbourne, said that “the pygmy possum has made a nest out of all these leaves and twigs, which is exactly the kind of habitat that reptiles love.”
But with or without geckos and spiders, these nest boxes help protect the pygmy possums.
“Tree hollows in Australia have been greatly depleted by logging, and the pygmy possum habitat has become extremely fragmented,” said Dr. Ritchie. “Even if we stop cutting trees now, it will take 100 years, and in some cases 150 years, for these cavities to form, depending on the tree and habitat.”
Bush Heritage Australia is regrowing Monjebup, which had been farmland for nearly a decade, not nearly enough time to form tree hollows. Lacking natural shelter, pygmy possums now inhabit about two-thirds of the nesting boxes in the reserve. This may even explain why the gecko decided to live with the pygmy possum family.
“Because this vegetation is so young and undeveloped – she’s only nine or ten years old – there are no habitats like natural cavities or crevices, so these nest boxes are the main habitat,” said Angela Sanders, wildlife ecologist at Bush Heritage Australia. “Animals that wouldn’t normally live together are actually forced together because there is so little habitat.”
It’s possible, of course, that cohabitants like this are less uncommon than undetected. “With the advent of modern technology, we’re picking up more of these really interesting natural history observations that may have been going on for a very long time,” said Dr. Ritchie.