When Alex Hams, a land conservation manager at Bush Heritage Australia, was giving a tour to visiting scientists and volunteers in mid-June, he got more than he bargained for. He opened the lid of a nest box for western pygmy possums at the Monjebup Nature Reserve, which his organization manages in southwestern Australia. There, among the leaves, he found not just a family of pygmy possums, but also a small, orange-eyed lizard known as a western spiny-tailed gecko.
“They were climbing all over each other and neither seemed to mind,” he said. “They were more concerned about the big human heads that were peering in through the top.”
He’d never seen anything like it, and neither had anyone he asked.
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 were not just passing through. They were genuine roommates, sharing a crowded space that was no more than eight inches deep and the same length across.
Inside the boxes, which replicate the kinds of natural hollows so many native Australian mammals and birds depend on for shelter, it’s a comfortable setup. “The pygmy possums use the eucalyptus leaves from nearby trees to establish the nest,” Mr. Hams said. “We provide the structure, they provide the interior design.”
Scientists tried to explain what they were seeing.
Conrad Hoskin, a gecko expert from 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 nestmates.
“The gecko will get some benefit from being in the warmth of those little mammals,” he said. “I suspect the presence of the gecko is neutral to the possum — a soft and harmless creature amongst them.”
Euan Ritchie, a professor in 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 sort of habitat that reptiles like.”
But with or without geckos and spiders, these nest boxes help protect pygmy possums.
“Tree hollows have been massively depleted around Australia because of logging, and pygmy possum habitat has become extremely fragmented,” Dr. Ritchie said. “Even if we stop cutting down trees now, it’s 100 years, and in some cases 150 years, before these hollows form, depending on the tree and the habitat.”
Bush Heritage Australia has been revegetating Monjebup, which had been farmland, for nearly a decade — nowhere near enough time for tree hollows to form. In the absence of natural shelter, pygmy possums now inhabit around two-thirds of the nest boxes at 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 — it’s only nine or 10 years old — there are no habitats like natural hollows or crevices, so these nest boxes are prime habitat,” said Angela Sanders, wildlife ecologist with Bush Heritage Australia. “Animals that wouldn’t normally cohabit are actually forced together because there is so little habitat.”
It is, of course, possible that cohabitations such as this one may be less unusual than unobserved. “With the advent of modern technology, we’re picking up more on these really interesting natural history observations that may have been happening for a long time,” Dr. Ritchie said.