Each year, bogong moths make an extraordinary journey over 1,000 kilometres from their breeding grounds in south-eastern Australia, to spend the spring and summer in high-altitude caves in the Snowy Mountains.
An international team of scientists is going to extraordinary lengths to understand how they find their way there.
The moths are the only known insect in the world to travel such a long distance, at night, from a broad geographical region to a very specific destination, in their first and only return journey.
A project started almost 10 years ago by Australian professor Eric Warrant has been uncovering the mystery behind the moths' exceptional navigation skills, and devising an innovative set of tools to do it.
In a purpose-built lab in Adaminaby in the NSW Snowy Mountains, Professor Warrant shows how he can insert a tiny electrode, measuring one thousandth of a millimetre at the tip, into an individual nerve cell in the moth's brain.
His team can then record the animal's response to simulations of the milky way and the earth's magnetic field.
"We've had to create our own equipment for projecting the stars, for controlling the magnetic field, and for tethering the moths in our behavioural set ups," Professor Warrant said.
"All of this is all home-invented, and that's half the fun."
The project has involved researchers from Australia, Germany, Finland, Sweden, Canada and China, and is based in Lund University in Sweden, where Warrant is Professor of Zoology.
Over the past 10 years, the researchers have travelled to the Snowy Mountains region each spring and autumn, setting light traps to capture the moths as they make their migration to and from the caves where they aestivate — the warm weather equivalent of hibernation.
Back in the lab, they have demonstrated the moths are able to use the earth's magnetic field, the stars, and even their sense of smell to locate the caves, which are at altitudes of 1,800 to 2,150 meters.
Working with such a small animal has benefits and disadvantages.
"Its brain has the volume of about a 10th ... of a grain of rice," Professor Warrant said.
"But nonetheless, we've been able to record from visual neurons in the central brain of the moth and have discovered the circuits which seem to be involved in processing the stars."
Unlike many birds that migrate extraordinary distances at night, the bogong moths make the journey with no prior experience of the route.
The moths' parents die months before their offspring emerge as adults.
"The entire route, the direction, the distance, it's a genetically inherited set of information, and this for me is absolutely fascinating," Professor Warrant said.
It's a mystery of natural history with implications for our understanding of other migratory species, with still more yet to be discovered.
Anna Honkanen, originally from Finland, is a postdoctoral researcher at Lund University.
She joined the project in 2019, focusing on electrophysiology to map which parts of the moth's brain detect and react to magnetic stimuli.
She describes the wonder of squeezing between boulders on Australia's highest peaks, and entering caves lined with sleeping moths.
"You might not even see them at first, they line the walls kind of like roof tiles, each one's head tucked under the wings of another moth, so they look like the granite they are sitting on," Dr Honkanen said.
"But once you see one, then you suddenly understand that there's thousands and thousands of them in the cool darkness of the cave.
"It's a pretty breathtaking experience when you can see that the entire cave is actually alive."
It is an enormously welcome sight for the scientists, who witnessed the same caves barren of moths just a few years earlier.
Australian alps' keystone species under threat
In 2017-2019, the driest, hottest conditions ever recorded in south-eastern Australia saw the bogong moth population collapse dramatically.
The researchers walked into alpine caves where 17,000 moths once crowded onto every square metre of cave wall, to find there were suddenly no moths at all.
"Approximately 99.5 per cent of the population was wiped out in a single year," Professor Warrant said.
In 2021, the bogong moth was placed on the International Union for Conservation of Nature's Red List of Threatened Species.
Plentiful rain in recent years has seen the bogong moth population recover to between two thirds and three quarters of the pre-drought population.
But it has become clear just how vulnerable the species is to fluctuations in climate.
The moths are a keystone species for the Australian alpine environment, bringing nutrients from their breeding grounds into the mountains, benefiting the entire ecosystem.
They are also a vital food source for alpine species, including the endangered pygmy possum.
"If the bogongs stopped coming to the mountains, entire ecosystems could collapse from the bottom up," Anna Honkanen said.
"Without the bogong, the mountains might no longer be what people are used to seeing."
