This Marsupial Dies After Marathon Mating. Now It’s Got Bigger Worries
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What if I told you that in Australia, a mouse-like marsupial called antechinus breeds so manically during its three-week mating season that the males bleed internally and go blind, until every male lies dead? And what if I told you that this isn’t the reason the species is facing an existential threat?

Reporting today in the journal Frontiers in Physiology, biologists from University of New England in Australia and the Norwegian University of Science and Technology present troubling evidence that antechinus might be ill-prepared for a warmer world. The researchers set out to look at something called phenotypic plasticity in the yellow-footed antechinus, one of the creature’s 15 known species. Think of your phenotype as your body’s hardware, or physiology: your height and skin color and metabolism. This is in part coded by your genotype, the genetic software that powers the hardware. Phenotypic plasticity is the ability of a species to respond to environmental stressors—like temperature swings—by altering their physiology without mucking with all the underlying genetics.

For the antechinus, the researchers were interested in the plasticity of its metabolism. This is highly influenced by temperature: An adult antechinus’ metabolism shifts to expend less energy when it’s cold during the winter, and there isn’t much insect prey for it to hunt. When it’s warm, an antechinus can afford to expend a lot of energy because prey is plentiful.

The researchers, though, were more interested in how temperature affects antechinus babies—that is, how being raised in cold or warm environments might affect how their metabolism works once they become adults. So they reared two groups of babies, one in colder temperatures, and one in warmer temperatures. They then flipped the thermostat, exposing the individuals reared in the cold to warm temperatures, and the warm-reared ones to the cold.

As the researchers expected, when the temperature switched from warm to cold, the animals decreased their activity levels, which the scientists were recording using infrared sensors that logged movements. This is perfectly natural for wild animals, since in winter they have fewer insects to hunt, and need to conserve their energy to keep from starving. In fact, in the dead of winter, antechinus can slip into a state called torpor, drastically lowering their body temperature and metabolic rates.

In the lab, the researchers also found that when turning up the heat on animals that had been reared in the cold, the animals increased their activity levels, just like they would in the wild as warmer spring temperatures bring more insects to hunt.

So far so good—until the researchers also looked at the metabolic rates, instead of just the activity levels, of the animals as they experienced temperature shifts. A metabolic rate is the measure of how much energy the animal needs to maintain function at rest. For a mammal like antechinus, that rate can change significantly when outdoor temperatures go up or down. Unlike a reptile, a mammal like antechinus has to constantly maintain its own body temperature, either spending energy to cool or warm itself.

This time, the researchers found that when the antechinus raised in the warm group shifted to the cold, they increased their metabolic rate only slightly. But those raised in the cold group that shifted to the warmth decreased their metabolic rate significantly. The discrepancy suggests that the babies brought up in cold conditions have more plastic phenotypes when it comes to adjusting to temperature changes.

“So we hypothesize that perhaps these results reveal that antechinus that are raised in cold conditions have more flexibility in their physiology than those that are raised in warm conditions,” says physiological ecologist Clare Stawski of University of New England in Australia and the Norwegian University of Science and Technology, lead author on the new paper. “Which might show you that in the future when it’s much warmer, and more consistently warm, that the antechinus might not be as flexible to changes in the climate.”

And that’s a problem, because the antechinus relies on torpor to survive the winter months. As Australia warms, this strategy may no longer be available to the species. “If it’s very warm, they can’t use torpor,” says Stawski. That might be fine if a warmer climate also ensures a steady supply of insects to eat all year round. “But if for some reason they lose all their food—for example, there’s a fire—they might not be able to deploy torpor, and then they would really struggle to have enough energy,” she says.

Bushfires are a perfectly natural component of the Australian landscape—every so often a mild fire sweeps through an area, and these animals can take refuge underground or in fallen logs. But climate change is creating ever more powerful wildfires. Instead of gently resetting an ecosystem, they wipe it out. Even if the antechinus in the fire’s area manage to survive, the ecosystem’s insects will have been obliterated. While insect populations will eventually rebound, all the vegetation will be gone—at least in the short term—so the insects will have less food. In other words, instead of leading to a year-round insect buffet, a warmer climate might actually create more summers in which the antechinus go hungry, because their food supply has been diminished by fire.

Australia has also been withering under a fierce drought; indeed, it was that lack of moisture that supercharged this season’s bushfires. Unfortunately for antechinus, food is closely linked to moisture availability, says Queensland University of Technology mammologist Andrew Baker, who wasn’t involved in this new work. “We found a decline in threatened antechinus numbers right across that drought leading up to the fires,” he says. “And that we believe is probably really closely related to lack of food availability.”

In fact, the animals’ three-week mating frenzy is so short because it’s timed with the availability of food. Females mate in the winter and give birth in the spring, when insect populations explode, providing the species with plenty of food. In the months leading up to that mating season, the males are sprinting all over the landscape, eating insects and packing on weight, since that they won’t even stop to eat once the sex frenzy starts. Once the orgy kicks off, the males’ testosterone levels skyrocket, which in turn glitches their bodies’ ability to regulate the stress hormone cortisol. An overload of this hormone makes the males’ bodies literally start falling apart. Their hair falls out, they develop open sores and they go blind, yet still stumble around in search of females.

The females, in turn, mate with as many partners as possible. Each carries sperm from perhaps dozens of males. By the time a female gives birth to around a dozen young that suckle in a depression on her belly, every adult male around her lies dead. None of them will have lived more than a year—they were all born after the previous year’s mating bonanza. Which is just as well, at least for the females and their offspring: It means more food for the mothers, who have to produce lots of highly-nutritious milk for their immature babies. (It seems bizarre, but it’s an evolutionary trade-off. Placental mammals like humans are born relatively mature, but take longer to develop in the womb. In marsupials like kangaroos, babies are born less mature, but have to finish developing in the mother’s pouch.)

For the antechinus, it’s a fast life filled with drama and death, and all of it depends on being able to eat enough food and save up enough energy for the big finish. But a warmer Australia will threaten its food supply, and leave it less able to adapt to change. If only its sole worry was finding a date.

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