Sex is more important than sleep for these marsupials

Marsupials are anything but a boring group of mammals. Kangaroos have some of the most powerful kicks in the animal kingdom, wombats are known for their poop cubes, koalas have a toxic diet, and the mouse-sized antechinus has its busy sex life. These small Australian marsupials will sacrifice multiple hours of sleep every night during their fast and furious mating season to make more time for reproduction. These new findings are described in a study published January 25 in the journal Current Biology and shows the first known direct evidence of this kind of sleep deprivation in a land-dwelling mammal.

[Related: These animals spend their whole lives waiting to have sex, and then they die.]

Strange breeding system for a mammal

Antechinuses are small carnivorous marsupials that live in wooded areas of northern and eastern Australia. There are currently 15 recognized species of antechinus, including the brown antechinus, swamp antechinus, and fawn antechinus. They are primarily nocturnal and eat insects, spiders, and even some small reptiles and frogs. 

Their unique breeding system is more reminiscent of the short-lived bugs that they feast on than other mammals. While females can live for two years, male antechinuses only live for a year. They can only reproduce once in that short lifetime and the males will typically all die at about the same time following their short and intense mating season. 

“The males have one shot at fathering offspring during a single three-week mating period,” study co-author John Lesku, a zoologist who specializes in sleep at La Trobe University in Melbourne, Australia, said in a statement. “We found that male, but not female, dusky antechinuses, become restless during their only breeding season.”

The sleep vs. sex trade-off

Males will trade off between sleep and reproduction that is likely driven by strong sexual selection. For animals–including humans–not catching enough Z’s can typically lead to numerous issues including irritability, lack of concentration, and increase the risk of high-blood pressure, and heart disease. During mating season, antechinuses lose sleep at a rate that would make an average human act as though they were intoxicated, according to the study’s authors.

“Using a combination of techniques, we showed that males lose sleep during the breeding season, with one male halving his sleep during this mating period,” study co-author and La Trobe University PhD student Erika Zaid said in a statement. “In humans and other animals, restricting the normal amount of sleep leads to worse performance while awake, an effect that compounds night after night. And yet, the antechinus did just that: they slept three hours less per night, every night, for three weeks.”

[Related: Why do people need to sleep?]

The team used accelerometers to track the patterns of movement of 15 dusky antechinus (10 males) from captive and wild settings, both before and during mating season. They took blood samples to measure hormonal changes and electrophysiological recordings from four of the males to determine how much they were sleeping. Additional blood samples were taken from 38 wild agile antechinus (20 males) to check if a biomarker for sleep loss called oxalic acid similarly decreased in mating season. 

Females lose sleep too

The results showed that the males were sleeping three hours less every night for weeks. The antechinus may have some unknown way to thrive with less sleep during their mating season. They also may simply accept the physical downsides of sleep deprivation in order to improve their chances of producing offspring. 

The decrease in oxalic acid suggests that the agile antechinus were sleep deprived during mating season, but there was no significant difference between the males and females. This could suggest that the females are also sleep deprived because of male harassment during mating season. 

Biologists are still not certain what causes the males to die after breeding season, but do not suspect that sleep loss alone is the culprit. The males that the team observed sleeping were not the ones in the worst condition. The team wants to learn more about how these marsupials handle their sleep loss. In part that’s because the males they saw sleeping the least were not the ones in the worst condition.

“Are antechinus equally compromised, but just get on with it?” they ask. “Or are they resilient to the negative effects of sleep restriction?”