Male antechinus bruno, a mouse-like marsupial that lives in Australia, sleep fewer hours a day during the three weeks in which they mate than during the rest of their lives, in order to increase their chances of reproducing. This is the conclusion of a multidisciplinary study published this Thursday in the journal Current Biology, which has managed, for the first time, to demonstrate this behavior in terrestrial mammals.
“In humans and other animals, restricting the normal amount of sleep leads to worse performance while we are awake, an effect that worsens night after night,” says Erika Zaid, a researcher at La Trobe University in Melbourne, Australia, and one of the authors of the study. “And yet Antechins do just that: they sleep 3 hours less per night, every night, for 3 weeks.”
The males, whose life expectancy is barely one year, die synchronously shortly after the end of the season of sexual frenzy. During their only window of opportunity to reproduce, they give up rest time in exchange for more hours of physical and sexual activity, to maximize the chances of transmitting their genetic inheritance. That is why researchers hypothesize that the ability to sleep fewer hours is an evolutionary characteristic of the species, a product of natural selection.
However, the Antechins continue to sleep for a good part of the day. On average, the specimens analyzed in the study went from sleeping about 15 hours to sleeping 12, but the times changed from one to the next. The one who limited his hours of sleep the most reduced them by half. This is something that partly surprised Zaid, who expected the animals to sleep even less. “It is an example of the essential functions that sleep provides,” he says.
The finding, which has also revealed that females do not reduce their hours of sleep, was possible after following the movement of 15 specimens in captivity, measuring their amount of testosterone in their blood and taking samples of their brain activity. This latest test has revealed that the antechinus only remain motionless when they fall asleep, so a simple way to know if they are awake or asleep is to record their movement.
The analysis of testosterone levels, on the other hand, has shown that the concentration of this hormone increases during the mating season, and that it does not do so uniformly in all specimens. The most active males, that is, those who sleep fewer hours, have higher levels than those who rest more.
Scientists believe that elevated levels of testosterone may help explain the intense level of activity that antechinas maintain during the mating season, but they doubt they will completely solve the mystery. The mechanism, if it exists, remains hidden for now. And they cannot rule out that the animal simply accepts the harm that losing hours of sleep entails in exchange for guaranteeing its paternity.
Another question that remains to be resolved is why the males of this species die after the mating season. One of the main hypotheses in recent years was that lack of sleep is to blame for the outcome. However, the specimens that slept fewer hours in the experiment were not the first to die, so the researchers rule out that the shorter rest time is the only culprit.
The authors point out that the cause is “an unknown trigger in the natural environment”, capable of triggering a synchronized death of the specimens.
