There is a small mouse-like marsupial that lives in Australia, South America, and Papua New Guinea and that will die for love. In a brief and frenzied mating season, the males of this species will compete desperately for the attention of the females, mate frantically with them, and get so stressed out by the experience that they will die, tragically, like an army of Romeos. The phenomenon is known as “synchronized suicidal reproduction,” or more technically, “semelparity.” It is more common among plants, fish, and spiders than mammals, although biologists have known about this particular marsupial’s reckless habits for at least 30 years.
What biologists haven’t known is why the marsupial would willingly subject himself to such heartbreak, year after year. There have been several hypotheses floated over the decades, but as Diana Fisher of the University of Queensland and her team of researchers show in a paper out this week, those hypotheses are implausible. Fisher and her team spent more than a decade observing the mating behavior of the marsupials and broke through years of clotted thinking about the phenomenon. In so doing, they inadvertently reveal how even something so straightforward as biological observation gets thoroughly distorted by our narrow human lens on gender dynamics and sex.
The researchers compared 52 different species of a creature of the Dasyuridae family of marsupials from different habitats. For the species who live in higher latitudes, the insects they eat are only available in abundance for brief periods, and the females synchronize their mating season to coincide with the food. They send out the signal and the males come swarming. The males try to mate with as many females as possible in sex sessions that can last up to 14 hours. During these marathon bouts of copulation, the males release high levels of hormones, including testosterone, which in turn elevates stress hormones. "If we humans get huge stress, we have a feedback system and we bring it down,” Fisher said. "But the marsupials just keep ramping it up more and more and are driven to spend all their time mating competitively.”
For years there were two reigning theories about this phenomenon, both of which made the males seem quite noble. The first was that the males fight for the females, and that elevates their stress hormones. “This has not turned out to be true,” Fisher wrote me. “They don't fight.” And even if they did, she pointed out, fighting would be fast and intermittent, not long and sustained. The second theory was that the males are altruistic, and die off to ensure that there is sufficient food for the next generation, a reason commonly cited in nature documentaries. But Fisher calls this one “implausible” as well. Natural selection, she writes, acts at the level of individuals passing on their genes, not populations of males acting for the good of the species. In this case, the males “mate themselves to death” says Fisher, in order to ensure that they, and not the next marsupial, will get as many sperm as possible into the female. They just won’t stop, until they are good and empty, and apparently they have very large testes so it takes a while.
In fact, what previous researchers have missed is that the mating behavior is entirely driven by the females. They synchronize their reproductive cycles to coincide with the available food, they determine the length of the mating season, and they are very, very promiscuous, mating with as many males as possible, indiscriminately—old, young, fit, not fit, any old marsupial will do. (In Fisher’s paper she calls the females “polyandrous.”) The males are powerless in this process and have very little agency. They have to adjust themselves to the schedule set by the females, and that schedule is so stressful that they die.
Apparently, overlooking female control is a common “oops” in animal mating research. In Daniel Bergner’s recent book, What Do Women Want?, he describes the great fallacy of monkey sex studies. For many years the reigning theory was that in rhesus monkeys, males initiate sex. But it turned out that this was only true in cages. Once they started to observe the monkeys in the wild, researchers saw something very different. The males would lurk at the edges of female-run domains. “The females invited them to serve sexually. The males remained—desirable, dispensable—until the females lost interest in them. Then they were dismissed, replaced.” Why did researchers fail to see for so long that females were the sexual aggressors? Because we want to believe that “the female libido is limited and that women are monogamy’s natural guardians,” writes Bergner.
Luckily the blinkers are coming off. Fisher says that when molecular techniques to do genetic fingerprinting became more available and affordable in the 2000s, researchers realized that, for example, bird pairs once thought monogamous were doing a lot of “extra-pair mating,” known in the human world as cheating, and that female promiscuity was fairly widespread in the animal kingdom. “It had not occurred to researchers that females were driving so much competition (and evolution) this way and it seemed surprising and needing explanation,” says Fisher. “Now this field of sexual selection from the point of view of females gets a lot of attention.” It takes years of patient observation to reverse received wisdom—a decade in Fisher’s case. But it seems only a matter of time before marsupials start burning their bras.