Why male marsupials are dying for sex

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. Hanna Rosin is the founder of DoubleX and a writer for the Atlantic. She is also the author of The End of Men. Follow her on Twitter. 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. Agile Antechinus, Antechinus agilis, trapped during a field trip, September 2005 Courtesy Mel Williams/Creative Commons 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.