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FEMALE cockroaches fancy anything in an exoskeleton once their fertility has
declined, say biologists at the University of Manchester. Unlike their choosy
younger sisters—who take more time to pick a mate—they will have sex
with any male.

Although they don’t know what triggers the behaviour, Patricia Moore and her
husband Allen Moore say there’s a good evolutionary reason for it. Older females
who do not mate early have fewer than half as many offspring during their
lifetime as roaches that mate early.

The older roaches’ carefree love life was uncovered while the team was
researching the creatures’ fertility. There may be some parallels with mammalian
fertility, since the researchers were studying the African cockroach,
Nauphoeta cinerea. Unlike many cockroaches, this species gives birth to
live young instead of laying eggs. Once born, the cockroach nymphs take around
100 days to develop into adults. The first day of their adult life was defined
as “day zero”.

By day 6, adult bugs are sexually receptive and eager to breed. The
researchers introduced adult females at days 3, 6, 9, 12, 15 and 18 to randomly
chosen males. Day 3 adults had no interest in mating, as expected. But they
found that females who first mated at 6 days had an average of 32 offspring in
their first litter, but those that waited 18 days to mate had only 24 young.

The differences became even starker over their 200-day life. A female mating
first at 6 days had on average 80 nymphs throughout her life, against only 40
for a female first mating at 18 days. “Waiting to mate has a severe effect on
their lifetime fertility,” says Patricia Moore.

The bug’s behaviour was surprising, too. At 6 days, says Moore, females
clearly discriminated between potential mates and took time over their choice,
which is probably based on the attractiveness of the males’ pheromones. But
between day 15 and day 18, females mated “very rapidly with any male” they could
find, she says. “They did not discriminate at all.”

“The cockroach’s ability to discriminate between mates closely follows their
fertility. We never expected to get such a beautiful data set,” Moore says. She
speculates that the dying off of the females’ egg cells may trigger the lack of
discrimination, perhaps controlled by a hormone.

Moore thinks her work may provide scientists with pointers to reproductive
behaviour in mammals. “Cycles of pregnancy in cockroaches won’t tell us what’s
happening in mammals,” she says, “but they will allow us to frame questions we
might want to ask about lifetime fertility in mammals.”