There is no stopping the male praying mantids of Sydney. Even
after their heads have been chewed off by their female partners,
these hardy insects continue to have sex for up to eight hours.
"It's pretty cool to watch," said a Macquarie University
scientist, Katherine Barry, whose research has led to an
explanation for how this extreme form of sexual cannibalism may
have evolved in false garden mantids, Pseudomantis
albofimbriata, a species common to suburbs including Epping,
Pymble and Turramurra.
By observing hand-reared mantids closely, Miss Barry has found
that females benefit substantially from consuming their male
partners, a job they complete after the lengthy mating has
finished.
In one of the first studies able to demonstrate this effect, she
found the cannibalistic females put on weight and produced more
eggs using the extra energy they got from a single meal of male
meat.
"Sexual cannibalism can boost the reproductive output of the
females by up to 40 per cent," Miss Barry said.
Not every sexual encounter ends in death for the males, which
helps explain why they keep trying. About 60 per cent of the time
they do not get eaten. "Those that approach from the rear are much
less likely to get cannibalised," she said.
And while about 40 per cent of trysts are fatal, in about half
of these cases the males manage to copulate while being
cannibalised.
The females attack first, grabbing the males by the head and
thorax and chewing off their heads, Miss Barry said. "But a lot of
males can use their legs to grab onto the female's abdomen and pull
themselves round while being eaten and start mating with her."
The males were able to sustain sex for hours, because they have
a second primitive brain in their abdomens, she said.
Her study with colleagues Gregory Holwell and Marie Herberstein,
which is published in the journal Behavioural Ecology, also
found that males should be wary of small, hungry-looking females.
"Females in poor condition were more likely to consume their
potential mates," she said.
The findings support a hypothesis for the evolution of sexual
cannibalism known as the adaptive foraging strategy, which suggests
the behaviour provides the females with nutrients that increase
their survival and fecundity.
Studies of other sexual cannibals, such as spiders, may not have
been able to demonstrate this effect because the males were much
smaller than the females.
But male mantids are about 40 per cent of the size of the
females, Miss Barry said. "So it's quite a good meal."
The bright green and brown garden mantids, which are closely
related to cockroaches, are total carnivores and tend to reside
near flowers so they can capture bees and flies to eat.
Deborah Smith- Science Editor
Pic: Kate Barry