Research finds species of female praying mantises who eat males after sex produce greater number of eggs than those who do not

Death by cannibalism might seem like a high price to pay for a fleeting moment of passion, but male praying mantises are doing it for the kids, new research suggests.

Scientists have discovered that female praying mantises who eat their mates after sex produce a greater number of eggs than those who do not, with the bodies of the ill-fated males used to aid their production.

Of the species of praying mantises known to exhibit sexual cannibalism it is estimated up to 28% of males are eaten by their partner. After mating, the female stores the male’s sperm and later uses it to fertilise the eggs that she produces.

The authors say the new study backs up a long-mooted theory that males could have evolved a behavioural trait of self-sacrifice to boost their reproductive success.

“There is an obvious cost – you are dead, you have lost all future mating possibilities,” said William Brown, of the State University of New York at Fredonia, who co-authored the research. “We measure costs and benefit in terms of offspring production,” he added. If, by dying, the male can boost the number of offspring produced by one female, the theory goes, it could outweigh the downsides of missing out on future conquests.

Published in the journal Proceedings of the Royal Society B by researchers in the US and Australia, the new study reveals how scientists unpicked the influence of cannibalism on the production of offspring in the praying mantis Tenodera sinensis, by tracking what happened to male ejaculate and bodily tissues after mating.

In the first step of the study, Brown and his colleague fed two groups of crickets with amino acids – the building blocks of proteins – that contained one of two types of radioactive atom. The insects were then served up to two groups of praying mantis with each group feeding on crickets bearing one type of radioactive marker. The upshot was that the “tagged” amino acids were incorporated into the mantises’ bodies, allowing the scientists to track their fate.

The scientists then allowed the males from one group of mantises to mate with females from the other. While 10 of the 21 couples were immediately separated after they had mated, the others were kept together until the female had devoured the male.

“The male praying mantis of this species is probably the largest potential prey item out there for the female,” said Brown. “So the female is seeing the male as a source of nutrition.”

By tracking the radioactive atoms within the mantises, the researchers found that that males that were eaten by their mates passed on nearly 89% of their tagged amino acids, while those who survived their romantic encounter passed on just over 25% – all of which came from their ejaculate.

When the researchers analysed the eggs, ovaries and other reproductive tissues of the cannibalistic females, they found the levels of radioactive atoms of male origin were almost double those found in females that did not eat their mate. That, the authors say, suggests that it is not only the male’s ejaculate that is used to help produce the eggs, but his bodily tissues too.

Cannibalism was also found to boost the numbers of eggs produced. Once the researchers discounted eggs thought to have been produced before mating, they found that females who ate their partners produced on average just over 88 eggs, compared to an average of 37.5 for those who did not.

While the study involved only a small number of praying mantises, Brown believes the research supports the theory that sexual cannibalism marks the ultimate male investment.

“What we are showing is that [the cost of the male] being killed by a female can be partially recouped if more offspring are produced with his body,” said Brown. “In the larger context, it could affect the willingness for males to take that risk.”