Sometimes life’s just not fair. It’s bad enough that male praying mantises in New Zealand are finding themselves more attracted to invasive South African females than to their own species. But of all the species in all the world to lust after, it had to be a sexual cannibal.
The hapless native, Orthodera novaezealandiae, is the only praying mantis native to New Zealand and is not sexually cannibalistic as far as anyone has observed. So the males are considered “naïve.” In their world, males and females mate and then go on about their business.
Not so for the invader, Miomantis caffra, also known as the springbok or South African mantis. Like many other mantis species, female springbok mantises regularly consume their mates. You can see where this is going. New girl catches boy’s eye, boy sidles up close …
“Females simply lunge and attack males as they get within striking distance and start consuming them immediately,” says Greg Holwell, who has been observing the carnage in his laboratory. “Sometimes they start at the head; sometimes they hold them widely in both arms and start in the middle, looking like they are eating a cob of corn.”
… And boy becomes cob of corn for hungry female.
Holwell and his colleagues at the University of Auckland in New Zealand have been testing which species is most attractive to male native mantises. Given a choice between females of their own or the invasive species in a Y-shaped maze, the males approached invasive springbok mantises more than 80 percent of the time. Mantis females commonly use pheromones to attract males, and the researchers suggest that the New Zealand and South African species may use the same pheromones. Such interspecies attraction has been seen before in other praying mantises.
Next, the team allowed males and females to mingle on a nice leafy branch. Female springbok mantises ate nearly 40 percent of males of their own species, and nearly 70 percent of native New Zealand males. Males that weren’t eaten tried to mate with the females, more evidence of their attraction. The results are reported November 26 in Biology Letters.
No one knows how many native males are being eaten by female invaders. The researchers have so far focused on laboratory experiments to demonstrate the fatal attraction but have not yet observed M. caffra cannibalizing the native species in the wild. “This is an obvious next step in our research,” Holwell says.
But if the cannibalism rate is anything approaching what Holwell has seen in the lab, it could spell big trouble for New Zealand mantises. Not only are native males wasting their time trying to mate with a different species, they are also at high risk of predation for their efforts. Males in sexually cannibalistic species often have risk-management behaviors to reduce their odds of being eaten, or at least their odds of successful mating before becoming dinner. But naïve natives have no such strategies, making them sitting ducks.
Since springbok mantises’ first recorded appearance in New Zealand in 1978, it appears that they have begun displacing native mantises, says Holwell. “Virtually anybody you ask in the Auckland area will say that they used to see the native species all the time but only see the invasive species now,” he says. “But there is no data.” There have been no systematic surveys of mantis numbers before or since 1978.
Some spiders, midges and small crustaceans called amphipods also cannibalize their mates. So it’s possible that other sexually cannibalistic species have gained a leg up as invasive species, Holwell says, especially if the species use similar pheromones.
If there’s one bright spot, it’s that female mantises rarely interact, so Holwell expects female natives are safe from the invaders. But they may be getting lonely.