The Praying Mantis: an unfairly maligned sexual cannibal?

Recently, probably due to the incredible macro photography of Igor Siwanowicz that I linked to recently, I’ve become rather obsessed with mantises. Before then, I’d paid very little attention to these weird creatures, and will admit to having been mildly terrified of them in the past. This stemmed from an unfortunate incident many years ago, when I tried to brush one off the door handle of a holiday apartment, only to learn that they can fly – and can propel themselves rapidly towards a human face. The neighbours came rushing out because they had heard what they assumed to be the cleaning lady screaming. Anyway, possibly due to this, I thought that there was just a ‘praying mantis’, and had no idea that the order could range from this:

Mantis by Igor Siwanowicz

to this:

Mantises by Igor Siwanowicz

…and many more incredible shapes, sizes and colours. The displays that a mantis can produce when threatened are really quite incredible, and I have been told that they are great for use in undergraduate practical classes. Mantises are apparently relatively short-sighted, and so two can be placed on opposing ends of a mounted piece of string, and it will suddenly become very clear when they notice each other. Unfortunately, the ‘loser’ will signify defeat by flying off to a remote corner of the classroom, delaying the end of the lesson as the tutor (or, more likely, a hapless TA) climbs up to gingerly retrieve it. Here are two videos of different mantis displays, the first relying on sound and wing movement, the other on the large peacock-style eyes on its wings:

So, what wonders does the mantis have in store for the intrepid sex researcher? Rather distressingly, the answer is sexual cannibalism. Males are smaller than females, which have voracious appetites and can eat up to 16 crickets per day, and so put themselves in a rather precarious position both prior to and during copulation. There is a lot of evidence, including the video below (complete with rather breathless commentary), of females devouring their mates during intercourse, but a controversy remains over the function of this.

Does the male, as in the case of the redback spider, allow himself to be eaten in order that the bearer of his children has a large meal when times are scarce, therefore helping to furnish his offspring with added nutrition? Some other insects allow females to ‘nibble’ on them during sex, thus lengthening the duration of copulation and enabling more sperm to enter the female and fertilise the eggs – could it be that this is a poorly-judged version of this adaptation? The latter seems unlikely, as females generally remove the head first, and although the body continues its final mindless act, studies have shown that ejaculation tends to happen more rapidly after decapitation, if at all.

A 2010 study by Kate Barry, of Macquarie University in Sydney, Australia, investigated the state of female nutritional status on mating dynamics in a sexually cannibalistic praying mantid. Previous studies have shown that scramble competition is important in these mating systems, and thus males are selected for their ability to detect and locate females. Given that each copulation holds the risk of death for the male (and the inherent fitness costs of this occurring), and that condition has been shown to be positively correlated with fecundity in female mantises, then theory would predict the evolution of strong male choice for females in good condition. A female in good condition is not only likely to have more eggs, making her a good choice for a male wanting to ensure that he makes the most of the copulation should it be his last, but is less likely to be starved of food. Interestingly, Barry found that long-distance male attraction to females is somehow linked to female fecundity, with ‘low condition’ females still being attractive to males as long as they had a base level of fecundity. Nutritional status seemed to have no bearing on male choice, and so it would appear that females may produce a pheromone that indicates fecundity. Indeed, in a species of cockroach there exists a relationship between ovarian development and pheromone production, meaning an honest signal is produced to demonstrate a female’s egg-bearing capacity.

It would seem, then, that the unfortunate decapitated males may be those that were unable to secure copulation with high-fecundity females, and sacrificed themselves in a final desperate attempt to pass on their genes. Or perhaps, as can happen in many species, they simply read the signals wrongly.

Read KL Barry’s paper here: http://dx.doi.org/10.1016/j.anbehav.2010.05.024

Note: since becoming more interested in the mantis, I’ve even come so far around as to having decided to place an order with Ornamental Insects for a ghost mantis, after listening to their interview on John F Taylor’s Reptile Living Room radio show. However, you may be able to tell from the picture below that, perhaps, I haven’t quite conquered my fear outright…

Ghost mantis by Ornamental Insects

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