Sexual cannibalism: a treat you can eat

When is a roll in the hay worth losing your head over? Suizilla

Sex is often viewed as a totally harmonious interaction between a male and female with the same innate goal – to produce offspring.

But it is becoming increasingly clear to biologists that the reproductive interests of males and females are rarely, if ever, the same.

We now know true monogamy is extremely uncommon in the animal world. Polygamy is the rule of thumb, where pairings are most often temporary and the lifetime reproductive success of mates is rarely equal.

In essence, individuals of each sex attempt to maximise their own reproductive fitness, irrespective of the potential negative effects on their partner(s).

This battle between the sexes and reproductive strategies is known as sexual conflict.

Fight for the right to party

Although sexual conflict can be imposed by either sex on the other, nature shows us it’s usually males doing most of the imposing. Indeed, there are countless examples of males harassing, guarding and even injuring females:

  • Male grey seals frequently harass females that resist their mating attempts. Increased levels of harassment result in reduced suckling time and slower-growing pups.

  • Male water striders guard females by remaining mounted after mating has finished. This decreases the chance of a female re-mating with another (potentially better) male, increases the energy expenditure of the female and increases the chance of the female being attacked by a predator.

  • The barbed penis of male bean weevils often inflicts damage within the female genital tract. This damage decreases female lifespan, increases the chance of infection and decreases the likelihood a female will re-mate.

There are many stories of this kind, but in the case of some insects and spiders the tables are turned and females gain the upper hand.

Aggressive females in some species attack and consume males during, after or even before they mate.

One of the best-known examples of sexual cannibalism occurs in the Australian redback spider: 12.5% of males are consumed during their first mating and a further 75% during the second mating.

Although this seems like an extremely bad scenario for the male, it is actually quite the opposite – at least from an evolutionary point of view.

First, cannibalised males mate for longer and fertilise more eggs than males that survive mating. Second, females are more likely to reject subsequent males after consuming their first mate.

From the female perspective, there seems to be little reproductive benefit gained from eating a single male partner because male spiders are typically less than 5% the size of females – not a particularly substantial contribution to a female’s diet.

Living on a prayer

The story is very different for praying mantids.

Female mantids can produce up to 40% more offspring as a result of consuming one male – the nutrients obtained from one large meal allow females to produce more eggs. Imagine the potential increase in reproductive output if females cannibalise multiple times throughout their lifetime.

Since female mantids usually attack males as they approach (not while they mate), there is a significant chance a male will be eaten without mating even once. As you can imagine, this is about as bad as it gets for males.

Unlike male spiders that can only mate twice, male mantids can mate many times if they get the chance. Consequently, self-sacrifice makes even less sense for them than for spiders.

Why? Because even when a male mantid is able to begin mating while being eaten (see video above) the 40% increase in reproductive output he might gain from the current mating is unlikely to outweigh the future reproductive output the male would have enjoyed had he not become dinner.

This results in one of the most extreme examples of sexual conflict studied thus far.

So how does a male mate while being eaten?

Male mantids have one brain in their head and another in their abdomen. This means they can continue to transfer sperm without a head.

So the moral of the story is …

Female praying mantids use sexual cannibalism as a foraging strategy to increase their reproductive output, with absolutely no interest in the huge costs imposed on their male partners.

Romantic, huh?

Would knowing you were dessert put you off the main course? Don’t be shy: leave your comments below.

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