Choosing Mates Wisely Is All The More Important When They Try To Eat You

Choosing our mates is among the most important decisions of our lives. We agonize over finding "the one", and for good reason. If we are going to spend the rest of our lives with one person and depend on that person to help create and raise our children, the stakes of choosing that person well are high. But at least we don't have to worry that if we choose wrong our partner will bite our head off... not literally, anyway.

Sexual cannibalism (eating your mate during or after sex) may seem unusual to us, but it is actually quite common in a number of spiders, scorpions and mantids. Sexual cannibalism is an extreme outcome of sexual conflict. Sexual conflict arises when the two sexes of a species have different optimum strategies to maximize the number of offspring (and grand-offspring and so-on) they have. In the case of sexual cannibalism, females benefit from eating their mate because the extra meal provides calories and nutrients that can be directed towards reproduction. For males, the benefit of getting eaten are less obvious, but surprisingly enough, there are benefits to the males too: Males are often eaten from the head-down, which leaves the male's reproductive bits to keep on copulating. Longer copulations generally result in more fertilized eggs.

Praying mantids are famous examples of sexual cannibals. However, praying mantid females only eat their mates roughly 30% of the time. Whether a female decides to eat her mate or not depends, in part, on how hungry and how skinny she is. Males seem to have figured this out, as they prefer to mate with females that are well-fed and fat. Although this could indicate that males have adapted to avoid females that are more likely to eat them, it could alternatively be because males are attracted to healthier and more successful females that will likely have better breeding success.

Romina Scardamaglia, Sandro Fosacheca and Lorena Pompilio from the University of Buenos Aires in Argentina tested whether male praying mantids choose their mates based on their risk of getting eaten by them. They set up triangular arenas in which a hungry aggressive female was tethered to one corner and a hungry but nonaggressive female was tethered to another corner (the unaggressive females were recovering from a brief anesthetic that made them less aggressive, but didn't seem to affect any other behaviors). Outside of the arena, a male was tethered to encourage the females to behave aggressively. The focal male (the animal of interest in this study) was placed in the third corner of the triangular arena and allowed to move freely. The researchers let the focal male observe while the females behaved aggressively towards the tethered male (none of the animals were hurt in this process, by the way). Then the tethered males were removed and the researchers measured the behavior of the focal males with the two females.

The males always approached a female, but they took a really long time to do it (on average, 2 hours!) And in the end, the researchers' hypothesis was supported: The males had a much stronger preference for the nonaggressive females over the aggressive ones. They were more likely to approach them; they spent more time with them; and they mated with them more.

We can't say for sure whether the males were responding to differences in aggressiveness between the females or some other effect of the anesthetic. But, combined with other studies that have found that male praying mantids prefer to mate with females that are fatter and well-fed (both of which also reduce aggression), suggests that it is likely that males are choosing to mate with females that will let them live to mate another day.

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Further reading:
Scardamaglia, R.C., Fosacheca, S. and Pompilio, L. Sexual conflict in a sexually cannibalistic praying mantid: males prefer low-risk over high-risk females, Animal Behaviour, 99, 9-14 (2015). DOI:10.1016/j.anbehav.2014.10.013.

Image Credits:
Praying Mantis Mating by Oliver Koemmerling at Wikimedia Commons .

Figure 1 from Scardamaglia, et al. (2015).