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Female Spider Kills and Eats Male to Attract a Partner

woman and spider manFemale praying mantis and black widow spiders are notorious due to their tendency to kill and eat males before, during and after sex. This behavior, from human’s viewpoint, is apparently risky, though – not least since the scent of a dead rival hardly entices other males to try their luck. However, for male Pennsylvania grass spiders (Agelenopsis pennsylvanica), they are willing to approach a female if she has killed and eaten a male recently.

Prior research found that this species of female spiders in urban settings can typically attract three males at most during their 2-week-long breeding season. Their cannibalism appears to leave themselves at risk of self-inflicted celibacy. In order to understand what was going on, Jonathan Pruitt and his co-workers from the University of Pittsburgh, Pennsylvania captured young spiders from hedgerows to the lab for further investigation.

The researchers selected 100 females and let 50 of them kill and eat a wounded male; the rest 50 dined on a cricket. After 20 to 24 days, the team gave a choice to 20 males: to approach the web of a cannibalistic female or that of a female that consumed cricket. As a result, three-quarters of the male spiders headed toward the cannibal. Pruitt says: “A cannibalism history makes the females even more attractive to the subsequent suitors.”

Furthermore, the cannibalistic female spiders were more likely than other females to produce egg cases after mating – and their egg cases were more likely to hatch. Pruitt notes: “It seems the benefits overwhelm the costs.”

 “For this species of spiders, cannibalism makes sense simply because females are typically larger than their partners, and it is highly likely that some males will be viewed as prey,” says Susan Riechert at the University of Tennessee, Knoxville.

But this does not suggest why males prefer to approach a cannibal’s web. However, another observation from the team could explain this. They found that females generally ate only one male, even though the following encounters gave them another opportunity to canibalise again. It is likely that males are aware that females are prone to cannibalise just once, and hence feel more confident of their survival by heading towards a known cannibal.

There are still a few unanswered questions. For example, why is a male a better dinner option for a breeding female than more standard prey? Pruitt says: “Perhaps males seem to be some super-stellar multivitamins.” But Riechert doesn’t think so. Another question is even bigger: if cannibalism is so advantageous, why do some females choose never to kill and eat a males? Pruitt says it may be simply that some females can’t afford to be picky. “Cannibalism might be particularly dangerous for those who are not very attractive, because the number of their would-be suitors is even lower.”

Reference:

  1. Pruitt, J. N., Berning, A. W., Cusack, B., Shearer, T. A., McGuirk, M., Coleman, A., Eng, R. Y. Y., Armagost, F., Sweeney, K., Singh, N. (2014), Precopulatory Sexual Cannibalism Causes Increase Egg Case Production, Hatching Success, and Female Attractiveness to Males. Ethology. doi: 10.1111/eth.12216

Source: NewScientist

Image source: marvunapp

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