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August 03, 2010

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Demystifying spider sex cannibalism

There’s no deep, com­plex rea­son why fem­ales eat the males—they’re just hung­ry, two re­search­ers say

Sept. 15, 2008
Courtesy University of Chicago Press Journals
and World Science staff

Fe­male spi­ders are vo­ra­cious preda­tors and eat many dif­fer­ent prey—some­times their own mates, af­ter sex. Sci­en­tists have pro­posed a range of ideas for why the fe­males do this, ideas that of­ten in­volve com­plex ev­o­lu­tion­ary bal­anc­ing acts of costs and ben­e­fits for each sex.

A fe­male wolf spi­der, Hog­na hel­luo, eats a male. (Cour­tesy U. Chi­ca­go Press)
But two re­search­ers say the an­swer may be sim­pler: the fe­males are just hun­gry, and many males hap­pen to be small enough to catch. 

The sci­en­tists, Shawn Wil­der and Ann Ryp­stra of Mi­ami Uni­ver­s­ity in Ohio, found that males are more likely to be eat­en if they are much smaller than fe­ma­les.

In one spe­cies of spi­der, Hogna hel­luo, large males were nev­er eat­en while small males were munched down 80 per­cent of the time, ac­cord­ing to the pair. 

This find­ing was con­firmed, they added, when they ex­am­ined pub­lished da­ta from a wide range of spi­der spe­cies. Males are more likely to be eat­en in spe­cies where males are small rel­a­tive to fe­ma­les, they con­clud­ed.

Much re­search on “sex­ual can­ni­bal­ism” has fo­cused on a few ex­treme cases in­volv­ing the sex­u­al as­pects of ev­o­lu­tion, the re­search­ers said. But by look­ing at da­ta on a wide range of spi­ders, Wil­der and Ryp­stra found that the size of the ma­le-female size dif­fer­ences are the key. “We were sur­prised to find that such a sim­ple char­ac­ter­is­tic… has such a large ef­fec­t,” said Wil­der.

Sur­pris­ing­ly, ev­o­lu­tion does­n’t seem to drive the rela­t­ion­ship, ac­cord­ing to the re­search­ers. For ex­am­ple, they rea­son, ev­o­lu­tion would­n’t pres­sure fe­males to be­come larg­er to eat more ma­les, be­cause each male would then be a smaller meal to them. And ev­o­lu­tion would­n’t drive males to be­come smaller to be eat­en more of­ten, be­cause then they would­n’t get to mate as of­ten. 

Rath­er, sex­u­al can­ni­bal­ism may be a byprod­uct of the ev­o­lu­tion of dif­fer­ently-sized fe­males and males for oth­er rea­sons, Wil­der and Ryp­stra pro­pose. The find­ings are pub­lished in the Sep­tem­ber is­sue of The Amer­i­can Nat­u­ral­ist.

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Female spiders are voracious predators and eat many different prey—sometimes their own mates, after sex. Scientists have proposed a range of ideas for why the females do this, ideas that often involve complex evolutionary balancing acts of costs and benefits for each sex. But two researchers say the answer may be simpler than previously thought: the females are just hungry, and many males happen to be small enough to catch. The scientists, Shawn Wilder and Ann Rypstra of Miami University in Ohio, found that males are more likely to be eaten if they are much smaller than females. In one species of spider, Hogna helluo, large males were never eaten while small males were munched down 80% of the time, according to the pair. This finding was confirmed, they added, when they examined published data from a wide range of spider species. Males are more likely to be eaten in species where males are small relative to females, they concluded. Much research on “sexual cannibalism” has focused on a few extreme cases involving the sexual aspects of evolution, the researchers said. But by looking at data on a wide range of spiders, Wilder and Rypstra found that the size of the male-female size differences are the key. “We were surprised to find that such a simple characteristic… has such a large effect,” said Wilder. Surprisingly, evolution doesn’t seem to drive the relationship, according to the researchers. For example, they reason, evolution wouldn’t pressure females to become larger to eat more males, because each male would then be a smaller meal to them. And evolution wouldn’t drive males to become smaller to be eaten more often, because they wouldn’t get to mate as often. Rather, sexual cannibalism may be a byproduct of the evolution of differently-sized females and males for other reasons, Wilder and Rypstra speculate. The findings are published in the September issue of The American Naturalist.