Redback spiders
Ken Jones/Copyright MCB Andrade 2003

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Mating with younger females helps male spiders dodge cannibalism

The world of spider sex is notorious for its cannibalism: Females often eat their mates following copulation. But this self-sacrificing behavior limits male reproductive potential—it’s extremely hard, after all, to pass on your genes after you’re dead. Now, a new study in Biology Letters suggests that some male widow spiders (above) have found a way to mate multiple times without being eaten. Their secret? Inseminating immature females. During the last 2 or 3 days leading up to their final molt, females have fully developed sperm storage organs called spermathecae that are still covered by their juvenile exoskeleton. In lab experiments and observations in the wild, scientists have discovered that some intrepid male spiders have learned to use their fangs to puncture the females’ abdomen in just the right place to access the spermathacae. The females store the sperm until reaching sexual maturity, after which their eggs are often fertilized by that same sperm. Scientists found that roughly a third of the immature females they dissected had been mated by males. Immature females were much less likely to cannibalize their mates but, after maturing, they produced just as many offspring as spiders fertilized after reaching maturity. This mating strategy not only lets the male escape with his life and go on to mate additional females, but it also grants him a higher likelihood of actually passing his genes to offspring—typically, the first male to mate a female has the best chance to actually fertilize her. The scientists suggest that this behavior may also be common in other invertebrates where males have access to immature females.