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Female dwarf spider.

Melanie Witthuhn

How male spiders keep females to themselves

Dwarf spiders don’t need to take paternity tests to know who the father is—for the most part. Right after copulation, males plug up the genital tract of females (red box in picture) to ensure that competitors can’t deposit sperm. Researchers studying the technique found that the larger the plug, the more difficult it is for subsequent males to remove. Described this month in Behavioral Ecology and Sociobiology, the “stoppers” effectively prevent 67.5% of males who show up later from breeding.

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