Dwarf Spiders’ “Chastity Belts” Explained

Reproduction is the engine of life—and some species go to greater lengths than others to ensure that their genetic material is passed on.

For instance, male dwarf spiders take special measures to guarantee their paternity by inserting a “mating plug” into a female to block the sperm of other males.

An adult female Oedothorax retusus. Photograph by Nigel Cattlin/Alamy

Now scientists have revealed more about how these arachnid chastity belts actually work. For example, the plugs vary in effectiveness based on size and age, according to a study published this week in Behavioral Ecology and Sociobiology. (See National Geographic’s spider videos.)

We caught up with study co-author Katrin Kunz, of the Zoological Institute and Museum in Greifswald, Germany, to learn more about dwarf spider mating.

What is this dwarf spider?

Oedothorax retusus is a common European spider species that’s less than three millimeters long. It lives in wet habitats such as salt marshes and riverbanks. (See “Small Spiders Have Big Brains That Spill Into Their Legs.”)

How do they mate?

From courtship to copulation, dwarf spider mating is a process that involves a complex string of actions.

Ventral view of an Oedothorax retusus female. Red square indicates the position of the genital region that is plugged by the male.
The belly of the female Oedothorax retusus. The red square indicates the genital region that is plugged by the male. Photograph by Melanie Witthuhn

“Courtship includes a series of behaviors: The male starts vibrating his opisthosoma [abdomen] when registering the female. The male then approaches the female while his first pair of legs trembles,” Kunz said by email. (Also see “Male Spiders Give ‘Back Rubs’ to Seduce Their Mates.”)

The spiders then position themselves across from each other to mate, which involves the male inserting his pedipalps—or male copulatory organ—into the female’s two copulatory ducts.

When the deed is done, the male then secretes a mating plug into the female’s copulatory opening.

So what exactly is a mating plug?

The plug is secreted into the female as a type of fluid, which then hardens, preventing other sperm from entering the female. (See “Wild Romance: Weird Animal Courtship and Mating Rituals.”)

The exact chemical composition of the mating plugs is yet to be determined, but Kunz and colleagues have found the gland that creates the plug material.

Kunz pointed out that since polyandry—a system in which a female mates with many males—is common in the animal kingdom, males of many species have developed creative ways to combat competition from other sperm, including “removal, displacement, [or] replacement of rival sperm.” (See “Sperm Tracked in 3-D—A First.”)

“Female O. retusus frequently mate with several males and store sperm within their genital tract. Thus, males face a high risk of sperm competion. In order to make sure that offspring are their own, males produce mating plugs as a copulation barrier,” Kunz said.

Furthermore, if the same male mates twice with a female and plugs both of her copulatory ducts, its chance of monopolizing that female is greatly improved.

What’s the most effective mating plug?

Previous research by Kunz’s colleagues determined that mating plugs significantly reduced the next male’s ability to mate with a female. (See “Male Black Widow Spiders “Twerk” to Avoid Being Eaten by Females.”)

But a key finding of the new study is that not all mating plugs are equal in their effectiveness.

For the study, Kunz and her colleagues staged “remating” trials in which males mated again with female spiders bearing plugs of various sizes and ages—a result of the time interval since the plug was inserted.

The results showed that larger and older plugs were more effective in blocking the entry of rival sperm, suggesting that the size and hardness of the plug is crucial.

Tell us: What weird animal mating rituals have you heard about?

Follow Stefan Sirucek on Twitter.

Changing Planet

Meet the Author
Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.