Spider-Like Creature Strips 'Skin' Off Female Before Mating

Researchers have documented for the first time a bizarre mating behavior among tiny spider-like creatures in which males "undress" the females before copulating.

A team of scientists observed the "exceptional" behavior in spider mites, which are small arachnids, the class of animals that includes spiders, scorpions and ticks, among others.

The team captured videos of spider mite males stripping off the hard exoskeleton of the females before mating, a study published in the Cell Press journal iScience describes.

Spider mites, which measure less than 1 millimeter in length, are found around the world, living and feeding on various host plants.

"In recent decades, they became model animals in diverse disciplines such as genetics, behavior, ecology, evolution and plant protection," said Peter Schausberger, an author of the study who is with the Department of Behavioral and Cognitive Biology at the University of Vienna.

"In agriculture, they are feared pests because of piercing the leaf tissue and sucking out the plant liquids," he told Newsweek.

In their study, Schausberger and his colleagues describe the unusual mating behavior. The focus of the team's research is investigating alternative reproductive tactics in male spider mites as well as male-male and male-female interactions.

Spider mites are so small that the researchers have to use microscopes in the lab to observe their behavior.

Schausberger first noticed the unusual "undressing" behavior while watching some videos his team had made of mating acts involving two-spotted spider mites—one of the more than 1,000 species of the mite.

In the videos, male spider mites can be seen guarding premature females that are about to molt and mature before stripping off their old exoskeleton (known as the exuvia) to make them accessible for mating sooner.

Besides being arachnids, spider mites belong to a larger, and extremely diverse, animal group known as arthropods. The creatures in this group, which includes arachnids, insects and crustaceans, have a hard shell known as an exoskeleton. They must go through stages of molting to keep growing, in which they shed their old exoskeleton to reveal a new one.

"Guarding spider mite males strip off the exuvia from females to get earlier access to their genital opening and inseminate them," Schausberger said.

For the males, this behavior increases their chances of reproductive success in a mating environment characterized by intense competition.

"In spider mites, it is of utmost importance to be the first mating partner because only the first mate sires the offspring of a female," Schausberger said. (In fact, the males sire only daughters because sons arise from unfertilized eggs.)

The female spider mites usually need just one mating partner in life to reach their full reproductive potential, according to Schausberger. Mating multiple times usually does not increase the number of eggs produced and can even decrease the quantity.

Therefore, mating multiple times can be detrimental to females unless their first male mate fails to transfer enough sperm. (The males, by contrast, can inseminate many females. And males still copulate with females that have been inseminated by another male, possibly to reduce the reproductive success of the rival male that inseminated the female before.)

"A female that has mated is no longer sexually receptive," Schausberger said. "Since un-mated females are rare, male competition for the few un-mated females is very intense. The pool of females that are fertilizable in a population is extremely small at any given point in time."

For this reason, male spider mites intensively compete for access to females that are close to becoming mature and defend these premature females against rival males. That is, the males guard the females before they become mature.

"The undressing behavior maximizes the chances of the guard to not lose his time and energy investment—they guard the female for several hours before the female molts to the adult stage—and that the guard is then indeed the one who inseminates the female once she is out from the old skin," Schausberger said.

He said that the observation of the previously unknown undressing behavior was a surprise and that he was "fascinated" by the finding.

The study highlights the stark differences between the mating strategies of the male and female spider mites.

"The males increase their reproductive success by having as many mating partners as possible," Schausberger said. "This contrasts with the female strategy: She needs just one mate, but this one should be an especially good one. Such a system selects for intense male competition.

"Owing to intense male competition, a male that succeeds in being the first mate to inseminate a female has superior competitive abilities," he said.

Two-spotted spider mites are the first creatures in which the undressing behavior is described in detail. But similar behaviors might also be found in other spider mite and arthropod species, according to Schausberger.

"There exist anecdotal reports from crustaceans that males help females during molting but no description, no details and no experiments," he said.

Among the study's implications is that sexual selection and intense male competition can lead to highly sophisticated behaviors and that being large is not necessary for the evolution of such behaviors, Schausberger said.