Male spider mites 'undress' females by pulling off their skin before copulating

When male spider mites are ready to mate, they strip off the skin of maturing females as part of a freakish mating ritual.

Scientists in Austria uncovered the creepy act for the first time while studying spider mites, the dust speck-size relatives of arachnids such as spiders and scorpions, in their lab. The researchers found that the males would guard the females, which typically reach sexual maturity at 10 days of age, and wait until their potential mates began molting their exoskeletons, according to a study published Friday (June 7) in the journal iScience.

"The males will guard the females for hours," study co-author Peter Schausberger, a zoologist and professor in the Department of Behavioral and Cognitive Biology at the University of Vienna, told Live Science. "The males are able to recognize when the premature females start molting because their exuvia [old, outer skin] turns silvery as air lodges between it and the new skin."

And this is when things get really weird. To make the female ready for mating sooner, the male then slips beneath the female and uses its pedipalps (needle-like mouthparts) to pull the skin off the female. Once the exuvia is removed, the male can insert his aedeagus (reproductive organ) into the female, according to a statement.

Related: Strange love: 13 animals with truly weird courtship rituals

The researchers also noticed that at times the males would use their forelegs to "drum" against the female, perhaps to trigger them to begin the molting process, according to the study. 

"It only takes a couple of seconds for copulation," Schausberger said. "This guarding behavior is high in energy and time, so the males want to ensure that another male doesn't take over a female."

This dedication to ensuring a mate is pivotal for the males, since the "first copulation partner of a female is the one that sires all the offspring," according to a statement.

Interestingly, the researchers discovered that at times the female spider mites would "undress" themselves when it came time for them to molt. However, the females pulled off the skin beginning from their heads, whereas the males would remove the hind part of the skin first.

While this is the first time that this skin-stripping behavior has been recorded in any species, spider mites aren't the only ones that conduct creepy mating rituals in the animal kingdom.

For instance, male butterflies will "penetrate the casing" of a female pupa, the stage in a butterfly's life cycle after it's a caterpillar and cocoons itself into a chrysalis, Schausberger said.

Both of these instances "show that intense mate competition can arise" and that these "sophisticated behaviors are driven by sexual selection even in the tiniest of animals," Schausberger said. 

The scientists hope to expand their research by seeing what happens when males have to contend with rivals during this undressing act.