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Immature mating as a novel tactic of an invasive widow spider

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Research explores male widow spider mating choices

Widow spiders from the genus Latrodectus defy conventional rules of mating. EU-funded researchers investigate why and how male widow spiders mate with immature females and how sacrificing themselves willingly during copulation with mature ones establishes their reproductive success.

Fundamental Research icon Fundamental Research

Recent studies have revealed that males of certain widow spiders genus Latrodectus mate with immature females – previously considered impossible. By definition, only adults were thought to be sexually mature, possessing developed genitalia necessary for reproduction. However, in the case of the L. geometricus and L. hasselti species, female copulatory organs are developed to a certain degree and are functional before they reach their adult stage.

Could males survive to mate another day?

This unconventional mating practice differs from normal mating in these two species. When mating with adults, males actively facilitate female cannibalism through a specific copulatory posture, but they do not engage in this behaviour with immature females. The male maintains a safe position throughout the entire copulation with immature females, positioned far from the female fangs. By contrast, when mating with adult females, the male ends up in a vulnerable position, prone to attack from the female. Other studies showed that male-triggered sexual cannibalism is advantageous for males of the L. hasselti species. Despite limiting themselves to a single mating event, males benefit as females become less receptive to additional mates after having cannibalised a male during copulation. Therefore, “sexual cannibalism in L. hasselti does not represent a case of sexual conflict. Instead, it is an adaptive male strategy to maximise male reproductive success by investing all efforts into mating with a single female,” notes Lenka Sentenska, coordinator of the Widow Spider Mating project. The purpose of this behaviour in the brown widow spider L. geometricus remains unknown. Funded by the Marie Skłodowska-Curie programme, the researcher examined the significance of male copulatory posture in both normal and immature mating. “We froze mating spiders using liquid nitrogen and scanned them with microtomograph to reveal the interaction between male and female copulatory organs during copulation,” explains Sentenska. “Despite the striking contrast in mating positions, genital coupling remains the same, whether the male mates with an adult or immature female. We concluded that the somersault performed with adult females does not provide any mechanical advantage during mating.”

Opting for mature females and blocking immature ones from remating

Sentenska explored a method of ensuring paternity in widow spiders – a process known as plugging – whereby males break off the tips of their copulatory organs and leave them into female copulatory ducts, thereby hindering remating. “Contrary to our expectations, our findings showed that the plugging success is higher in immature females. This indicates that the genitalia of immature females is sufficiently developed to hold and retain plugs”, remarks Sentenska. To investigate male mate choice, scientists exposed male spiders to isolated silk from both adult and immature females simultaneously. Despite the apparent advantages of mating with immature females – no risk of cannibalism, more insertions and higher plugging success – males consistently showed preference for the silk of adult females. “Earlier studies in L. hasselti indicated that male self-sacrifice reduces female receptivity to future matings. This led us deem that immature L. geometricus females, who had not experienced male self-sacrifice, would be more likely to re-mate than adults. However, our findings contradicted this prediction,” states Sentenska. Females who first mated as immatures were less likely to mate again compared to those who first mated as adults. “Interestingly, the occurrence of male self-sacrifice did not affect the re-mating probability of adult-mated females. Consequently, the role of self-sacrifice in L. geometricus differs from its congener and remains unknown,” concludes Sentenska.

Keywords

Widow Spider Mating, mating, immature females, widow spider, cannibalism, plugging, genus Latrodectus

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