Male redback spiders at risk of being cannibalised by their sex partners have developed a survival tactic: mating only with females too young to eat them.
New research has identified the adaptive behaviour among some males of the widow spider genus. Adult females of the genus regularly cannibalise their partners during or after mating.
Male Australian redback and brown widow spiders facilitate monogyny – the practice of males mating only once – through “self-sacrificial mating behaviour” in a bid to increase their chances of paternity.
But a study published in the journal Biology Letters has found that some males will avoid becoming a post-coital meal by mating with immature females that are not yet experienced in eating their partners.
The benefits of immature mating were twofold, researchers concluded: not only did it “rarely end in cannibalism”, it also increased male spiders’ chances of successful insemination – and even copulating with a second female as they usually survived their first mating.
Though the young females are also not mature enough to conceive immediately, the sperm is stored in their two sperm storage organs until they reach adulthood, when it fertilises their eggs.
A third of the immature redback spiders collected by researchers were already mated.
The study, titled “Copulation with immature females increases male fitness in cannibalistic widow spiders”, was co-written by five researchers from Canada and Israel.
The study concluded the behaviour may have arisen from the same “extreme sexual selection” that was thought to have led to the evolution of male self-sacrifice in Australian redback and brown widow spiders.
Maydianne C B Andrade of the University of Toronto Scarborough, one of the co-authors, told the Guardian the behaviour was instinctive, not learned, and widespread.
“Even in this extreme system where females usually hold all the cards, males have evolved a way to shift the balance to favour their own reproductive success,” she said.
Andrade had been studying widow spiders for two decades, and the discovery had been quite by chance.
“I suspect there is quite a lot going on in nature of which we are unaware.”
But, she warned, the “challenging” tactic was not without risk for the male spiders.
It was effective only during a brief developmental window after female spiders’ genitalia had developed, but before they had fully matured.
Misjudging that window could be fatal, because females without developed sperm storage organs had been observed cannibalising adult males.
Despite that, the study said redback males “successfully find and mate one out of three immature females in nature”.
Female redback spiders can store sperm for up to two years and use it to lay several batches of eggs. Young spiders are also cannibalistic and will eat unhatched eggs and their siblings.
Females take an average of about four months to mature, and may live for two to three years. About the size of a large pea, they are significantly larger than males, which mature after about three months and live for only about six or seven months if they are not killed during mating.
According to a 2002 report by Andrade, more than 80% of redback males die without finding a potential mate in nature, and 65% of matings end in the male being eaten.
Males approaching females are also sometimes mistaken for prey and eaten before mating occurs.
The same 2002 study found that some male redbacks had evolved to “somersault” during copulation, twisting their abdomens on to the fangs of their mates in what was construed as an “adaptive male strategy of self-sacrifice” that, if they were eaten, increased their chances of paternity.
Dr Robert Raven, an expert in redback spiders at the Queensland museum, was effusive about the latest finding.
“These animals are literally beside us, in our houses and so forth, yet we know so little about them.”
