Female Madagascan Darwin’s bark spidersare 14 times heavier than males on average and 2.3 times larger.
The species was recently discovered by Matjaž Kuntner, the senior author on the paper published today in Scientific Reports, and chair of the Biological Institute ZRC SAZU.
While studying the mating behaviour of these spiders, the researchers weren't surprised to observe sexual cannibalism and genital mutilation.
But, they were not expecting to witness oral sexual contact, in which the males routinely salivate onto the females’ genitalia.
‘Oral sexual contact seems to be an obligate sexual behaviour in this species as all males did it before, in between, and after copulations, even up to 100 times,’ said Matjaž Gregorič, a research associate at the Biological Institute, Scientific Research Centre of the Slovenian Academy of Sciences and Arts.
According to the researchers, oral sex isn’t common in the animal kingdom.
Mammals are the few who are typically seen to do this, and ‘fellatio-like’ behaviours have been observed in macaques, lemurs, bonobos, hyenas, cheetahs, lions, dolphins, and bats.
The behaviours seen in the C. darwini spiders, which the researchers describe as ‘cunnilingus-like,’ are even rarer.
SEXUAL COMPETITION IN SPIDERS
The team speculates that the 'cunnilingus-like' behaviour observed in Madagascan Darwin’s bark spiders may work to signal the male's quality or reduce sperm competition.
They also suggest it may be a means of cannibalism avoidance, or assessment and manipulation of mating plugs, though they note that these scenarios are unlikely.
Male spiders have developed gruesome tactics to ensure successful fertilization of a female, and secure paternity.
Some have been known to guard the female after copulation, or manipulate her appeal through chemical substances.
They've even been observed using a 'mating plug' to block the reproductive opening.
And recently, researchers discovered another, more effective strategy.
Some types of orb-weaving spider have been observed to mutilate the genitalia of females after sex.
Preventative methods weren't always effective, however, and the researchers witnessed the females successfully capturing and cannibalizing males on numerous occasions.
Some male spiders took to a safer approach known as opportunistic mating.
To ensure the female could not attack, the small male would approach a freshly molted female, mating with her while her exoskeleton is still soft.
‘Our field and laboratory study uncovers a rich sexual repertoire that predictably involves cannibalism, genital mutilation, male preference for teneral females, and emasculation,’ the authors wrote.
‘Surprisingly, C. darwini males engage in oral sexual encounters, rarely reported outside mammals.’
The team speculates that this type of sexual behaviour may work to boost the chances of paternity by signalling the male's quality or reducing sperm competition.
They also suggest it may be a means of cannibalism avoidance, or assessment and manipulation of mating plugs, though they note that these scenarios are unlikely.
SPIDERS KNOWN TO EXHIBIT 'SEXUAL CANNIBALISM'
Sexual cannibals, like the female raft spider, have long been thought to chow down on their mates as a result of uncontrollable aggression.
A recent study from the University of Melbourne suggests that this behaviour is actually more calculated than reputation suggests, and spiders are putting potential suitors to the test by attacking them.
'Females are sexually cannibalistic because they are testing the males, rather than just being inherently aggressive,' said Dr. Mark Elgar of the University of Melbourne.
Researchers say there are many factors that may contribute to this behaviour ‘including the size of the males in comparison to the female, the female's age and whether or not she is a virgin.'
They were also more likely to attack smaller, easier to capture males.
Raft spiders are not the only arachnid species known to cannibalise their mates, and other studies reveal that females are not the only ones who are guilty.
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