Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Caerostris darwini
Habitat: Across bodies of water in Madagascar
Mating can be a deadly act for spiders – females often end it by eating their partners.
But the Darwin's bark spider may have found an unusual way around this risk: keeping the juices flowing. During copulation, males orally lubricate females' genitals. The reason may be to prevent themselves from becoming a meal.
Darwin's bark spiders build some of the largest webs, and they do so using silk that is ten times as tough as Kevlar, but their mating behaviour was largely unknown. So, during a two-week survey in Madagascar, a group of researchers led by Simona Kralj-Fiser at the Slovenian Academy of Sciences and Arts tried to demystify the spiders' sex lives.
"Darwin's bark spiders are a rather enigmatic species, but even we were in for a surprise when we observed their peculiar courtship behaviour," says Kralj-Fiser.
Spider kiss
The team observed that females mate with multiple males, and that males are ultra-competitive as a result. They also found that 76 per cent of females overall were aggressive towards males, with 35 per cent cannibalising them after copulation. So males need a tactic to prevent getting eaten – and one way is to linger around younger females.
"Males cohabitate with young females until their final moult to adulthood. While a female is young, soft and defenceless, the guarding male can copulate with her for a long period of time. When a female's cuticles harden and she can move and attack, she is able to prevent long copulations," says Kralj-Fiser.
So sticking with young females that can't yet turn cannibal is a good way for males to avoid becoming dinner, but what about when they mate with adult females?
During their normal mating routine, the males attach their chelicerae – the first pair of fang-like appendages near the mouth – to females' epigyne, the external genital structure. But when males moved their mouthparts away from adult females' genitals during courtship, researchers could see an unusual drop of liquid on the epigyne. This behaviour wasn't seen when males mated opportunistically with young females.
"Males nibble on female external genitals using their fangs, and then we observed that there was a liquid coming out of the fangs. We do not know what this liquid is, but it looks like digestive juices, which they usually secrete when eating," says Kralj-Fiser, who presented the study at the Ethological Society's "Causes and consequences of social behaviour" conference in Hamburg, Germany, last week.
Web of desire
Kralj-Fiser suggests the oral lubrication relaxes adult females so they are less likely to engage in sexual cannibalism – which would explain why the males don't make such an effort with the younger females that are unable to eat them.
Other species have their own strategies to deal with female aggression. For example, males of Nephila pilipes calm females by giving them "back rubs" and depositing silk on their bodies. Male Darwin's bark spiders one-up this species: they engage in what researchers described as intensive courtship behaviour, employing mate binding tactics as well as the oral lubrication trick.
"Oral lubrication may be one way the female evaluates the male quality, or it may have to do with the 'plug' that the male will subsequently place in the female genitalia, making it harder for other males to mate," says Ingi Agnarsson at the University of Vermont, who discovered the species in 2010.
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