麻豆传媒

The smaller male placates his mate to avoid becoming her post-coital dinner (Image: Matja啪 Gregori膷)

Species: Caerostris darwini
Habitat: Across bodies of water in Madagascar

Mating can be a deadly act for spiders 鈥� females often end it by eating their partners.

But the Darwin鈥檚 bark spider may have found an unusual way around this risk: keeping the juices flowing. During copulation, males orally lubricate females鈥� genitals. The reason may be to prevent themselves from becoming a meal.

Darwin鈥檚 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鈥� sex lives.

鈥淒arwin鈥檚 bark spiders are a rather enigmatic species, but even we were in for a surprise when we observed their peculiar courtship behaviour,鈥� 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 鈥� and one way is to linger around younger females.

鈥淢ales 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鈥檚 cuticles harden and she can move and attack, she is able to prevent long copulations,鈥� says Kralj-Fiser.

So sticking with young females that can鈥檛 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 鈥� the first pair of fang-like appendages near the mouth 鈥� to females鈥� epigyne, the external genital structure. But when males moved their mouthparts away from adult females鈥� genitals during courtship, researchers could see an unusual drop of liquid on the epigyne. This behaviour wasn鈥檛 seen when males mated opportunistically with young females.

鈥淢ales 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,鈥� says Kralj-Fiser, who presented the study at the Ethological Society鈥檚 鈥淐auses and consequences of social behaviour鈥� 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 鈥� which would explain why the males don鈥檛 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 鈥渂ack rubs鈥� and depositing silk on their bodies. Male Darwin鈥檚 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.

鈥淥ral lubrication may be one way the female evaluates the male quality, or it may have to do with the 鈥榩lug鈥� that the male will subsequently place in the female genitalia, making it harder for other males to mate,鈥� says Ingi Agnarsson at the University of Vermont, who discovered the species in 2010.