The eight legs of Christmas (Image: Ciara Phelan)
Packaging their presents in silk means more time between the sheets for some eight-legged lovers
FOR those of us with clumsy hands, Christmas is a nightmare. It is a time when we encase the gifts that we so thoughtfully bought in an embarrassing mess of crumpled paper and badly placed tape. Perhaps our jobs would be easier if we had eight eyes, eight hands and could extrude our own wrapping paper.
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Meet the male nursery web spider. This rather drab looking fellow spends his life in rough grassland and woodland across Europe hunting for insects, which he gives to females in the mating season. When he finds a suitable present, a juicy fly or a mealworm, he paralyses it with his venom. He then carefully wraps the fresh insect in reams of silk, crushing it into a round white ball, before heading off in search of a mate.
Many male insects and spiders woo females with nutritious “nuptial gifts”. There are many benefits to doing so. The gifts advertise a suitor’s qualities as a skilled hunter and forager. They provide nourishment for the mother of their future children. And perhaps most importantly, they distract a potentially cannibalistic female from devouring them instead.
But none of these benefits explain why the nursery web spider wraps its gifts in silk. It certainly isn’t necessary – females will often accept males that give them unwrapped prey. And it isn’t to immobilise the prey as the venom does that. So why bother? It’s only recently that biologists like in Denmark have begun to unravel the mystery.
Gift wrapping is just the beginning of a long courtship for a male nursery spider. He walks about in search of a mate, often following lines of silk spun by mature females. When he finds a female, he rears up, waves his feeler-like palps in the air and presents his gift.
If she is impressed by his efforts, she grabs the parcel and engages in a gentle tug-of-war with him. She will try to steal the gift if possible, but if he resists, she sticks around and starts to eat it. Even then, she can’t unwrap the gift – she can only drool digestive fluids into it and suck up the resulting liquid. While she does, he uses one of his palps to inseminate her, and the other to keeps a tight hold of the silk parcel. After an hour, the male finally lets go and the female runs off, taking both his gift and his sperm.
It’s a wrap
Bilde and her team have spent years watching nursery spiders mate in the laboratory. They have found that compared with just 10 minutes for those that don’t.
Gift wrap means even more time between the sheets. Females take more time to digest a thickly wrapped gift than a poorly wrapped one, giving a male more time to transfer more sperm. The silken strands also give him a better grip upon his gift, which stops the female from simply snatching it and running off. In fact, by simply wrapping their gifts, males can increase the length of their liaisons by another 30 per cent or so.
However, the males aren’t always as generous as they appear to be. Maria Albo at the Clemente Estable Institute for Biological Research in Uruguay discovered just how deceitful the males can be when she unwrapped the parcels using forceps. “Nobody had really looked at what was inside these gifts,” she says.
Albo found that a third of the presents were worthless. Rather than juicy, pristine insects, they contained either plant parts or the empty husks of prey that the male had already eaten. “Normally, you can see the prey exactly as it is,” she says. “But when you open the worthless ones, you don’t recognise anything.”
These worthless gifts have clear advantages for the male. He doesn’t waste a meal that he could devour himself, and he might not even need to go to the effort of capturing prey at all. But for the female, it must be like hoping for diamonds and receiving an empty Tiffany box instead. Because all the gifts are wrapped in silk, she can’t tell if she’s in for riches or disappointment until she starts to feed.
“It’s like hoping for diamonds and receiving an empty Tiffany box”
“After 30 minutes, if they realise there is nothing inside, they run away,” says Albo. That’s still enough time for the male to pump her with thousands of sperm. And yet when Albo and Bilde poured the equivalent of cold water over a mating couple, they found that the females who left early only contained about a third of the sperm of those who finish the job. Fewer of their eggs hatched too.
Nursery spiders aren’t the only gift-wrappers. When Albo studied a more obscure species from South America, Paratrechalea ornata, she found that the female was attracted to the colour of the wrapping.
She also found that she could turn a male spider into an irresistible catch by dabbing a white spot onto his mouthparts, mimicking the presence of a white silk parcel. With this subtle beauty spot, females paid more attention to him and and more frequently. That might be because P. ornata hunts along the edges of ponds and lakes during dusk and darkness, when white objects are easier to spot from a distance. So if gifts are properly wrapped, they are more likely to lure her in from afar.
Once she gets closer, the gift also acts as a way of showcasing the quality of a male’s genes. Albo found that males in rude health are more likely to wrap their prey and wrap them well, producing beautiful, opaque white packages. Weaker males offered sloppy-looking gifts, with bits of black prey visible through meagre strands of silk.
To P. ornata, what’s on the outside seems to matter more than what’s inside. Albo found that 70 per cent of their gifts are empty, meaning deception is the norm. Unlike nursery spiders, however, the females don’t seem to care. For this spider, gifts are just silk purses for sows’ ears. They neither prefer genuine gifts, nor punish males that offer hollow ones by giving them the heave-ho. It is unclear why. It could be that their habitat is so rife with prey that an extra morsel makes little difference to them. Or they might surreptitiously punish males that offer worthless gifts by getting rid of their sperm later. Either way, in this species, it seems that the ritual of gift-giving is evolving into little more than the exchange of fancy wrapping.
Perhaps it’s best you’re not a spider. After all, it is what’s inside the parcel that really counts.
This article appeared in print under the headline “The eight legs of Christmas”
