Black widows sniff out cannibals

6 July 2011 Last updated at 12:43 By Victoria Gill Science reporter, BBC Nature The female black widow spider is much larger than the male Male black widow spiders can sniff out a well-fed female simply by walking on her web, according to researchers.

The males appear to use this technique to avoid hungry females, which are more likely to cannibalise them.

This habit of eating the male after mating earned the spiders their name.

But this research shows that males can mitigate that risk by sniffing the silk to find out if a female is ravenous and therefore too risky to mate with.

The study is published in the journal Animal Behaviour, and is the first to show that the spiders are able to pick up "chemical cues" from each others' silk.

James Chadwick Johnson from Arizona State University, US, who led the study, hand-fed female black widow spiders to make sure he had a well-fed group of females for the experiment.

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By waving his legs and plucking the web, he's providing vibrations to tell the female, 'I'm not prey'”

End Quote James Chadwick Johnson Arizona State University "Strictly speaking, it was forcep-feeding," he told BBC Nature. "We held a cricket in the forceps and left it sitting on the female's web. Eventually the female would run over and start throwing silk and wrapping up the cricket."

The team gave these females one cricket per week.

"By the fourth week, it was difficult to convince her to eat it," he said. "But we wanted to get as much cricket into her as possible."

The other group of females in the test were starved for several weeks. This did not endanger their lives, but they were "visibly smaller".

In the test, the researchers placed male spiders onto the different females' webs to see how they would react.

To make sure the males were only taking cues from the female spiders' silk, they also put males onto a small bundle of clean silk - with no debris from consumed prey - taken from the webs of both well-fed and hungry females.

"We also swapped the females," recalled Professor Johnson, "tricking the male into thinking he was with a starved female when he was with a well-fed one."

The male spiders, which pick up scents through their feet, were able to tell the difference just by walking on the silk. They carried out their typical courtship dance much more actively when they were on the silk of a female who had been force-fed on an ample diet of crickets.

"This courtship display is a long drawn-out process," explained Professor Johnson. "Before the male mounts the female, he will spend an hour or two walking around plucking and tapping the web.

"It's like spider tai chi; by waving his legs and plucking, he's providing vibrations that are very distinct to tell the female, 'I am not a prey item'.

"This continues until he eventually climbs onto the abdomen of the female. Then he'll transfer sperm and get out of town as quickly as possible."

One aim of Professor Johnson's studies of spider behaviour is to understand more about animals, including spiders, that share our urban, human-dominated environment.

He told BBC Nature: "We change the world like no other species and so it seems to me the onus is on us to understand what happens when we change environments.

"It's not just an altruistic goal either. For example, we are finding a lot of cannibalism in urban infestations of widows compared to desert populations and understanding this... could tell us whether a population is likely to grow or not."

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