Male black widow spiders piggyback on the work of their rivals to find female mates faster

A new University of Toronto study shows that male black widow spiders will divert the silk tracks left by rival males in their search for a potential partner.

Black male widows will follow the chemical signals - bpheromones - emitted by potential partners, but it has always been assumed that they would avoid rival males because the competition is so fierce.

"We expect men to use female tails to find a potential partner and to use those left by their rivals to avoid them," says Catherine Scott, PhD candidate in Professor Maydianne Andrade's laboratory at Scarborough University and lead author of the study.

"We have found black male widows to follow rather than avoid rival males, using their silk trails as highways that quickly lead them to a female's canvas, even if they cannot feel the female themselves."

The Western Black Widow Spider is a poisonous spider that originates from large areas of western North America. A man's life is brutal and short. Finding partners is risky - only 12% of males survive and few females are receptive to potential partners each night. Surviving males may also be cannibalized by much larger females before they have a chance to mate.

Andrade, a renowned expert on the mating habits of cannibal widow spiders, says that this dual tactic of using female tails and following their rivals seems to benefit males because speed is essential.

"It is better for men to fight over the same female than to risk missing her completely," she says.

"The courtship can also last for hours, so even if a male is not the first to arrive on the web, if he arrives fast enough, he may be able to win the competition to be the first to mate."

Scott conducted much of the research for this field study with co-author Sean McCann, partly through a crowd fundraising campaign, where she followed hundreds of black widows from the West on a beach on Vancouver Island.

She discovered that males were able to locate females up to 60 metres away, even if they were only the size of a grain of rice. Surprisingly, males with access to the silk trails of other males moved at higher speeds than those with access only to the chemical cues of the females.

It is not clear whether males detect chemical clues left on their rivals' silk, but this is probably the case since they only follow the silk of other black widows and not that of other species.

What is clear is that the presence of a rival's silk indicates that a female was waving with pheromones at some point earlier in the evening. The first male to find this female will turn off the signal by destroying her web, but Scott notes that other males will "listen" to this previous conversation by following the silk trail left behind, even if they can no longer feel the female themselves.

The immediate goal of this research is to see how male black widows use information to win the race to find a companion, but understanding the chemical communication of black widows could have important practical applications. The results could potentially help develop a pesticide-free method to control black widows in situations where they are considered parasites, which is important given that their venom is a neurotoxin in humans.

Scott, who will become a postdoctoral fellow at Arcadia University in Pennsylvania in the fall, was surprised to see men following their rivals. Generally, if males have a choice between competing for a companion or avoiding competition, they avoid it.

"When we studied these spiders in the field, we realized that they probably never make that choice," she says.

Although much of the laboratory research tends to focus on how chemical cues affect yard behaviour or allow females to attract potential partners, she says much less is known about how males find females in the wild.

"It is very difficult to do this type of study in the wild," adds Andrade, whose laboratory has discovered how male black widows choose well-nourished females to avoid being eaten alive, and how they also seek immature females as a breeding strategy.

"When we discover a behavior that seems counter-intuitive, it shows that we really need to study this mating behavior as it occurs in nature."