Black Widow Spiders Declining Due to Even Deadlier Brown Widows, Suggests Study

Black widow spiders are known for being one of the deadliest spiders in the world. They are distinguished by an hourglass shaped marking on the abdomen, and have a venomous bite which often produces muscle pain, nausea, and mild paralysis of the diaphragm, making breathing difficult. While most victims of black widows survive without serious complications, a bite can be lethal to very small children and the elderly. 

In the past 20 years, researchers have noticed black widow spiders commonly being displaced by the brown widow (Latrodectus geometricus), a fellow species in the same genus, Latrodectus. However, new research suggests that this is not just a simple case of one species winning the competition for food or habitat. Instead, a new study shows that brown widow spiders have a natural inclination to seek out and kill nearby black widows. 

Origin of brown widows and their appearance

The brown widow spider became established in South California in early 2000, and has become well entrenched as part of the local spider fauna in urban Los Angeles and San Diego, according to the Center for Invasive Species Research. 

The brown widow, unlike its starkly black-and-red coloured relative, the black widow, has shades of tan and brown, and black accent marking. The brown widow has an hourglass shaped marking, but it is typically an orange shade rather than the vivid red of a black widow.

Brown widows 6.6 times more likely to kill southern black widows than other related species

Researchers at the University of South Florida conducted experiments pairing brown widow spiders in container habitats with related cobweb spider species. Black widow is one of the varieties of cobweb spiders. The researchers found that the brown widows were 6.6 times more likely to kill southern black widows than other related species. 

The study was published March 13 in the Annals of the Entomological Society of America

In a statement released by the Entomological Society of America, Louis Cotichhio, who led the study as part of his undergraduate research at USF, said the team has established brown widow behaviour as being highly aggressive towards the southern black widows, yet much more tolerant of other spiders within the same family. 

Where are black widows native to?

Suspected to have evolved in Africa, brown widow spiders have been introduced on all continents except Antarctica. Meanwhile, black widow spiders are native to North America. Black widow spiders comprise two closely related species, the western black widow (Latrodectus hesperus) and the southern black widow (Latrodectus mactans). 

How the study was conducted

In the first part of his career, Coticchio served as a zookeeper specialising in venomous animals in California and returned to Florida to earn a degree in biology, where he channelled his passion for spiders into his research projects. While collecting wild spiders in Florida, Cotichhio noticed brown widows displacing black widows but not other related species, which got him wondering. 

Cotichhio said he had a sneaking suspicion that Florida in particular provided plenty of food and habitat for both the brown and black widow, and that there was possibly some other area such as behavioural differences that were playing a role. 

Cotichhio explained that his observations in the field showed that brown widows appeared to be much more tolerant of other species outside of their genus, and so if resources were the main factor, the same behaviour should have been observed with other spiders competing for the same resources. But that did not seem to be the case, he said. 

Cotichhio, together with researchers at University of South Florida, devised a three-part study to explore the potential drivers of brown widows displacing black widows.

The researchers applied mathematical modelling to the risk factors to survival that brown and black widow spiders face. 

Black and brown widows are more likely to die by predation than by starvation

The team observed that both species are far more likely to die by predation than by starvation. The researchers noted in the paper that “competition for scarce resources is not a significant cause of mortality among spiderlings for either species”.

Brown widow females were more fertile than their black widow counterparts

The researchers also compared rates of growth and fertility between brown and black widows, and found that sub-adult brown widow females were 9.5 per cent larger than black widows, and adult female brown widows reached reproductive maturity 16 per cent sooner than black widows. 

According to the study, adult male brown widows were 25 per cent smaller than adult male black widows, but reached reproductive maturity sooner. 

Brown widow females were about twice as fertile as black widows, and often produced multiple egg sacs at a time. Meanwhile, the black widows produced just one egg sac at a time. 

In some cases, black widows defensively killed brown widows

The researchers obtained the clearest results on placing brown widows in proximity with black widows and other spider species. In 50 per cent of pairings, sub-adult brown widow females simply cohabitated with red house spider (Nesticodes rufipes) females. In 40 per cent of pairings, sub-adult brown widow females were killed and consumed by the red house spiders. 

In 80 per cent of the pairings, brown widows cohabitated with triangulate cobweb spiders (Steatoda triangulosa), and in 10 per cent of the pairings, brown widows were killed. 

However, when sub-adult brown and black widow females were paired, the brown widows killed and consumed the black widows in 80 per cent of pairings. 

In 40 per cent of the trials involving pairings of adults, black widows were killed. In 30 per of the trials involving pairings of adults, black widows defensively killed brown widows. In the remaining 30 per cent, black widows and brown widows cohabitated. 

Brown widows showed “bold” behaviour, black widows never observed as aggressors

The researchers said brown widow spiders regularly ventured into black widow webs, throughout the experiments. While red house spiders and triangulate cobweb spiders also showed “bold” behaviour, black widows were never observed as aggressors. 

Deby Cassill, one of the authors on the paper, said the researchers did not expect to find such a dramatic and consistent difference in the personalities of the brown widow and the black widow. If there is no resistance from a neighbour, brown widows, which are boldly aggressive, will immediately attack the neighbour. 

Black widows are extremely shy

When there are two bold spiders, the initial attack is often resolved by both individuals going to separate corners and eventually being fine with having a nearby neighbour. However, the black widows are extremely shy, counterattacking only to defend themselves against an aggressive spider, Cassill said.

Brown widows are aggressive towards black widows, but not humans

However, the characterisation of brown widow spiders as “aggressive” is a relative term, according to the researchers. The term “aggressive” reflects the stance of brown widows toward black widow spiders, but not toward humans.

Widow spiders are synanthropic, which means they are commonly found around human-made structures, such as barns, garages and sheds. However, widow spiders are very shy when harassed by humans or larger animals that are not considered prey, according to Cotichhio. 

He also said widow spiders will run or roll up into a ball and play dead when being attacked or harassed by most other animals outside of their prey range. Compared to black widows, brown widow venom causes less severe reactions to humans. Also, brown widow spiders rarely bite people. 

Predation by invasive species on its native relative is rare in the animal kingdom

The evident aggression of brown widow spiders towards black widows raises many questions, including why it shows such behaviour, and what drives it to be aggressive toward a closely related species.

According to the researchers, invasive species typically outcompete natives through advantages in factors such as fertility, growth, dispersal or defences against predators, and direct predation by an invasive species on its native relative, across the animal kingdom, is rare.

What is next?

Cotichhio said he wants to find out how brown widows interact with other species of spiders, more specifically black widows in Africa, where brown widows are believed to have originated.

He also said he would love to see if the behaviour of brown widows and their displacement of black widows is something they have adapted only in North America, or if this behaviour is something they exhibit naturally even in areas where they have co-evolved with black widows for much longer periods of time.

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