Crowdfunding black widow research

For the past six months, Sean and I have been spending most of our nights observing black widows in their natural habitat on Vancouver Island, BC. We did a couple of short experiments during our time in the field, but the vast majority of our work involved simply observing the spiders as they went about their business. The goal was to get a better understanding of the natural behaviour and mating dynamics of this population. This kind of basic natural history research (as opposed to experiments designed to test specific hypotheses) is not often done because it can be challenging, time consuming, and expensive, and is looked on by some as not as important as hypothesis-driven research. I think this is a shame, because there is still so much waiting to be discovered if we only take the time to look. And it is easy to overlook amazing and potentially important phenomena if we don’t take that time. It’s also easy to make incorrect conclusions about the way the world works when we rely only on experimental data and don’t already have a good understanding of the natural history of the organisms we are studying.

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Sean and I doing black widow research at Island View Beach. Drone photo: Sean Lambert (used with permission)

Let me tell a quick story. You may recall that last year our study about web reduction behaviour in black widows was published. (Here’s a plain language summary of the research). Based on observations of sexual behaviour of black widows in the laboratory, we knew that males often engage in web reduction when courting on the webs of virgin female. The male cuts up sections of the web, bundles them up, and wraps them with his own silk. We wanted to know the function of this behaviour, so we ran some carefully designed experiments in the field, and concluded that web reduction allows males to avoid competition by decreasing the attractiveness of the female’s web. We assumed that this is a common tactic used by the first male to arrive at a female’s web, in order to avoid other males from finding the female and interrupting his courtship efforts. I was looking forward to learning more about web reduction this summer as we observed black widows behaviour across the course of a mating season. Guess how many times we observed web reduction in the field? Exactly once. All our laboratory observations were of males introduced onto the webs of adult females. It turns out that in the field, males mature before females do, and most of the time they arrive at a female’s web before she is sexually mature (and before she has the attractive chemicals on her web that trigger web reduction behaviour). Our previous results were not wrong, but without this year’s fieldwork we might never have realized that by focusing on sexual interactions between males and adult females we were missing a big part of the story. How males find immature females, and their behaviour once they do, is likely much more important for avoiding competition than web reduction. There is so much we still don’t know about black widows, that’s just waiting to be discovered!

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Male western black widow (Latrodectus hesperus) engaged in web reduction behaviour on an adult female’s web. Photo: Sean McCann

I feel extremely lucky to have had the opportunity to do this fieldwork as part of my PhD research. Spending six months in the field is very expensive, and a bit of a risk scientifically, because exciting results are definitely not guaranteed. I will likely not have the opportunity to do extended natural history fieldwork like this again, because funding for basic research is increasingly hard to come by. Government funding for science is more and more focused on applied work that has clear benefits for the public. The problem with this model is that future applications and benefits of basic research are often difficult to foresee.

In my case, there are some obvious potential applications of studying chemical communication in widow spiders. Some species are invasive or considered pests in certain areas (including vineyards in BC), so a way to control them without using harmful pesticides would be very useful. Once we understand how male and female black widows respond to one another’s chemical messages, and the identity of the chemical compounds involved, we may be able to develop ways of using these naturally occurring messages to trap and remove spiders from areas where they are a problem. Partly because of this, I think, I was successful in securing an NSERC scholarship to do my PhD (thank you Canadian taxpayers!!!) but even so, I am not exactly drowning in money to support my research.

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Female black widow on her web at Island View Beach. Photo: Sean McCann

The rest of my PhD work will take advantage of the excellent understanding we now have of how black widows actually behave in nature. I will be able to design laboratory experiments that are as naturalistic in context as possible, and use what we now know based on this year’s field observations to make well-informed conclusions. However, to really understand how chemical information affects black widows over the course of their development and their mating interactions, the best place to do the work (both experimental and observational) is in their natural habitat, and this is where the title of the post comes in.

If I had my wish, next summer I would go back to the field for four months (the full mating season) to do experimental and observational studies of black widows that will improve our understanding of their chemical communication. Our lab’s funding is sufficient to pay for travel to and from the field site and for the basic equipment we’ll need to do the research*, but there’s one problem. It’s simply not safe to do the work I have planned (often at night) alone. I will need a field assistant, and that field assistant will need a salary. (Volunteer field assistantships for this kind of work do happen, but they are bad for science, and not an option we would consider.) I can and will apply for several graduate student research grants, and if I am successful these will help defray the costs of the planned fieldwork. Unfortunately, most of these explicitly do not allow the funds to be used to pay salaries. That’s where you come in!

*UPDATE: Here is a note on our campaign page where my supervisor explains the funding situation and why we’re asking for money in a bit more detail.

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The logo and hashtag for the project. Logo designed by The Vexed Muddler.

For the next 30 days, Sean McCann (who was my field assistant this year, and who will continue to collaborate on the project one way or another), my supervisor Maydianne Andrade, and I will be running a crowdfunding campaign on Experiment.com as part of their Arachnid Challenge. We hope to raise the $6000 USD (the salary of a full time field assistant for four months) that we would need to make my plans for another season of black widow fieldwork a reality. This is an opportunity for anyone to support and participate in our research. If we are successful, we will post regular updates about our plans and progress, and share stories and photographs from the field.

Please check out the campaign page for more details, and consider donating. Even $5 will make a difference, and everyone who contributes will receive our heartfelt thanks and be acknowledged in all publications and presentations resulting from the research. If you donate $15 USD (about $20 CAD) or more, we are offering various tangible tokens of our very deep appreciation, including swag with our fantastic logo (designed by the brilliant Vexed Muddler) and prints of Sean’s beautiful photographs. Even if you cannot support the project with a donation, I would be so grateful if you would consider sharing the campaign with your friends and colleagues on social media, email, or in real life. The more people we reach, the more likely we will be to reach our goal!

Thank you so very much in advance for your support – we really appreciate it!!!

Web reduction for rival obstruction!

This post is about a new paper just published online in the journal Animal Behaviour, titled Web reduction by courting male black widows renders pheromone-emitting females’ webs less attractive to rival males by Catherine Scott, Devin Kirk, Sean McCann, and Gerhard Gries! You can read the full text here (free access until 28 August, 2015).           All photos and video are copyright Sean McCann

This short video shows a male western black widow engaged in web reduction behaviour – a common element of the complex courtship rituals males perform on females’ webs. You can see him cutting some silk lines, then pulling silk out of his spinnerets with his last pair of legs, wrapping it around a bundled up section of the female’s web.

Web reduction behaviour is somewhat puzzling. This male black widow is attempting to convince a potentially cannibalistic female several times his size to mate with him. Destroying large areas of her home – which she relies on for both prey capture and protection from predators – is not the most obvious approach. So why do males do it (I know, the title of the paper probably gives this one away), and why do females let them?

Before we address the mystery of web reduction, let’s take a step back and set the scene for this story. It’s a pretty juicy one – there’s attraction, courtship, rivalry, and manipulation! Or maybe not manipulation… we’ll see!

L hesperus pair

Black widows are sexually dimorphic: the familiar female is on the left, and the much smaller and more brightly coloured male is on the right. My study species is the western black widow, Latrodectus hesperus.

Island view beach, on the Saanich Peninsula of Vancouver Island, BC, is a beautiful place to visit. The site shown below is on the lands of the Tsawout First Nation, who have kindly allowed me to collect spiders and do field work here over the past few years.

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This coastal sand dune ecosystem supports a great diversity of organisms, but the black widows are the dominant web-building spiders. Females build their tangle-webs under the driftwood logs at a density of 2-3 webs per square metre of available habitat. There are many logs on this beach; correspondingly, there is a huge population of widow spiders.

female_under_log

At our field site, black widows build their webs under driftwood logs – often several females can be found living under a single piece of wood!

Female black widows almost never leave their webs, so when it comes time to mate, they need males to come to them. The solution to this problem comes in the form of pheromones on their silk. These chemical messages are kind of like scent-based personal ads, that provide the male with information about the female’s mating status and whether or not she’s well fed (sexual cannibalism is rare in this species, but starving females will sometimes eat males!).

Female_AD

If the female’s silk pheromone was actually a personal ad, it might say something like this!

Adult male black widows have only one goal in life: find a female to mate with (and become the father of as many offspring as possible). Once they mature, males stop capturing prey and abandon their webs for a more a nomadic lifestyle. If they detect an attractive female’s pheromone in the air, they follow it to her web.

Male_following_pheromone

A nomadic western black widow male traverses the sand dunes in search of a mate. Note: “pheromone trail” added for dramatic effect.

Once he arrives on the female’s web, the male begins an elaborate courtship display, dancing on the web to transmit vibratory messages (including “male, not meal!”) to the female. It may take many hours of courtship before copulation finally occurs.

male_courtingSoon after he arrives, the male may start to destroy the female’s web. He cuts out sections, bundles them up, & wraps them in his own silk. Males usually reduce the area of the web by about 50%, and gather the destroyed sections up into loose silk-wrapped packages. These can be rope-like (as in the photo at the top of this page) or a tighter ball that has been wrapped extensively by the male like the one below.

silk_ball_smallNow to address our earlier question: why do the males engage in this behaviour? 

Well, black widows are not the only spiders who do web reduction. Other species in the same family (Theridiidae) and others (including Linyphiidae) have similar behaviour. Watson showed in 1986 that web reduction in the sierra dome spider makes the female less attractive to rival males. He concluded that by bundling up the female’s web, the male decreases the surface area from which the silk-borne sex pheromone is released. Some researchers have assumed that it works the same way in other species. Others have suggested that web reduction could function in communication between the male and the female, perhaps improving vibration transmission through the web, or transmitting a male silk pheromone to the female. It could have one or both of these functions in black widows, but until now, no one had ever investigated!

Neriene litigiosa

The sierra dome spider, Neriene litigiosa (family Linyphiidae).

We set out to determine whether web reduction decreases female attractiveness to male black widows in a natural setting. We were pretty sceptical that reducing the web surface area by only about 50% could limit pheromone emission. Usually you have to decrease the dose of a pheromone by an order of magnitude to see any difference in attractiveness. However, males aren’t just bundling up the female’s pheromone-laden silk – they are also adding their own. We thought maybe it could be the addition of the male’s silk (and associated pheromones) that keeps other males away. We designed an experiment to try to find out.

First, we put a bunch of female spiders in cages and allowed them to establish webs. Then we loaded up the cages and took them, and a batch of males, to our field site.

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This is what (part of) a truckload of black widows looks like. Here, my coauthor Devin is loading the cages containing females and their webs into the back of the lab pickup.

Once we got to our field site, we removed the female spiders from their cages (because we wanted to look at the attractiveness of silk only, not the females themselves) and set up four treatments: intact webs, mechanically reduced webs (with half the silk cut out), male-reduced webs (with about half the silk, on average, bundled and wrapped by the males we brought) and empty cages as controls (to confirm that captured males were actually attracted to the silk in the cages, not just wandering randomly).

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Four treatments: intact web, scissor-reduced web (50% of silk removed entirely), male-reduced web (courting males bundling up about 50% of silk on average), and no-web control.

We then turned the cages into traps that would capture any wild males attracted to the silk inside by surrounding them with sticky strips. Then we set the traps out on the beach in groups of four – one for each of the treatments.

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The experimental setup. Each of the four traps contains a different treatment, and the white sticky strips surrounding the cages trap males that are attracted to the silk inside the cage.

We set the traps out at sunset (black widows are nocturnal) and waited to see what would happen. Soon the data started strolling in. We checked the traps every three hours, collecting and preserving any captured males. The sheer number of males out on the prowl was incredible – some webs attracted more than 10 males overnight!

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Close-up of a trap containing a pheromone-laden female’s web, with a male black widow captured outside on the sticky strip.

After 24 hours, just by looking at the vials full of male spiders we had captured, the results were clear (if you want to see statistics, you can read the paper!). Male-reduced webs caught only about one third as many males as intact webs, so web reduction does in fact decrease attractiveness! As we suspected, however, removing half of the silk entirely did not significantly reduce a web’s attractiveness – we captured almost as many males outside scissor-reduced webs as intact webs. (A few spiders got trapped outside the empty control cages – they may have blundered into the sticky strips on their way toward an an attractive web.)

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Beautiful data. It’s not often that the raw data tell the whole story, but here they do!

Evidently, when a male black widow reduces a female’s web, whatever he is doing is much more effective at decreasing its attractiveness than removing half of the pheromone-laden silk entirely. And he’s not actually removing any of the female’s silk – he’s just bundling it up into a ball. There are a couple of potential explanations for how web reduction works. Perhaps the female’s pheromone is not evenly distributed on the web, and the male targets the pheromone-rich silk for web reduction. Then, by wrapping those sections up in his own silk, he creates a barrier that limits the emission of the female pheromone. Another possibility is that the male’s silk has its own pheromone on it, one which other males detect and avoid. Or it could be a combination of both these mechanisms – we’re still not sure. We did another experiment to test the second idea, but the results neither supported nor completely ruled it out (see the paper for more details!). We will try to get to the bottom of this in the future.

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Time for a cute male widow interlude! Look at him peeking out from behind that blade of grass.

For now though, let’s come back to the questions we set out to answer. Why do male black widows do web reduction? It allows them to monopolize the female, by making her web less attractive to other males. Courtship and mating last several hours, so if a male reduces the female’s web as soon as he arrives, he can decrease the likelihood of rival males arriving and interrupting. This may be very important at our field site, where competition for access to females appears to be fierce – during our second experiment, we had one intact web attract over 40 males in a single night! So web reduction is good for the male, because it helps him to avoid competition.

What about the female? Losing her web may be costly – she has to spend time and energy rebuilding it. However, we think she might actually benefit from web reduction too, and that the benefits may outweigh the costs. Sure, her web is important for prey capture and protection, but it’s also really attractive. So attractive, in fact, that even if she doesn’t add more pheromone, it will continue emitting its “come-hither” message for several days. Given number of males we saw arriving at each web during our experiment (40 in one night! even 10 is pretty extreme!), remaining attractive once she has already found a mate might not be so great. Having a choice between multiple males might be a good thing, but the female really only needs to mate once to fertilize all of her eggs. By “muting” her chemical signal though web reduction, the male might be doing her a favour: allowing her to rebuild her web without attractive pheromones (female sex pheromone production shuts off immediately after mating in black widows, but they don’t ever take down their existing webs). Rather than having to waste time and energy chasing off superfluous suitors, this may allow her to get on with the business of producing egg sacs!

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Female western black widow guarding her egg sac. These spiders are very protective mothers!

References and further reading (also linked in the text)

Baruffaldi, L., & Andrade, M. C. (2015). Contact pheromones mediate male preference in black widow spiders: avoidance of hungry sexual cannibals? Animal Behaviour, 102, 25-32.

MacLeod, E. C., & Andrade, M. C. (2014). Strong, convergent male mate choice along two preference axes in field populations of black widow spidersAnimal Behaviour, 89, 163-169.

Salomon, M., Vibert, S., & Bennett, R. G. (2010). Habitat use by western black widow spiders (Latrodectus hesperus) in coastal British Columbia: evidence of facultative group living. Canadian Journal of Zoology, 88(3), 334-346.

Stoltz, J. A., McNeil, J. N., & Andrade, M. C. (2007). Males assess chemical signals to discriminate just-mated females from virgins in redback spidersAnimal Behaviour, 74(6), 1669-1674.

Watson, P. J. (1986). Transmission of a female sex pheromone thwarted by males in the spider Linyphia litigiosa (Linyphiidae). Science, 233(4760), 219-221.

Sex pheromone on the silk of black widow females – more complicated than we thought

The first paper from my MSc has just been published online in the Journal of Chemical Ecology! This study was a collaboration with colleagues Sean McCann (bioassay designer, photography/videography master, and all-around awesome assistant), Regine Gries (analytical chemistry wizard), Grigori Khaskin (synthetic chemist extraordinaire), and my supernatural supervisor Gerhard Gries. If you don’t have access to the journal, you can read the accepted manuscript here.

Here’s the story of the paper.                                                                                             Note: all photos and the video are copyright Sean McCann.

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A female western black widow (Latrodectus hesperus) on her web. The silk is impregnated with sex pheromones that attract males and trigger courtship behaviour.

When I started my MSc, one of the goals for my research was to “find the pheromone” of the western black widow. What does that mean exactly? Well, we already knew that female black widows (spiders in the genus Latrodectus) produce sex pheromones that are somehow incorporated into the silk of their webs. These are sort of like chemical personal ads – they can provide information about things like the species, sex, age, mating history, and body condition of the individual producing them. When a male black widow matures, his only goal in life is to find a female to mate with. He abandons his web and follows his nose (not literally – we don’t really understand much about how spiders smell but their “noses” are most likely on their legs and pedipalps!) to a nearby female’s web. Given a choice among multiple available females, male black widows will go for a well-fed virgin based on the smell of her silk alone. Once he arrives at her web, he contacts the silk and “tastes” (again with receptors on his legs/pedipalps) the pheromone, which triggers courtship behaviour. We wanted to find out the chemical structure of the female’s sex pheromone.

We had a pretty good idea of what to expect, because other researchers had already identified a pheromone of the Australian redback spider (Latrodectus hasselti). It looks like this:

Lhasselti_pheromone

N-3- Methylbutanoyl-O-(S)-2-methylbutanoyl-L-serine methyl ester. Contact pheromone of Australian redback spider (Latrodectus hasselti) females.

Male western black widows are actually attracted to the webs of redback females, implying that the structure of the pheromone is similar, if not identical, in these two species. (It’s not necessary for males to discriminate between Australian redback and North American western black widow females in nature, because they never encounter one another, so it wouldn’t be that strange if they shared the same pheromone). So we set out to analyze the silk of our western black widow females, and see if we could find a similar compound.

better frame

We persuaded females to provide us with clean silk by allowing them to build webs on glass frames for three days. We then collected the silk and extracted it for use in behavioural experiments and chemical analysis.

Some silk collection, extraction, and analytical chemistry* ensued (I’ll leave it to you to read the paper for details if you’re interested), and just as we had hoped, our western black widow females had a compound on their silk that was very similar to the redback pheromone above:

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N-3-Methylbutanoyl-O-methylpropanoyl-L-serine methyl ester. Candidate pheromone of western black widow females (Latrodectus hesperus).

Not only is this chemical similar to the redback pheromone, it is also present in small amounts on redback females’ silk. So it seemed like an ideal candidate for the western black widow pheromone, and provided a potential explanation for the attraction between the two species. Now all we had to do was make a synthetic version* of the pheromone and test it on actual males.

Before we could determine whether the compound we had found was in fact the pheromone we were looking for, we needed to come up with a way of comparing its effects with the real thing. We knew that contact with a female’s silk triggers courtship behaviour, but black widow courtship is long and complex, and involves several different kinds of behaviour, some of which are very subtle. The male’s courtship dance sends vibrations through the web to the female, possibly providing information about his quality and identity (including that he is a potential mate, not a meal!). It also involves the production of copious amounts of silk by the male. This male silk carries its own pheromones, and is deposited all over the web and onto the female herself in the form of a “bridal veil” during courtship.

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Male black widow (L. hesperus) engaging in silk-wrapping on a female’s web during courtship. Here the male is wrapping a section of web that he has destroyed during web reduction behaviour, which I will discuss in a future post.

We designed an experimental setup to assess male responses to silk pheromones. We constructed this high-tech device out of bamboo barbeque skewers, laboratory labeling tape, and a paper cup filled with floral foam. The skewers form a “T” and at each end of the horizontal arm we slid on little envelopes made of squares of filter paper folded in half and stapled. This simple and inexpensive device was one of the big successes of the project.

Tsetup

Our simple and inexpensive T-rod for testing male behavioural responses to contact silk pheromones.

The T-rod design makes it easy to compare an individual male’s response to a test stimulus on one side (for example, a female’s silk wrapped around the paper envelope) to a control (blank paper) on the other.

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Wrapping silk around a filter paper for behavioural experiments.

A male spider is introduced at the base of the “T” and climbs up to the top. Once he gets to the intersection, he can decide whether to go left or right. Males almost always began the experiment by investigating both sides of the “T”, but they spent much longer in contact with the silk-wrapped paper than the blank paper. Not only that, but they spent much of their time wrapping the female silk-wrapped paper with silk of their own – obvious courtship behaviour.

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A male black widow silk-wrapping on a filter paper with silk extract on it.

Knowing that males would respond to female silk in this way on the T-rod, we were now ready to confirm that the behaviour didn’t depend on the structure of the silk itself, and to see if males would respond to our synthetic candidate pheromone in the same way as they would respond to the real thing.

We prepared female silk extract using methanol as a solvent (this is the same idea as vanilla extract, but instead of extracting the flavour of vanilla beans into ethanol, we extracted the chemicals on the silk into methanol) and applied it to one of the filter papers on the T-rod, and methanol alone to the other.

Slide1

We tool silk from a glass frame like the one above and submerged it in methanol to extract the pheromones into the liquid, which we then used in behavioural tests.

Males responded in exactly the same way to silk extract as they did to silk itself, spending most of their time on the filter paper impregnated with extract, and wrapping it extensively with silk. Here’s a video showing what that looks like (first at full speed, and then slowed down):

This told us that a pheromone that can be extracted from the silk triggers courtship behaviour, and the structure of the silk itself is not necessary. But when we tested male responses to our candidate pheromone (dissolved in methanol, using methanol alone as a control), things were not so clear-cut. Males spent more time on the pheromone-impregnated paper than methanol alone, but they didn’t prefer it as much as they had preferred the silk extract to methanol. A few males engaged in silk-wrapping when they made contact with our compound, but not the majority, like we had seen for the extract. This means that although our “pheromone” elicited some male activity, by itself it is not enough to consistently trigger courtship behaviour. It seems to be a pheromone component – meaning that the pheromone is a mixture of one or more compounds in addition to the one we identified, and more work will need to be done to figure out what they are.

It would have been nice to be able to say we found the pheromone. But our results suggest that the chemical communication system of black widows is more complicated than we originally thought, and even more fascinating.

In the study that identified the redback pheromone, the researchers measured male activity (the amount of time they spent moving around when in contact with a filter paper impregnated with pheromone), not courtship behaviour. It could be that this pheromone too is only one component of a more complex chemical cocktail. Like our pheromone component, it may be responsible for eliciting searching behaviour, but not quite enough on its own to consistently trigger courtship behaviour by males.

If multiple compounds are involved in these spider pheromones, they might each have different functions. We don’t yet know whether the same pheromone that attracts males is responsible for triggering courtship, or if different compounds provide different kinds of information, about things like a female’s mating status and feeding history. We have learned that the scent-based sexual communication system of black widows is likely more sophisticated than we originally thought, and that there is much more to discover!

L hesperus pair

Male and female western black widow on a female’s web.

*Neither the analytical chemistry nor the synthesis of the candidate pheromone were trivial tasks – rather they required the expertise and generous efforts of my very talented coauthors Regine and Grigori. I hope they will forgive me for glossing over the details here! 

Here’s the full citation for our paper:

Scott C, McCann S, Gries R, Khaskin G & Gries G. 2015. N-3-Methylbutanoyl-O-methylpropanoyl-L-serine Methyl Ester – Pheromone Component of Western Black Widow Females. Journal of Chemical Ecology. DOI: 10.1007/s10886-015-0582-x

 

Dinner or date?

A comparison of the vibrations transmitted by courting males and ensnared prey in two web-building spider species.

Today, I am excited to publish my first blog post about some of my own spider research! Our paper, “A meal or a male: the ‘whispers’ of black widow males do not trigger a predatory response in females”, has just been published in Frontiers in Zoology (freely available online).

This study is part of the PhD work of my friend and collaborator Samantha Vibert. In fact, we did some of the data collection and analysis for this paper during my very first semester in our lab, when I was working as an undergraduate research assistant. That was when I first began to really look closely at spiders and their incredible behaviour. My experience working with Sam that summer sparked my passion for the complexity and beauty of all of the various aspects of the private lives of spiders, which so often go unnoticed by humans.

Here is a plain-language summary of the paper, written with Samantha Vibert, and with photos by Sean McCann:

Spiders are fascinating but largely overlooked creatures, with sophisticated signalling systems involving chemical, vibratory, tactile, and in some species visual communication. A spider’s web is essentially an extension of her exquisitely tuned sensory system, allowing her to quickly detect and respond to vibrations produced by entangled prey. Not only is the web a highly effective prey-capture device, but it is also the dance floor on which prospective mates must demonstrate their desirability. The first moments after a male spider steps onto a female’s web may present a great risk, since spiders are often cannibalistic. We were interested in how a dancing male spider avoids a potentially deadly case of mistaken identity. One way that he might deal with this challenge is by transmitting vibratory signals that are very different from the vibrations produced by ensnared prey.

webs are good for this

Spider webs are highly effective prey-capture devices, so how does a courting male avoid the fate of flies like this one?

Our study species were the western black widow and the hobo spider, which are both found in British Columbia.

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A western black widow (Latrodectus hesperus) hanging from her tangle-web under a log at Island View Beach on Vancouver Island.

Black widows are in the family Theridiidae, and build complex, three-dimensional tangle-webs, while hobo spiders (family Agelenidae) build dense sheet-webs. Female black widows are much larger than males, while hobo spider males and females are closer in size.

Hobo web

A hobo spider (Tegenaria agrestis) female on her sheet web at Iona Beach, in Richmond, BC.

The purpose of our study was to describe some of the vibratory courtship signals of males in these two species, and to determine which aspects of these vibrations might allow females to discriminate between prospective mates and their next meals.

First, we recorded the vibrations transmitted through the web by courting males in both species using a laser Doppler vibrometer. At the same time, we video-recorded the male’s courtship behaviour. This allowed us to describe and analyze the different kinds of vibrations that were transmitted through the web during specific behavioural elements of each male’s courtship display. We then recorded the vibrations produced by the struggles of two types of common prey insects (house flies and crickets), on both black widow and hobo spider webs.

We found that male and prey vibrations differed more in the black widow than in the hobo spider. Hobo spider male vibrations contrasted with prey vibrations only in terms of their duration – the courting male moves around almost continuously on the female’s sheet web, while prey struggles are generally brief and intermittent. Black widow male courtship vibrations were also longer than prey vibrations on tangle webs (for the same reason), but they were also distinctive based on their generally lower amplitude and higher dominant frequencies.

To our surprise, we also found that most courtship behaviours in both species did not generate the kind of very stereotyped, complex and distinctive “songs” that have been reported in several other spider species. These species tend to court on substrates like leaf litter and plants, which most likely transmit vibrations quite differently than webs. Some male orb-weavers also produce highly rhythmic patterns during their vibratory courtship displays. So our finding leads us to wonder to what extent web architecture and complexity might constrain the transmission of the male courtship signals, and therefore the design of these signals.

One very interesting exception to the rule turned out to be the vibrations generated by the male black widow’s abdomen tremulations (an up-and down waggle of the abdomen, performed as the male hangs upside down from the female’s web). These vibrations were always very distinct from anything produced by prey: they were long-lasting and of very low amplitude, like a constant humming.

Here’s a short video of a male western black widow vibrating his abdomen on a female’s web (Supplemental File 1 from Vibert et. al 2014):

To learn more about these particularly stereotyped, ‘whisper-like’ male signals, we built our own custom web vibrator by modifying a loudspeaker. We were then able to play recorded vibrations of a male’s abdomen tremulation or a fly’s struggles back to females and observe their responses. Black widow females were much less likely to respond aggressively to vibrations played back at the “whisper-like” low amplitude of male abdomen tremulation, but attacked when we turned up the volume to levels typical of prey vibrations. This was the case regardless of which type of vibration we played. So we speculate that the males vibrate their abdomens either to avoid triggering a female’s predatory response, or even to turn it off.

Is it possible that the females that didn’t attack low-amplitude vibrations simply couldn’t detect them? We don’t think so. First, spiders are specialists when it comes to detecting even faint vibrations, and second, some females actually responded with courtship behaviour: abdomen ‘twitches’ which are similar to the male’s abdominal movements, but more emphatic. These abdomen twitches undoubtedly transmit their own vibrations through the web, and it would be very exciting to further investigate the female’s side of the vibratory ‘conversation’ during courtship.

Abdomen vibration seems to be a relatively common type of courtship behaviour and has been described in several spider families (‘abdomen wagging’ in an orb-weaver, and what has recently been described as ‘twerking’ in jumping spiders are a couple of examples). If indeed the “whispers” caused by these vibrations are involved in lowering female aggression, this might explain why such behaviour is fairly common among spiders.

The orb-weaver Argiope keyserlingi’s courthip also involves abdomen vibration, but in this species another vibratory signal was recently implicated in reducing the risk of cannibalism. The ‘shuddering’ of a courting male delays the female’s predatory response. One of the common features of black widow abdomen tremulation and these ‘shudders’ is that they are the first courtship behaviour performed by males after they enter a female’s web.

widow pair

A male western black widow courting a large, potentially dangerous female. Abdomen vibration is performed on and off throughout the male’s courtship display, starting just after the male steps onto the web, and featuring prominently during attempts to approach and mount the female.

Very little is known about the kinds of vibratory courtship signals that male web-building spiders transmit to females through their webs, except for in orb-web weaving species. We hope that this new information about vibratory communication in tangle-web and sheet-web building spiders will contribute to better overall understanding of the function and evolution of web-borne vibratory courtship signals.

References:

Vibert, S., Scott, C., and Gries, G. (2014). A meal or a male: the ‘whispers’ of black widow males do not trigger a predatory response in females. Frontiers in Zoology, 11(4).  doi:10.1186/1742-9994-11-4

Wignall, A. E., & Herberstein, M. E. (2013). The Influence of Vibratory Courtship on Female Mating Behaviour in Orb-Web Spiders (Argiope keyserlingi, Karsch 1878). PloS one8(1), e53057. doi:10.1371/journal.pone.0053057

Wignall, A. E., & Herberstein, M. E. (2013). Male courtship vibrations delay predatory behaviour in female spiders. Scientific reports3doi:10.1038/srep03557

Red-throated Caracaras: awesome birds that eat wasps

This post is not about spiders (sorry for any disappointment this may cause). Instead, it’s about some amazing (crazy?) neotropical raptors that specialize in preying on the brood of social wasps!

Red-throated Caracaras (Ibycter americanus)

Red-throated Caracaras (Ibycter americanus) in French Guiana. These birds are members of the family Falconidae, and they are specialist predators of social wasps. (photo: Sean McCann)

Getting close enough to a wasp nest to eat its contents is no mean feat–as anyone who has ever been stung by a worker wasp defending its nest can probably attest. I was privileged to be involved in a study of these birds in French Guiana, led by Sean McCann.

Today, Sean and I and all our co-authors got a belated Christmas present with the publication of our paper in PLOS ONE.

Here’s a quick video summary of our research, narrated by me and Sean:

For more information, videos, and photographs, head over to Sean’s blog, Ibycter.