Proceedings of the Royal Society B: Biological Sciences
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Male black widows parasitize mate-searching effort of rivals to find females faster

Catherine E. Scott

Catherine E. Scott

Department of Biological Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada, M1C 1A4

[email protected]

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Sean McCann

Sean McCann

Department of Biological Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada, M1C 1A4

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and
Maydianne C. B. Andrade

Maydianne C. B. Andrade

Department of Biological Sciences, University of Toronto Scarborough, Scarborough, Ontario, Canada, M1C 1A4

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Abstract

Mate-searching success is a critical precursor to mating, but there is a dearth of research on traits and tactics that confer a competitive advantage in finding potential mates. Theory and available empirical evidence suggest that males locate mates using mate-attraction signals produced by receptive females (personal information) and avoid inadvertently produced cues from rival males (social information) that indicate a female has probably already mated. Here, we show that western black widow males use both kinds of information to find females efficiently, parasitizing the searching effort of rivals in a way that guarantees competition over mating after reaching a female's web. This tactic may be adaptive because female receptivity is transient, and we show that (i) mate searching is risky (88% mortality) and (ii) a strongly male-biased operational sex ratio (from 1.2 : 1 to more than 10 : 1) makes competition inevitable. Males with access to rivals' silk trails moved at higher speeds than those with only personal information, and located females even when personal information was unreliable or absent. We show that following rivals can increase the potential for sexual selection on females as well as males and argue it may be more widespread in nature than is currently realized.

1. Introduction

Sexual selection arises when the reproductive success of one sex is limited by access to potential mates [1]. In most sexually reproducing species, males compete to fertilize the relatively limited number of eggs produced by females, and the form of competition depends on ecological factors including the distribution of potential mates in space and time [2]. In many taxa, females become sexually receptive at unpredictable spatial or temporal intervals and males increase their fitness through sequential mate acquisition rather than monopolization. The resulting mating system is ‘scramble competition polygyny’ [35]. If windows of female receptivity are brief, or the first male to mate fathers the majority of a female's offspring, selection on scrambling males is particularly intense [5]. Scramble competition polygyny can broadly affect the evolution of traits, including sexual dimorphism in development time and body size, and male speed, agility and sensitivity to female signals [4,5]. More mobile males encounter more potential mates [6], but also pay high costs [5], including energetic costs [7] and elevated mortality rates [8].

Mate-searching success is a critical precursor to mating, and only those males that successfully find females will be exposed to sexual selection via contest competition and/or female choice. Nevertheless, while studies of competition and choice are common [9], those investigating traits and tactics that confer a competitive advantage in scramble competition are scarce [1012]. This is problematic since mate searching will often function as the first filter in the episodes of selection that determine reproductive success [13]. Moreover, mate searching may affect the importance of other components of sexual selection, particularly when males' search tactics are tuned to the risk of competition over potential mates [1417]. Overall, the dearth of studies of mate searching leaves a considerable deficit in our understanding of sexual selection in nature.

Mate-searching performance will often depend on how males use information that indicates the location and status of females. Males may use personal information, obtained directly via exploration or observation, and they may also acquire social information indirectly by observing or detecting the behaviour of other animals [18]. Integrating both types of information may increase the fitness payoff for behavioural decisions [19,20], although some species use social information only when it complements personal information [19,21]. In other species, social information may be preferred [22], particularly when acquiring individual information is risky [23], or when variable environmental conditions decrease the reliability of personal information [24]. For example, spatio-temporal variation in the environment can alter the transmission or detection of information [25,26], with differential effects on the availability of personal and social information. How and when animals use personal and social information to make mating-related decisions will depend on conditions including temperature/weather, physical structure of the habitat, and ecological variables such as predation risk and the spatio-temporal distribution of conspecifics. These factors will affect the fitness benefit and feasibility of acquiring and using social compared to personal information, so information use is expected to be context-dependent [22,27].

Understanding when social information is used in mate searching is important because the range of genotypes accruing high fitness via choice or competition can be altered relative to contexts where only personal information is used [28]. For example, females commonly use social information to copy the mate choices of other females [29], and the result can be a strong reproductive advantage for relatively few preferred males [30]. By contrast, male mate-choice copying is generally unexpected, because it will increase the risk of sperm competition, particularly in species with first-male sperm precedence [29,31]. Typically, males are expected to use social information (i.e. cues of the presence of rivals) to avoid or reduce investment in females that are likely to have already mated, even if these females are otherwise preferred [1417]. In contrast with copying, this can decrease inter-male competition over fertilization and reduce variation in female mating rates. Rare exceptions occur in cases of sex role reversal (e.g. pipefish [32]), in some unusual mating systems (e.g. sailfin mollies, where males are targeted by a gynogenetic congener [33]) and in cases where the presence of other males may be a marker of costly to ascertain female receptivity (e.g. meadow voles [34]). However, most studies showing male mate-choice copying were conducted in the laboratory (e.g. [34,35], but see [33]). It is not clear whether these patterns would manifest under complex environments in the field, or in a wider range of taxa. Thus, while the use of social information by males may be context-dependent, can change the direction or intensity of selection and affect evolutionary processes [36], it is largely unknown how mate-searching males use social information in nature.

We examined mate searching and information use by males in a species with intense competition over access to females [37]. By pairing insight from observational and experimental studies in the field with focused laboratory experiments, we provide a rare, replicated examination of information use in male mate searching in complex environmental conditions. We studied the western black widow spider (Latrodectus hesperus), a sexually dimorphic species with first-male sperm precedence mediated by sperm plugs [38]. In this species, males typically develop on solitary webs, mature before females and must search for sedentary mates over complex terrain [38]. In three congeners, mate searching leads to high mortality (greater than 80% [3941]), imposing strong selection on efficient detection of and movement towards signalling females [6,42,43]. Black widow females signal receptivity using a short-lived, volatile sex pheromone and cease to be attractive or detectable during or immediately after mating [38,44]. Thus, males detecting airborne pheromones receive reliable personal information about the presence and location of unmated females. Social information is also available because male spiders trail silk ‘safety lines’ behind them as they move through their habitat [45]. Contact with the silk of other males could thus provide chemical and/or tactile social information [46] about the presence and movement paths of rivals [15].

Here, we investigated the use of these forms of personal and social information during mate searching as a function of variables affecting costs and benefits of information use. We hypothesized that mate-searching males should make context-dependent decisions about whether to use social information, and about whether to avoid or follow competitors. The detection of personal information provided by female airborne pheromones is likely to be affected by environmental variables [25,26] that will not affect the availability of social cues based on physical contact with the silk of rivals. Moreover, the fitness effect of following or avoiding rivals should depend on the local competitive environment, and trade-offs between the risk of sperm competition (favouring the avoidance of rivals), and benefits of efficient searching (favouring use of all available information to find females quickly).

We report the results of a longitudinal field study showing that scramble competition polygyny is the most likely mating system at our field site (consistent with congeners [38]). We use census data to estimate (i) the mortality rate during mate searching as a measure of the potential intensity of selection [47] on mate-searching males and (ii) the operational sex ratio (OSR; the ratio of sexually active males to fertilizable females [3]) as a measure of the degree of competition to acquire mates across the season [48]. At high OSR, competition is inevitable [49], and rapid, efficient searching for females may be favoured. At low OSR, with fewer competitors, avoiding rivals may yield the highest payoffs.

We then use experimental field studies to show that male black widows detect and respond to both personal and social information in the context of mate searching. We confirm predictions of this inference via laboratory experiments that manipulate the availability of different information sources. In these experiments, we predicted (i) in the presence of both kinds of information, males would avoid females when male silk cues (social information) suggested the presence of a competitor [15], but that (ii) when personal information was not available, males should follow the draglines of rivals as using this social information would be superior to a random search of the habitat. Thus, we expected to find plasticity in ‘mate-search copying’, despite strong first-male sperm precedence.

2. Material and methods

(a) Mating system and the intensity of sexual selection

We studied a population of western black widows (L. hesperus Chamberlin & Ivie, 1935) inhabiting a roughly 20 × 400 m area of coastal sand dunes above the high-tide line on Vancouver Island, British Columbia, Canada (48°35′10″ N, 123°22′17″ W). At this site, driftwood logs, woody debris and rocks provide microhabitats, and these features combined with low-lying vegetation covering the dunes provide challenging terrain for male locomotion. The population is dense, with on average 2–3 subadult or adult females per square metre of microhabitat during the late summer mating season [50].

From April to September 2016, we surveyed the population (see electronic supplementary material, S1 for methodological details; each data point in figure 1 represents a survey date), tracked the movement of marked males among microhabitats and webs, and used these data to estimate mortality and distances travelled during mate searching. We also used demographic data from our surveys to calculate the ratio of adult males to final-instar (subadult) or sexually receptive adult females (OSR) over 2–10-day intervals across the late summer mating season. The OSR is a widely used metric predicting the direction and intensity of competition over mates [3,48,49,51].

Figure 1.

Figure 1. Demographic data for a natural population of L. hesperus at a coastal field site on Vancouver Island, British Columbia, Canada (48°35′10″ N, 123°22′17″ W). The population was assessed in regular nocturnal surveys (individual data points) and full censuses (arrows) over the 2016 late summer mating season. We recorded the number of (a) adult males, (b) sexually receptive females, and estimated (c) the OSR (ratio of males to receptive females; see electronic supplementary material, S1 for calculation details). (Online version in colour.)

(b) Field experiments on male mate searching under varying information availability

We asked how distance from pheromone-emitting females and availability of social information affected male mate-searching success in two male-release field experiments (see electronic supplementary material, S1 for details about animal collection and husbandry). Released males moved over natural terrain (in an area without any naturally occurring females), searching for unmated, pheromone-emitting females and their webs contained in mesh cages (pheromone traps) [37,44]. We ran experiments on nights forecasted to have a southerly wind, which would place males upwind of pheromone traps (figure 2).

Figure 2.

Figure 2. Patterns of arrival at female pheromone traps under differing wind conditions in field tests of mate searching by L. hesperus males. (a,b) Number of males attracted by each caged female (black bars) relative to the male-release transect (vertical dashed lines). (c,d) Wind speed and direction during the experiment. Numbered arrows represent the wind velocity over each hour (spiders stopped arriving after 4 h in 2016 and 6 h in 2017).

Prior to release, each male was marked with a unique colour code (electronic supplementary material, figure S1) using quick-drying modelling paint (Testor's enamel), which does not decrease survivorship [39]. Male size and condition affect mate-searching success and speed in spiders [42,43], so we measured males from photographs (size) and weighed males to calculate size-corrected mass (an index of condition, 2017 only, see electronic supplementary material, S1 for details).

Mate-searching experiments (7 September 2016; 11 September 2017) commenced after sunset (L. hesperus is nocturnal), when we placed 11 pheromone traps containing unmated females and their webs along a 20 m (2016; spaced 2 m apart) or 40 m (2017; spaced 4 m apart) east–west transect (approximately perpendicular to the forecasted wind direction; figure 2). We released uniquely marked males in groups (n ∼ 20 per group, see electronic supplementary material, table S1) at 10 m intervals along a 60 m north–south transect (2016: n = 117 males; 2017: n = 130 males), starting at the 60 m point (farthest from females). In this design, the probability of encountering silk draglines of other experimental males (social information) increases with distance from the line of females.

We continuously monitored male arrival at pheromone traps by walking along the line of traps and collecting males that were on or within 15 cm of each trap (2016), or that had crossed beyond the line of traps (2017; they were considered to have found the female nearest to the crossing point). The 2016 data were informative, but using this design, males would often remain still or slowly wander between two cages, likely because they could not localize the source of the pheromone when competing sources overlapped. We changed our design in 2017 by spacing females every 4 m instead of every 2 m. Both spacing patterns are realistic challenges for mate-searching males in nature, where females may share the same microhabitat (webs within 2 m [50]) or may be spaced across different microhabitats (mean nearest-neighbour distance = 4.2 m; C.E.S. 2016, unpublished data). After each experiment, we obtained hourly records of the average wind speed and direction from a nearby weather station.

(c) Laboratory tests of context-dependent male information use

We tested the effects of personal compared to social information in the laboratory using an X-maze choice apparatus made of string which allowed males to choose or avoid the cues left by other males (figure 3a; see electronic supplementary material, S1 for experimental protocol). This set-up reduced the mobility advantage of following silk relative to field conditions by allowing spiders to walk continuously on a small-diameter substrate [52].

Figure 3.

Figure 3. Latrodectus hesperus males detect and follow the draglines of rival males in the laboratory on an X-maze choice arena, and the presence of this species-specific social information increases their speed when personal information is available simultaneously. (a) In the X-maze choice assay, the male released first (labelled ♂1) is attracted to a caged pheromone-emitting female (arrows indicate wind produced by electric fans), with choice of route determined by the male. A second male (♂2) is released after removal of the first male. The second male can choose to follow or avoid the silk of the first male. (bf) The number of L. hesperus males choosing to follow silk of conspecific males (providing social information) or heterospecific (S. grossa) males over controls. The presence of a female (providing personal information) and/or a fan producing wind is indicated by a picture of a caged female and/or grey dashed arrows, respectively. Significant differences (binomial tests; p < 0.05) are indicated by asterisks (*). (g,h) The relationship between male phenotype (leg length and body condition index) and male speed after reaching the intersection of the X-shaped choice apparatus (decision point) during the experiment shown in (b). Grey-filled circles and solid lines: males with access to silk cues of a rival (=social information); empty circles and dotted lines: males without access to silk cues.

In three laboratory experiments, we gave males the option to use or avoid the silk trails of a rival male (social information) when choosing a path of movement. We used naive males reared in the laboratory (n = 22 in each experiment). We first asked (figure 3b) whether males use social information when they simultaneously have access to personal information (fan-generated ‘wind’ blowing female pheromone towards the male), mimicking the conditions of the 2016 field experiment (figure 2c). Second (figure 3c), we tested male responses to social information in the absence of personal information but the presence of wind, which represents profitable conditions for seeking personal information (travelling upwind may result in the male encountering a pheromone plume in the field). Third (figure 3d), we asked if males would respond to the social information in the absence of personal information and wind, such that social information represents the only available cue of the location of a potential mate.

Two additional X-maze experiments examined whether males respond to the silk of conspecific male competitors in particular (with clear implications for sexual selection), or to spider silk in general (which may reflect general attraction to habitat suitable for spiders [18]). We tested male response to the silk of syntopic, confamilial Steatoda grossa (C. L. Koch, 1838) (Theridiidae) males collected from our field site (see electronic supplementary material, S1 for more details about experimental animals). First, in the presence of a pheromone-emitting conspecific female, male black widows (n = 32) were given a choice between heterospecific silk and a string-only control path (figure 3e). Next, following trials (n = 16) in which the test male avoided the heterospecific silk, a second test male was introduced to the apparatus and allowed to choose between the silk of the heterospecific male and the conspecific male (figure 3f). These trials took place outdoors in Saanich, British Columbia, at a location with no naturally occurring L. hesperus.

(d) Statistical analyses

Mate searching in the field. We ran general linear models in R (v. 3.2.0 [53]) to determine the effects of distance from females on (i) probability of arriving at a pheromone trap and (ii) average speed of males that successfully found females (time elapsed/linear distance travelled). We included male size (2016, 2017) and body condition index (2017) in these models as covariates. Models with probability of recapture as the response variable used a binomial distribution and logit link, and models with speed as the response variable used the normal distribution and identity link. We examined the residuals of each model to determine whether assumptions of normality and homoscedasticity were violated and we log-transformed male speed to meet model assumptions.

X-maze choice trials. We used binomial tests to determine whether males preferred one side of the X-maze, and multiple regression to determine whether the silk of another male affected average male speed (figure 3b). Using order (whether the male was first or second to enter the maze) as a predictor allowed us to test the effect of silk left by the first male on the second male's speed, with male size and body condition as covariates. We subsequently ran separate models using leg length and body condition as predictors because they were negatively correlated for males used in this experiment (R2 = 0.36; F1,36 = 20.21; p < 0.0001).

3. Results

(a) Operational sex ratio, mate-searching success and mortality in nature

We recorded the location and developmental stage of 461 females, and marked and tracked 290 males that arrived at females' webs throughout the active season for this population (April–September 2016). Our data show that males mature earlier than females (protrandry; figure 1), consistent with a scramble competition mating system [5]. Most sexual activity occurred in August, when males were most abundant. During this period, the OSR was consistently male biased (greater than or equal to 5 : 1; figure 1c), and a conservative estimate indicates fewer than 15 receptive females were available on most nights (figure 1b; electronic supplementary material1). The number of mate-searching males decreased in September and the OSR was lower, but remained male-biased (1.2 : 1–7.5 : 1) until just before the end of the active season when few males remained and the OSR became slightly female-biased (figure 1c).

The risk of mortality during male movement between webs was approximately 88%. We estimated male survival in three ways: (i) 2/16 males (12.5%) first marked on their juvenile web were later found on females’ webs, (ii) 33/274 (12%) males first marked on a female's web were later found on a second female's web, and (iii) 4 of those 33 males (12%) were subsequently found on a third female's web. Most males were found on webs within 60 m (=more than 9000 body lengths for males of maximum size 6.5 mm) of their point of origin, but some moved more than 200 m (electronic supplementary material, figure S2).

(b) Mate-searching success, speed and social information use in the field

During the male-release study in 2016 (figure 2), the wind speed was high (mean speed 13 km h−1) and its direction relatively consistent (as forecast, figure 2c). Mate-searching success rates were high (62%), with males at all distances (up to 60 m) equally likely to find females (figure 4a; electronic supplementary material, tables S1 and S2). Successful males were distributed across the webs of females such that 73% of available females attracted at least one potential mate, with females most directly upwind from the male-release vector attracting the most males (figure 2a,c). This pattern is consistent with males using personal information (volatile pheromones) to find females over long distances. By contrast, during the 2017 replicate, the wind was weak (mean speed 4 km h−1) and its direction variable, so consistent personal information was not likely to be available (figure 2d) [54]. Overall searching success was much lower (26%) and decreased with distance such that no males released at 50 or 60 m were successful (figure 4b; electronic supplementary material, tables S1 and S2). Moreover, only 46% of females attracted at least one male, and there was strong coincidence of male attraction, with 82% of successful males arriving at the web of the single female directly in line with the release vector (and the wind direction during the first hour of the experiment; figure 2b,d). In this second replicate, males had access to personal information primarily during the first 2 h of the experiment, after which wind conditions would have limited transmission (figure 2d) [54]. Nevertheless, more than half of all males (53%) that found females arrived after the first 2 h, when personal information was unavailable or inconsistent. A third, modified version of this experiment showed comparable results under similarly weak and variable wind conditions (electronic supplementary material, figures S3 and S4 and table S4).

Figure 4.

Figure 4. Distance from females and the size of L. hesperus males affect mate-searching success and speed in the field. Graphs show relationships between distance from pheromone-emitting females and (a,b) recapture rate, and (c,d) average speed of recaptured males. Lines are predicted fits (solid) and approximate 95% confidence intervals (grey dashed) from general linear models of the data. (a,c) Results from the 2016 replicate when the wind was strong and relatively consistent in direction (figure 1b). (b,d) Results from the 2017 replicate when the wind was relatively weak and highly variable in direction (figure 1d). (c,d) Speed was log-transformed for analysis with back-transformed predicted fits displayed on the raw data.

 The maximum average speeds attained by males under poor wind conditions (less than 0.25 m min−1 in 2017) were much lower than those of males searching in strong, consistent wind (maximum speeds of greater than 1.25 m min−1 in 2016; figure 4c,d). Surprisingly, males moved more quickly when released further away from females, particularly in 2016 when personal information was consistently available (figure 4c,d; electronic supplementary material, table S2). Since we released males in sequence from 60 to 10 m from females, those males released from the furthest points were more likely to encounter silk and had access to longer pathways of rival silk (produced by males released slightly later, but closer to females). We frequently observed males moving along silk at a rapid pace, much more so than when traversing the ground or vegetation. We infer that the availability of social information and/or silk pathways facilitates efficient locomotion.

(c) Context-dependent male responses to social information in the laboratory

When males were released in the X-maze in the presence of directional personal information (pheromones and wind) and given the choice between a string-only path and string covered in conspecific male silk, 95% (n = 22) of males chose to follow the silk of the rival male (p = 0.00001; figure 3a,b). When only social information was available (silk and wind but no pheromones), all males (n = 22) readily moved upwind but showed no preference for following silk (59% chose the silk path; p = 0.53; figure 3c). In the absence of wind and pheromones, however, 82% (n = 22) of males followed silk trails left by conspecific rivals rather than the control path (p = 0.004; figure 3d).

These responses were specific to conspecific rivals. Latrodectus hesperus males showed no response to the silk of S. grossa males relative to control strings (p = 0.5; figure 3e), and 87% (n = 15; one male did not make a choice) preferred to follow conspecific male silk when both were present (p = 0.007; figure 3f).

In the laboratory, males with access to silk draglines moved towards females more quickly than those with only personal information (model with size as covariate: t = 1.9, p = 0.065; model with body condition as covariate: t = 2.3, p = 0.028; figure 3g,h; electronic supplementary material, table S3), indicating that social information coupled with personal information allows males to find females more rapidly, as in the field.

4. Discussion

Our results show that L. hesperus males follow the silk of male conspecifics during mate searching, that this response is specific to information left by potential sexual rivals and that males use social information to determine movement pathways if there is no possibility of acquiring personal information (e.g. no wind). Furthermore, social information combined with personal information allows males to find females most efficiently. Contrary to our predictions, when males paid attention to social information, they consistently followed the silk of other males, with little evidence for variation in how the information was used.

Despite increasing the risk of contest and sperm competition, following the silk trails of rivals is likely to be adaptive for males in this population since it increases the speed and efficiency of finding females, but is not likely to significantly increase the risk of competition. The extremely male-biased OSR (greater than 10 : 1) during the peak mating period, along with very few receptive females available on any given night, means intense competition over females is inevitable. Moreover, few males (12%) survive the trip to find a female, so there is strong selection on males to succeed, even if they then have to compete with an accumulation of males for the few females that are sexually receptive on a given night. Although being first to mate is important for success in sperm competition in this species, being the first male to arrive at a female's web is not critical. Courtship may last several hours before the first copulation occurs [38], so later-arriving males may still be successful at mating if they win in contest competition or are chosen after assessment by the female. In fact, later-arriving males may be more likely to mate if the courtship effort of rivals has already induced female receptivity by the time they arrive at the web [55].

Our results indicate that social information (chemical and/or structural cues) on the silk of earlier searchers can allow a male to find a female efficiently even when access to personal information (volatile sex pheromone) is minimized. This is consistent with the prediction that animals should use social information when personal information is out of date, unreliable or expensive to obtain [2224,27]. Male black widows apparently move upwind to seek personal information (figure 3c), but when weak or absent wind makes detection of a pheromone source difficult or impossible, they can nonetheless find females by following the silk trails of rivals (figures 2b,d and 3d). After initially detecting a female using personal information, social information produced by an early-arriving rival will allow a following male to rapidly catch up, even if wind conditions change. This may be of particular benefit to L. hesperus males because the signals of sexually receptive females are ephemeral. Courting males can interfere with transmission of the female's signal by altering the web [37], rendering the female unattractive or undetectable during and after mating [38,44].

Social information increased male mate-searching efficiency when personal information was available simultaneously. Males that had access to conspecific silk moved more quickly; an effect that was suggested in the field and reproduced experimentally in the laboratory (figures 3g,h and 4c,d; electronic supplementary material, tables S1 and S2). Similarly, synergistic effects when combining social with personal information have been found for foraging ants and bumblebees [20,21] and for birds gauging brood parasitism risk [19]. The recency of social information is also likely to be important [56,57]; the benefit of following a rival should decrease with time since his trail was produced. Chemicals on draglines may provide reliable information precisely because the cues are transient; in other species, silk-borne chemicals are water soluble and washed away daily by dew [58]. Regardless, in the absence of any personal information about the location of receptive females, the expected payoff from following an old trail to an already-mated female and obtaining some share of paternity is likely to exceed the payoff from searching randomly for unmated females with the attendant risk of mortality or failure.

In scramble competition polygyny, males maximize their fitness via sequential mate acquisition rather than mate monopolization [12,34]. In these systems, more active males that move longer distances will encounter more females [4,5], but because mate searching is energetically costly [7] and increases predation risk [8], mating success should be higher for males that spend less time searching. Scramble competition can severely impair the evolution of choosiness [59] and mate-choice copying can be adaptive when mate choice is costly [28,30]. We expect that using social information to find females efficiently should also be adaptive in other taxa with scramble competition polygyny, particularly when the costs of searching are high. Since scramble competition is common across taxa, including arthropods [5], amphibians [6], reptiles [60], fishes [61] and mammals [34], this may be a common tactic that is currently unappreciated.

The use of inadvertent social information (produced as a byproduct of locomotion) by male black widows to locate females is best described as an example of local enhancement, where an observer's attention is drawn to a location or resource (here a potential mate) by the activity of demonstrators [18,23]. Mate-choice copying requires animals to observe mating interactions directly and is typically discussed in terms of animals using public information (a subset of social information) to assess the quality of a potential mate (whose location is already known) [18,29]. Nevertheless, the consequence of social information use by L. hesperus males (mate-search copying) is likely to be similar to that of mate-choice copying. Male movement towards a particular female will increase the rate at which subsequent males approach, and ultimately attempt to mate with that same female [29]. Widespread use of this tactic may result in some females remaining unmated, increasing the variance in female mating success ([28,30]; figure 2a,b) and altering the strength and direction of selection. This has implications for the evolution of both male and female traits and the potential for sexual selection on female signals [62].

Data accessibility

Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.cv66j09 [63].

Authors' contributions

C.E.S. and S.M. designed the study and collected the data with consultation from M.C.B.A. C.E.S. analysed the data and made the figures. C.E.S. and M.C.B.A. wrote the manuscript with input from S.M. All authors gave final approval for publication.

Competing interests

The authors declare no competing interests.

Funding

Funding was provided by the Natural Sciences and Engineering Research Council of Canada (Discovery grant no. RGPIN-2017-06060) and the Canada Research Chairs Program (both to M.C.B.A.); C.E.S. was supported by an NSERC CGS-D during the study. The 2017 fieldwork was funded by the Toronto Entomologists’ Association Eberlie Grant (to C.E.S.), the Experiment.com Arachnid Challenge Grant and many generous individual donors to the ‘Team Black Widow’ crowdfunding campaign.

Acknowledgements

We thank the Tsawout First Nation for permission to do fieldwork on their lands. J. Carrière, M. Guarrasi, J. Hoac, Z. Kerami, V. Nguyen, B. Robinson, R. Santos, M. Swift, T. Truong and N. Wong collected the data for the first laboratory experiment. P. K. McCann and C. and D. Copley provided housing and other logistical support during fieldwork. We also thank M. Boers for help with data wrangling, J. T. Cullen for access to his microbalance and G. I. Holwell, G. S. Blackburn, D. T. Gwynne, A. C. Mason, L. Baruffaldi and Z. Luo for helpful comments on the manuscript. This study was completed in partial fulfilment of the requirements for a PhD in the Department of Ecology and Evolutionary Biology (University of Toronto).

Footnotes

Electronic supplementary material is available online at https://dx.doi.org/10.6084/m9.figshare.c.4576433.

Published by the Royal Society. All rights reserved.

References