DOI: 10.5061/dryad.bg79cnpjg
title: Vibrating aggression: Spider males perform an unusual assessment strategy during contest displays

description: A recurrent question in animal contests is whether individuals adopt a
 self or mutual assessment rule to decide to withdraw from a contest.
 However, many empirical studies fail to find conclusive support for one of
 these two possibilities. A possible explanation is that assessment
 strategies vary between individuals. In the contests of the orb-web spider
 Trichonephila clavipes, males perform a vibrational display on webs that
 may escalate to physical contact. Since all individuals perform the
 vibrational phase and only some of them escalate, we proposed two
 hypotheses: 1) all individuals perform mutual assessment during the
 vibrational phase, or 2) some individuals that do not escalate adopt
 self-assessment, while individuals that escalated adopt mutual assessment.
 To evaluate these hypotheses, we investigated the relationship between the
 duration of the vibrational phase and frontal leg length (a proxy of male
 fight capacity) of loser and winner males in contests that escalated and
 did not escalate to the physical contact phase. We found a non-significant
 relationship between duration and losers leg length for both contests that
 escalate and did not escalate. While we found a positive relationship
 between duration and winners leg length, particularly in contests that did
 not escalate. These results do not provide support for mutual assessment
 or for a mix of different assessment rules among individuals. We suggest
 that in T. clavipes, the dynamics of the vibrational phase may be
 explained by two different contest strategies (opponent-only assessment or
 size-based aggressiveness) that are dependent on intruder motivation to
 escalate.
description: <strong><em>Study
 area</em></strong> We conducted this study
 between December 2018 to March 2019 and between February 2021 and March
 2021 in the Ecological Station of the Federal University of Minas Gerais
 (19° 52’ 25’’ S, 43° 58’ 21’’ W), Minas Gerais State, Brazil. The
 Ecological Station is a 114 ha area composed of a mixture of
 semi-deciduous Atlantic forest, Brazilian savanna (i.e., Cerrado), and
 invasive plant species (Antonini and Martins 2003). Individuals of
 <em>T. clavipes</em> are abundant in this area between
 December and June (Almeida and Peixoto 2021; Silva et al. 2021).
 <strong><em>Field
 sampling</em></strong> We searched for
 female webs along three trails of approximately 20 m, 300 m, and 400 m in
 length. We selected female webs with mature females or females in their
 last instar because males of <em>T. clavipes</em> defend
 females in both ontogenetic stages (Almeida and Peixoto 2021). We
 considered females that had a swollen and sclerotized epigynum with two
 distinct copulatory openings to be mature (Higgins 2000; Rittschof 2011),
 and females with an abdominal diameter greater than 6 mm (Moore 1977) and
 without a sclerotized epigynum to be in their last instar. Furthermore,
 for last instar females, we also observed them for seven days after the
 sample to confirm they did not make an additional molt. We sampled only
 female webs with at least two males established (16 webs with two males,
 10 webs with three males, seven webs with four males and one web with five
 males). We defined the central male as the individual located up to 5 cm
 from the female's abdomen (Christenson and Goist 1979) and peripheral
 males as the other males that were established inside the female web up to
 5 cm distant from the last web spiral (Christenson and Goist 1979; Brown
 1985). We did not sample female webs in which males were all established
 between the central and the peripheral position, because this avoided the
 identification of the central male. In addition, all sampled males were
 sexually mature as assessed through dark brown coloration and sclerotized
 pedipalps (Myers and Christenson 1988). In other
 empirical studies that reported male contests in <em>T.
 clavipes</em>, the rival with the larger body size typically wins
 the fight (Christenson and Goist 1979; Vollrath 1980; Rittschof 2010). In
 these studies, the measure chosen as a surrogate of body size is the
 cephalothorax width (Rittschof 2010; Constant et al. 2011). However, it is
 important to note that <em>T. clavipes</em> males do not use
 their cephalothorax directly in fights, and for this reason, it may be
 that cephalothorax width is related to winning chances due to its
 correlation with other traits that determine the outcome (Vieira and
 Peixoto 2013). A possible surrogate of fighting capacity that is directly
 used in <em>T. clavipes</em> fights is the male frontal legs
 (Christenson and Goist 1979; Constant et al. 2011). Males use the frontal
 legs to perform vibrations on the web and also during the physical contact
 phase. In fact, there is empirical evidence that central males have longer
 frontal leg length than peripheral males in webs of females (Silva et al.
 2021 – Supplementary Material). Furthermore, in our data, we also found
 that longer frontal legs increased the chances of victory (Supplementary
 Material, Fig. S1). Therefore, we considered the frontal leg length as a
 reliable trait to estimate the fighting capacity of <em>T.
 clavipes</em> males. We estimated the leg length of males by
 photographing (using a Canon EOS Rebel T5) the male with a ruler
 positioned adjacent (but without touching the male). Then we used the
 ImageJ software to measure the leg length based on the scaled photograph
 (Schneider et al. 2012). The agonistic interactions
 between <em>T. clavipes</em> males generally occur when the
 female is feeding on a prey (Farr 1976; Christenson and Goist 1979;
 Christenson et al. 1985; Cohn et al. 1988). Therefore, to induce
 interactions between males, we carefully placed a grasshopper on the
 female web (Silva et al. 2021). After placing the grasshopper, we waited
 for 10 min to observe if the female captured the prey and the males
 started to interact. To define the beginning of the interaction between
 males, we used the description provided in other studies on agonistic
 interactions of <em>T. clavipes</em> males (Christenson and
 Goist 1979; Rittschof 2010; Constant et al. 2011; Silva et al. 2021). The
 interaction started when the first male began vibrating the female web
 (i.e. when a male used the first pair of legs to pull and release web
 threads) and the other male started to vibrate in response (Silva et al.
 2021). We considered the end of the interaction when one of the males fled
 to the periphery of the web or was knocked down from the female web. We
 recorded all the interactions between males using a voice recorder and
 registered the duration of each phase of each interaction. At the end of
 the interaction, we also registered which individual won (i.e. established
 in the central position) and lost (i.e. established in the peripheral
 position) the contest. To avoid pseudoreplication, we sampled only one
 interaction per female web. We also marked males and females with
 non-toxic paint on their abdomens after the end of the interactions to
 avoid resampling them (Christenson and Goist 1979; Constant et al. 2011;
 Rittschof 2011). We did not mark the males before they interacted to avoid
 stressing them before the observations. In addition, males of this species
 move very slowly during the interaction and it was possible to track the
 winner and the loser without marking them. Even in interactions that
 escalated to physical contact, males rarely changed positions on the web
 (the peripheral male won the interaction in 2 contests), which allowed us
 to correctly identify each one. In specific cases in which females
 disrupted the male-male interaction or males ran into other female webs
 before returning to the web in which they were fighting, we discarded the
 observation from our sample. We observed a total of 34 interactions
 between males. Half of these interactions occurred in webs of mature
 females and the other half in webs of females in the penultimate
 instar. <strong>Statistical
 analysis</strong> To evaluate whether all (or
 most) males perform mutual assessment or whether there is a mixture of
 self- and mutual assessors during the vibrational phase, we built two
 general linear mixed models (GLMMs) - one for winners and the other for
 losers. We considered the duration of the vibrational interaction as the
 response variable, and the male front leg length and escalation occurrence
 as explanatory variables. We also log-transformed the duration of the
 vibrational phase to reach homogeneity of variances. To control for a
 potential effect of female quality on the motivation of each individual to
 fight (Silva et al. 2021), we inserted female age (juvenile or mature) in
 both models as a random variable (the inclusion of female age as a fixed
 factor provided similar results).<strong>
 </strong>In<strong> </strong>each model, we inserted
 frontal leg length and escalation occurrence both as independent factors
 and an interaction term between them. Based on this, if all males perform
 mutual assessment during the vibrational interactions, we expect the
 interaction term to be non-significant, while the relationship between the
 duration of the vibrational interaction and frontal leg length should be
 positive for loser males and negative for winner males (Taylor and Elwood
 2003). However, if some males perform mutual assessment while others
 perform self-assessment, we expected the interaction term to be
 significant. The relationship between the duration of the vibrational
 interaction and frontal leg length should be positive for loser males and
 negative for winner males in interactions that escalated to the physical
 aggression phase and positive for losers, but non-related to winner’s
 frontal leg length in interactions that did not escalate. To provide a
 measure of how much variation in the duration of the vibrational phase is
 explained by our predictor variables, we calculated the marginal and
 conditional pseudo-R<sup>2</sup> for mixed-effects models as a
 non-dimensional standardized measure of goodness of fit (Nakagawa and
 Schielzeth 2013). As additional information to
 investigate whether all males performed mutual assessment or whether the
 population was composed of a mix of self and mutual assessors, we
 performed a second analysis. If the fights that do not escalate are
 composed of mutual and self-assessors, the variation in the contest
 duration explained by winner leg length should be higher when compared to
 escalated contests that are predicted to be composed of mutual assessors.
 To estimate this difference, we calculated the coefficient of
 determination R<sup>2</sup><sub>resid</sub> (Ives
 2019) separately for two general linear mixed models (GLMMs) using the
 front leg length of winner males as an explanatory variable - one
 considering the initial contest phase of fights that escalated and the
 other the initial contest phase of fights that did not escalate to the
 physical contact phase. We also used the female age as a random factor to
 control possible differences in interactions for females in different
 reproductive stages. The
 R<sup>2</sup><sub>resid</sub> is used to report
 the variance explained by a fixed variable in GLMMs (Ives 2019). Values of
 R<sup>2</sup><sub>resid </sub>closer to 1 indicate
 a stronger correlation between the predictor variable and the response
 variable used in the model (Ives 2019). We chose
 R<sup>2</sup><sub>resid</sub> because it is
 appropriate to compare different data sets using the same model and is
 minimally affected by differences in sample size (Ives 2019). According to
 our first hypothesis, if all males perform mutual assessment, we expect
 contests that escalate and that do not escalate to have a similar
 relationship between initial phase duration and winners' fighting
 capacity. Therefore, we expect a similar
 R<sup>2</sup><sub>resid </sub>for the models
 testing the relationship between the duration of the vibrational phase and
 front leg length considering non-escalated fights and the same model
 considering only fights that escalated to the physical aggression phase.
 On the other hand, according to our second hypothesis, the vibrational
 phase in contests that do not escalate should be composed of a mix of self
 and mutual assessors, while contests that escalate should be composed of
 mutual assessors. This would lead to a smaller
 R<sup>2</sup><sub>resid </sub>value for the model
 testing the relationship between the duration of the vibrational phase and
 front leg length for non-escalated fights when compared to the same model
 considering only fights that escalated to the physical contact phase. To
 perform our analyses, we used the packages "lme4" (Bates et al.
 2014), “MuMIn” (Barton 2022), “rr2” (Ives 2018) in the R software (R Core
 Team 2023).
description: # Vibrating aggression: spider males perform an unusual assessment
 strategy during contest displays
 [https://doi.org/10.5061/dryad.bg79cnpjg](https://doi.org/10.5061/dryad.bg79cnpjg) In this study we evaluated whether an initial non-contact vibrational interaction performed by males of the spider *T. clavipes* represents a contest phase in which all rivals perform mutual assessment or a mixture of self- and mutual assessors. If our hypothesis that all rivals perform mutual assessment was correct, we expected to find a positive relationship between the duration of the vibration interaction and losers frontal leg length and a negative relationship between the duration of the vibrational interaction and frontal winners leg length. We would also expect a similar R2resid for the model testing the relationship between duration and winners front leg length for non-escalated fights when compared to the same model considering only fights that escalated. On the other hand, if our hypothesis about a mixture of self- and mutual assessors was correct, we expected different patterns for contests that escalated and did not escalate. For contests that escalated, the relationship between the duration of the initial contest phase and individual leg length should be positive for losers and negative for winners. For contests that did not escalate, this relationship should be positive for losers and non-significant for winners. In addition, we also expect a smaller R2resid value for the model testing the relationship between the duration of the vibrational phase and winners front leg length for non-escalated fights when compared to the same model considering only fights that escalated to the physical contact phase. We built two general linear mixed models (GLMMs) - one for winners and the other for losers. In addition to two general linear mixed models (GLMMs) using the front leg length of winner males as an explanatory variable - one considering the initial contest phase of fights that escalated and the other the initial contest phase of fights that did not escalate to the physical contact phase. For these models, we calculated the coefficient of determination R2 resid (Ives 2019) to estimate the difference in variation in the contest duration that do not escalated compared to escalated contests. We found that, for loser males, the duration of the vibrational interaction was unrelated to the front leg length for both interactions that escalated and did not escalate to a physical contact phase. For winner males, the greater R2resid in contests that escalated in relation to contests that did not escalate matched with our prediction for the mixture of self- and mutual assessors. However, the duration of the vibrational interaction was unrelated to the front leg length in contests that escalated, but positively related to front leg length in interactions that did not escalate, which does not match our predictions. Therefore, the results we found do not follow the predictions from self or mutual assessment (Taylor and Elwood 2003). We suggest that in T. clavipes, the dynamics of the vibrational phase may be explained by two different contest strategies (opponent-only assessment or size-based aggressiveness) that are dependent on intruder motivation to escalate. ## Description of the data and file structure Data used is included in two csv files: leglength_2 and fight. ·      “leglength_2” was used to analyze the probability of males of Golden orb-web spider (*Trichonephila clavipes*) winning a contest in relation to the frontal leg length (cm). The dataset contains 9 columns: **1.**    **length**: Leg length of each male in centimeters. **2.**    **idmal**: Each male identity **3**.    **idweb**: The identity of each web that the contest was recorded. ·      “figth” was used to analyze 1) whether all (or most) males perform mutual assessment or whether there is a mixture of self- and mutual assessors during the vibrational phase. The dataset contains 17 columns: **1.**   **Lengthcentral:** Leg length of the central male in millimeters **2.**  **LengthcentralCM**:Leg length of the central male in centimeters **3.**  **Lengthperi:** Leg length of the peripheral male that fought with the central male in millimeters **4.**  **LengthperiCM:** Leg length of the peripheral male that fought with the central male in centimeters **5.**  **moduledif**: The difference in centimeters between Leg length of central and peripheral male in value in module **6**.  **lengthdif:** The overall difference in centimeters between Leg length of central and peripheral male in value **7**.  **escalation:** Information if the contest of that particular male escalates to a physical contact phase **8**.  **time:** Duration of the contest in seconds **9.  winnerID:** Identity of the winner of the contest based on his initial position in females web (central or peripheral male) **10. loser:** Leg length of the loser male in millimeters **11.**  **loserCM:** Leg length of the loser male in centimeters **12. winner:** Leg length of the winner male in millimeters **13.**  **winnerCM:** Leg length of the winner male in centimeters **14**.   **femalestatus:** The reproductive status of the female on the web in which the contest occurs **15**.   **time_esc:** The duration of the physical contact phase in seconds, if it occurs (in case that the contest did not escalate "NA"s are filling the cells) **16.**   **nmales:** Total number of males on the web in which the contest occurs ## Code/Software The annotated R script used in the study De Almeida et al. " Vibrating aggression: spider males perform an unusual assessment strategy during contest displays." is provided. The R packages used are: "lme4" Bates D, Mächler M, Bolker B, Walker S (2015). “Fitting Linear Mixed-Effects Models Using lme4.” Journal of Statistical Software, 67(1), 1–48. doi:10.18637/jss.v067.i01.\ Barton, K., & Barton, M. K. (2015). Package ‘mumin’. Version, 1(18), 439.\ Ives, A. R., & Li, D. (2018). rr2: An R package to calculate R2s for regression models. J. Open Source Softw., 3(30), 1028. The R version used were R version 4.3.2 (2023-10-31) "Eye Holes": R Core Team (2023).