Unexpected male choosiness for mates in a spider Bel-Venner et al. Electronic Supplementary material Size-Assortative Pairing as an unbiased Result of Male Mate Choice




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Unexpected male choosiness for mates in a spider

Bel-Venner et al.
Electronic Supplementary material
Size-Assortative Pairing as an unbiased Result of Male Mate Choice
In our study, alternatives other than male mate choice (i.e. temporal and spatial heterogeneity, contest competition between males, female choice, female density, and sampling bias) that might explain assortative pairing could be excluded as follows (Crespi 1989; Rowe & Arnqvist 1996):

Temporal heterogeneity. Adult emergence of males and females was normally distributed over the mating season at both sites (July-September). By means of polynomial regression models, we fitted the size of adult males against the first day of their settlement near a female and the size of females against the day of their adult moult. Early-emerging adults were larger than late-emerging ones, which seems to be a common pattern among spider species (Table A1; Miyashita 1993; Schneider 1997; Higgins 2000). In all our analyses, we removed the temporal covariation in size of males and females by using residuals computed from polynomial regression models (Table A1) instead of absolute body size values. This allowed us to determine the actual competitive ability of a male relative to other males of the site, and the actual quality of a female about to moult relative to females available in the site.
Spatial heterogeneity. We checked for spatial homogeneity of both male and female body sizes at each study site. Since females were sedentary, the dataset comprised only one position (x,y) per individual. At each site, we set up a neighbourhood based on Gabriel’s graph (Jaromczyk & Toussaint 1992). To test spatial autocorrelation of residual sizes, we then computed Moran’s index based on 999 permutations under the null hypothesis of random distribution. The males’ neighbourhood matrix was built up in a slightly different way since several positions could be sampled per male at various dates. For each male, we built up his trajectory composed of all his locations irrespective of his activity, and defined a two meter-wide buffer zone around each position, which reflects the median distance a male can travel per night. Two males were considered to be neighbours whenever their trajectories overlapped during the same day. Adult males, as well as subadult females, were randomly spatially distributed according to their residual size at both sites (Females at LC: N=72, Moran’s Index=0.033, P=0.32; Females at HC: N=53, Moran’s Index=-0.0544, P=0.63; Males at LC: N=68, Moran’s Index=0.0489, P=0.19; Males at HC: N=186, Moran’s Index=0.0236, P=0.09).
Contest competition between males. In analyzing size-assortative pairing among all pairs sampled during our recordings, we considered only those formed by a female and the first male to guard her. This pairing pattern does not depend upon competitive interactions. Because males moved mainly at night, they were also more likely to encounter females at that time. At both sites we conducted nocturnal scans (held on 10 non consecutive nights from 0000 to 0500 hours) to detect encounters between roaming males and subadult females that had stopped building webs and had never been visited by any male. Whenever an encounter was detected, we identified the male and the female through their visual markings. We observed 15 nocturnal male-female encounters and pairings (6 and 9 at the low and high-competition sites, respectively) and checked the pairing pattern the next morning. All of the males settled near their mates without interacting with any other male, revealing low competition between males in the course of the first settling. Twelve out of 15 males involved in nocturnal visits were marked and all of them were still guarding their mate on the following morning. Therefore, we considered that the first pairs diurnally sampled did actually not depend upon competitive interactions between males.
Female choice. Among spiders, adult females may be selective of males or their sperm (Eberhard 1996; Kotiaho et al. 1996; Elgar 1998; Schneider & Lubin 1998; Elgar et al. 2000; Maklakov et al. 2005; Prenter et al. 2006). Resistance, aggressiveness, and even cannibalistic behaviour of adult females towards males are commonly reported, mainly among dimorphic species where females are larger than males and whenever females can interact with males courting them on their webs (Elgar 1991; Vollrath & Parker 1992; Maklakov & Lubin 2004; Prenter et al. 2006). By contrast, very few studies documented interactions between subadult females and adult males, despite the common occurrence of pre-copulatory mate guarding of pre-moult females in spiders. It seems, however, that subadult females exert little impact on the outcome of pre-mating encounters (Fahey & Elgar 1997; Holdsworth & Morse 2000). In our study, males competed to guard subadult females which were smaller than adult males (Table A2). Furthermore, females were no longer building webs at the time males visited them (Bel-Venner & Venner 2006). Subadult females therefore, seem unable to chase males that settled down close to their retreat. This was confirmed by our field observations (more than 500 and 50 hours of diurnal and nocturnal observations, respectively); we never observed any agonistic interaction between subadult females and their visitor or guarding male. Finally, newly moulted adult females that still had no functional web offered no resistance to males. Males started courting them soon after moult completion and achieved copulation when they were still vulnerable (Bel-Venner & Venner 2006, Schneider & Lubin 1998).
Female density. We computed a weekly score of female density at both sites through the ratio of the total number of females that moulted as adults within a week on one site, to the total surface area of the site. For consistency in our results, we considered female density only for the period when adults were most abundant (see operational sex-ratio). Density of females was homogeneous between the two sites (NLC=NHC=6; Mann-Whitney U-test: U=13, z=-0.80, P=0.42).
Sampling bias. The paired, as well as solitary, spiders sampled in this study were representative of their sites because: 1) the two-dimensioned sites offered no hiding place for spiders; and 2) pre-copulatory mate guarding is a long-lasting (usually several days), conspicuous behaviour (Bel-Venner & Venner 2006).
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Table A1. Seasonal variation of spiders’ adult body size at the Low- and High-Competition sites (LC and HC, respectively).

Site

Sex

N

Best-Fitted Model*

R2

df1, df2, F

P-value

LC

Male

48

size=-0.0067.day+2.1637

0.117

1, 46, 6.11

0.017

Female

74

size=0.0003536.day2-0.0495.day+3.574

0.466

2, 71, 30.17

<0.0001

HC

Male

95



0.009

1, 93, 0.80

0.372

Female

50

size=-0.0003279.day2+0.0112.dayt+2.0872

0.238

2, 47, 7.05

0.002

* We regressed body size of males against the first day of their settlement near a female, and body size of females against the day of their adult moult; day 1= 1st July. Body size of guarding males did not change throughout the season; mean body size +/-s.e.=1.856+/-0.026 mm.

Table A2. Body size characteristics of adult males and subadult females at the Low- and High-Competition sites (LC and HC, respectively)

Site

Body size Parameter

Median +/-interquartile mm (N)

Mann-Whitney test

P-value







Subadult females

Adult males

U

z




LC

Prosoma Width

1.54+/-0.07 (74)

1.75+/-0.40 (68)

1386

-4.62

<0.0001

Tibia Length*

2.09+/-0.21 (74)

3.50+/-0.90 (67)

4

-10.22

<0.0001

HC

Prosoma Width

1.56+/-0.06 (50)

1.73+/-0.40 (188)

1857

-6.59

<0.0001

Tibia Length*

2.09+/-0.24 (49)

3.50+/-0.83 (188)

135

-10.47

<0.0001

* Tibia length of the first pair of legs (right leg)


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