The Roles of Feeding State, Aggression and Habitat Structure on Group Foraging in A

The Roles of Feeding State, Aggression and Habitat Structure on Group Foraging in a

California Orb-Weaving Spider

A thesis submitted to the

Graduate School

of the University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Master of Sciences

in the Department of Biological Sciences

of the College of Arts and Sciences

Mark L. Tiemeier

B.S. University of Dayton, 2005

November 2011

Committee Chair: George W. Uetz, Ph.D.

Cincinnati, OH

Abstract:

Spiders tend to be solitary and aggressive, thus species which exhibit social behaviors offer opportunities to explore the evolution of sociality. Metepeira spinipes is an orb-weaver from Mexico and the California coast that displays facultatively colonial foraging behavior, as the frequency and size of colonies vary with food availability. This species appears to exhibit risk-sensitivity in foraging, as past studies have shown that colonial foraging reduces variance in the amount of prey captured. As a result, solitary behavior is seen in prey poor sites and aggregation is seen in areas of high food availability. However, these population-level phenomena are likely the consequence of individual foraging decisions and interactions between spiders. In this thesis, I have explored the role of feeding state in the spiders’ decision whether to forage solitarily or colonially. In field experiments, I manipulated the food intake of spiders and found that well fed individuals ( ad lib feeding) are significantly more likely to join a nearby colony than starved spiders. Additionally, I examined the interactions between colony resident spiders and diet-manipulated spiders added to their webs. I found that aggression is not associated with joining colonies and that feeding state and body condition are not associated with the likelihood to escalate encounters or overtake webs. As aspects of the microhabitat such as food availability and vegetation structure are known to affect foraging behavior in web building spiders, I also undertook a pilot study exploring the effect of vegetation structure on grouping in M. spinipes . I found that colonies are located in larger cavities within vegetation that is less dense and complex than solitary webs and colonies use fewer silk attachments per spider. Results from this study indicate that individual M. spinipes exhibit risk sensitive behavior in foraging, which leads to occurrence of colonial web groups in ii

areas of higher food availability. While aggressive behavior on the part of residents and/or intruders plays no apparent role in colony formation, the role of habitat structure in grouping may be a productive area for future study.

iii

Acknowledgements:

This manuscript and the research it reports were made possible through the support of many people. First, I am extraordinarily grateful to Dr. George Uetz for the chance to work on this project. He provided knowledge, support and assistance throughout my time at the

University of Cincinnati and his mentoring has helped me become a better scientist. I would also like to thank Dr. Ann Rypstra and Dr. Eric Maurer for serving on my committee and providing valuable feedback. I owe a large debt of gratitude to the Zaidain family who housed my spiders and me during my research trips to Half Moon Bay. In addition, I would like to express appreciation to my lab mates throughout my time at U.C. Jenai Rutledge, Brian

Moskalik, Shira Gordon, Andrea Plunkett Orton, Alex Sweger, Rachel Gilbert, Brent Stoffer and

Liz Kozak each offered intellectual, logistical and emotional support in their own unique ways and this manuscript would not be what it is without their help. I am also deeply appreciative of my roommates, friends and fellow members of the biology department who offered advice, encouragement and occasionally distraction from the task at hand when it was necessary.

Thank you to my family, who have always been supportive and understanding no matter what odd undertaking I commit myself to, for listening to me talk so much about spiders these past few years and inviting me over for dinner twice a week anyway. In conclusion, I would like to thank the spiders that were manipulated in this study, the brilliant researchers who built the body of knowledge upon which my work rests, and those of you interested enough to read this!

Table of Contents

Abstract ...... ii

Acknowledgements ...... v

List of Tables & Figures ...... viii

Introduction ...... 9-15

Spiders and Social Behavior ...... 10

Food Availability and Colonial Foraging ...... 11

Rationale and Study Objectives ...... 13

Study Site ...... 15

Chapter I (Obj .1 and 2): Feeding state, aggression and group foraging decisions in a

colonial web-building spider ...... 16-24

Experiment 1 – Test of Feeding State on Decision to Join a Colony or live solitarily

...... 16-19

Methods ...... 17

Statistical Analysis ...... 18

Results ...... 18

Experiment 2 –Influence of feeding state and aggression on colony web attachment

...... 19-24

Methods ...... 19

Statistical Analysis ...... 21

Results ...... 22

Discussion – Objectives 1 and 2 ...... 23 vi

Chapter II (Obj. 3) – Natural History of M. spinipes on the California Coast ... 25-34

Habitat Structure and Colonial Web Building ...... 25

Methods ...... 27

Spider Census ...... 27

Microhabitat Data ...... 27

Statistical Analyses ...... 29

Results ...... 30

Discussion –Objective 3 ...... 32

Conclusions and Future Directions ...... 34-37

References Cited ...... 38-45

Figures & Tables ...... 46-77

vii

List of Tables & Figures:

Table 1: Ethogram of Aggressive Behaviors

Table 2: Effects of Feeding Treatment and Social Experience on Grouping

Table 3: Effects of Feeding Treatment on Body Condition, Aggression, and Persistence

Figure 1: Sample Grayscale Image of Aerial Cover

Figure 2: Roosevelt Beach Colony Size Distribution

Figure 3: 2009 Body Condition and Feeding Treatment

Figure 4: 2010 Body Condition and Feeding Treatment

Figure 5: Aggression Escalation Score by Feeding Treatment and Residency Status

Figure 6: Persistence in aggressive web defense or takeover

Figure 7: Structural Profile of Vegetation of Roosevelt Beach Habitat

Figure 8: Vegetation Complexity of Roosevelt Beach Habitat

Figure 9: Comparison of Web Characteristics between Colonial and Solitary Webs of Roosevelt Beach Habitat

Figure 10: Web Distribution Within Height Categories (25cm-50cm, 50cm-75cm, 75cm-100cm and >100cm)

Figure 11: Comparison of Silk Attachments between Colonial and Solitary Webs

Figure 12: Estimation of Aerial Vegetation Cover

Figure 13: Microclimate Data

viii

Introduction

Social behavior has long been a central focus of study in evolutionary biology (Darwin

1859, Wilson 1975). Why are some animal species highly social and tolerant of conspecifics while other species are aggressive and solitary? Under what conditions is social grouping beneficial to animals, and under what conditions is it not? These questions have been explored at length in many well-known social animal model systems, and much insight has been gained.

Both primates (Smuts et al 1987, Sterck et al 1997, Kappeler and Schaik 2002) and non-primate mammals (Solomon and French 1997) have been focus of extensive research, as have birds

(Emlen 1978, Cockburn 1998,) fish (Pavlov and Kasumyan 2000, Parish et al 2002, Brown and

Laland 2003) and hymenopteran insects (Hamilton 1963, 1964, 1967; Wilson 1971, 1975; Seeley

1995; Korb and Heinze 2004; Wilson and Holldobler 2005). Most of these model organisms exist within largely social taxa. To explore these fundamental questions, however, it is sometimes useful to study social animals that are “exceptions to the rule”, i.e., social animals within a typically asocial taxon or phylogenetic lineage, as insights often arise from these unique situations (Whitehouse and Lubin 2005, Costa 2006).

There are many examples of the fitness benefits of social foraging in animals (Wilson

1975, Wittenberger 1981, Giraldeau and Caraco 2000). For example, some animal groups

improve their ability to hold and to acquire territory that was previously unattainable as a result

of either competition, as in lions (Mosser and Packer 2009), or as a result of physical

constraints, as in the spider Metabus gravidus which can, in a colony, stretch across streams

that would be unbridgeable by a solitary web (Buskirk 1975, 1979). Benefits of grouping may 9

also include reduced cost of territory maintenance as has been shown in some spiders (Uetz

1988, Jakob 1990). Additionally, grouping often reduces predation costs through behaviors such as dilution (Hamilton 1971, Foster and Treherne 1981) or mobbing (Curio 1978, Dominey

1983, Tamura 1989, Passamani 1995). Lastly, more effective foraging, achieved through a variety of mechanisms, is a common advantage of sociality in animal systems (Clark and Mangel

1986, Krause and Ruxton 2002).

Social behavior in spiders

Spiders are generally solitary predators with little tolerance for conspecifics and a reputation for cannibalism. However, a small number of species demonstrate varying degrees of sociality (Aviles 1997; Uetz and Hieber 1997; Whitehouse and Lubin 1999, 2005; Lubin and

Bilde 2007). Spider social behavior ranges from temporary aggregations of webs in typically nonsocial species (Rypstra 1989) to cooperative prey capture, feeding and parental care in permanently communal groups such as Mallos gregalis (Witt et. al. 1978, Jackson 1979, Tietjen

1986), Anelosimus spp. (Rypstra and Tirey 1991, Aviles and Tufino 1998, Jones and Reichert

2008, Yip et al 2008), Stegodyphus spp. (Johannesen et al 2006, Salomon and Lubin 2007) and

Theridion nigroannulatum (Aviles et al 2006). Some social species demonstrate extremely biased sex ratios and even reproductive division of labor, indicating the possibility of eusociality

(Vollrath 1986, Rypstra 1993).

Colonial web-building, which includes individually defended foraging orbs and retreats for each spider in a three-dimensional communal framework built cooperatively by all of the spiders in a colony, is an intermediate form of spider social behavior found in several previously explored orb weaving genera including Cyrtophora (Lubin 1974, Rypstra 1979), Metabus 10

(Buskirk 1975, 1986), and Metepeira (Uetz et al. 1982; Uetz & Hieber 1997). Metepeira spinipes, one such colonial orb weaving spider, is found in a variety of habitats in Mexico and coastal California (Uetz et al. 1982; Uetz & Hodge 1990). Like most colonial orb-weavers, M. spinipes display both solitary and colonial foraging depending on conditions and, as such, are a

useful model organism for exploring factors that may influence the evolution of sociality (Uetz

et al., 1982; Uetz & Hieber 1997; Uetz 2001).

Food availability and colonial foraging

The literature provides much evidence that habitat food availability is a factor affecting

the likelihood that spiders will live colonially (Uetz et al. 1982, Uetz 1988; Rypstra 1983, 1985,

1986, 1989; Gillespie 1987). Increased levels of prey availability have been shown to increase

spider density (Rypstra 1983) and reduce cannibalism (Rypstra 1986). Indeed, social spiders are

mostly found in areas with high food availability and large prey items, such as tropical lowlands,

whereas higher latitudes and elevations support primarily solitary spiders (Aviles 1997, Powers

and Aviles 2007, Purcell and Aviles 2007). Past research has also shown that Metepeira spiders

in areas of Mexico with high prey availability are more likely to forage in colonies than those in

desert areas (Uetz et al 1982). Likewise, colonies of Metepeira spinipes experimentally

removed from a high-food habitat (a feed-lot waste dump) were reduced in size unless food

availability was supplemented (Uetz et al 1982). Colonial Metepeira are known to gain foraging

advantages by living in groups, including improved foraging efficiency achieved through the

“ricochet effect,” where prey insects able to escape from one orb web are captured when they

bounce into another (Uetz 1986, 1989; Uetz & Hieber 1997).

This, however, creates a paradox: if group foraging increases prey capture efficiency, why do spiders living in prey-poor areas, i.e., the spiders which would most benefit from such a behavior, tend to forage solitarily? This apparent contradiction can be explained by risk sensitivity theory (Real and Caraco 1986, Uetz 1988, 1996). Prior research has shown that in addition to increasing prey capture efficiency, foraging in groups also decreases the variance in prey captured (Pulliam and Caraco 1984; Uetz 1988, 1996). In areas with high average prey abundance, low prey variance would reduce the probability of chance starvation events.

However, in areas where average prey abundance does not meet the energetic needs of the spiders, a high food variance would increase the probability of having at least a chance encounter with a food source. Reducing the variance (via group foraging) around a low mean level of prey, however, would merely ensure starvation. Thus, risk sensitivity theory predicts that spiders in prey rich environments should choose the risk-averse tactic of foraging colonially to reduce chances of starvation, while spiders in prey poor environments should choose the risk-prone tactic of foraging solitarily to increase the low chance of apprehending abundant prey (Uetz 1988, 1996). Patterns of group foraging in Metepeira spp. follow these predictions, as the frequency of occurrence of groups varies with food availability in habitats across Mexico

(Uetz et al 1982; Uetz 1988, 1996; Uetz & Hieber 1997) and spiders in groups have lower variance in prey capture rates (Uetz 1988, 1996). However, colonial behavior - at the level of species, population and/or colony - is the result of foraging decisions at the individual level

(Caraco et al. 1995, Uetz 1996), and selection would therefore favor individuals that make risk- sensitive foraging choices based on their current state (Real and Caraco 1986; Uetz 1988, 1996;

Uetz and Hieber 1997). This prediction has not been tested experimentally. 12

Rationale and Study Objectives

Cyclical fluctuations in environmental conditions, e.g., changes in moisture regimes associated with the El Niño / Southern Oscillation (ENSO), can have ecosystem level effects, and create bottom-up “pulses” of prey abundance that affect predatory arthropod populations

(Polis et al 1997, 1998 ). Colonial Metepeira in desert areas of Mexico have shown El Niño- related fluctuations in social grouping tendency, presumably related to prey availability (Hieber

& Uetz 1990; Polis et al 1998). Several M. spinipes populations along the central coast of

California, which had once consisted primarily of solitary spiders, began to increase in the frequency of colonies after the 1997 El Niño brought more moisture and therefore food abundance. This set of circumstances has provided an opportunity to study both the long-term impact of climatic cycles as well as the individual decisions that result in facultative colonial web-building.

Objective 1 – Feeding state and group foraging decisions

I conducted a series of experiments designed to explore colony formation in M. spinipes and to test the hypothesis that individuals forage in a risk sensitive manner – i.e., they are sensitive to the starvation risk arising from variance in their food supply. If these spiders exhibit risk sensitive foraging, I predicted that well fed individuals would be more likely to join colonies than poorly fed individuals. This prediction was tested with field experiments that manipulated food intake of M. spinipes and tracked the tendency to join colonies or remain solitary.

Objective 2 – Aggressive behavior and group foraging decisions

The prediction tested in Obj. 1 was made under the assumption that any differences seen between treatments in the frequency of spiders joining a colony would result from the decision of the introduced spider. It is entirely possible, however, that territorial resistance from members of the colony is the cause of any difference observed. Do territorial interactions between spiders influence which individuals join a colony and which do not? Are the outcomes of these interactions between spiders affected by feeding state? It is possible that whether or not an individual forages colonially is influenced less by its decision to join a colony and more by its ability to overcome resistance from resident spiders.

In order to test this assumption, I conducted a second set of experiments to tease apart the process of colony formation further by more closely examining aggressive and territorial interactions between introduced and resident spiders of a colony. If it is true that the likelihood of joining a colony is a function of aggression and well fed spiders are better able to fight their way into a colony while poorly fed spiders are “evicted,” then I predicted that well fed spiders introduced to colonies should show a higher escalation of aggression, more persistence and win more contests against residents than starved spiders. I also predicted that more aggressive spiders should join colonies at a higher rate than spiders which show less escalation during aggressive interactions.

Objective 3 – Natural History of Metepeira spinipes on the central California coast

Web building spiders are affected by the structure of their habitat in a more direct way than many other animals, as so many aspects of life revolve around the web and construction of that web is limited by the substrate that is available to build on. Structural features of the surrounding vegetation have been shown to affect web placement in orb weaving spiders 14

(Rypstra 1983, Greenstone 1984, McNett and Rypstra 2000). Additionally, the greater size of colonial webs and the ability of these spiders to form structural attachments with neighboring silk instead of simply vegetation may translate into different requirements from their local habitat structure. As such, further attention to the natural history of M. spinipes and description of their habitat in coastal California was warranted. I therefore conducted an additional set of pilot observations to gather microhabitat data and further explore the role of environmental factors (e.g., vegetation structure, cavity size, aerial cover, temperature, humidity) in the location of colonial webs of M. spinipes .

Study site

Research studies were conducted in August of 2009 and 2010 at Half Moon Bay State

Beach in Half Moon Bay, California (37° 27' 54.288", -122° 26' 40.272"). Located in the central coast of California, Half Moon Bay State Beach consists primarily of coastal scrub habitat.

Abundant plant species include coyote brush ( Baccharis pilularis ), California sagebrush

(Artemisia californica ), and coast buckwheat ( Eriogonum latifolium ) (Corelli et al 1995).

Summer temperatures are generally cool (average highs around 18° Celsius and lows around

11°) and the summer climate is arid (average monthly precipitation around 0.5cm) with heavy fog in the morning and dry afternoons with a westward wind coming off of the ocean.

Chapter I: Feeding, Aggression and Group Foraging Decisions in a Colonial Orb-Weaving

Spider

Experiment 1 – test of feeding state on decision to join a colony or live solitarily.

Risk sensitivity theory predicts that spiders in prey rich environments should choose the risk-averse tactic of foraging colonially to reduce chances of starvation, while spiders in prey poor environments should choose the risk-prone tactic of foraging solitarily to increase the low chance of apprehending abundant prey (Uetz 1988, 1996). Patterns of group foraging in

Metepeira spp. follow these predictions, as the frequency of occurrence of groups varies with

food availability in habitats across Mexico (Uetz et al 1982; Uetz 1988, 1996; Uetz & Hieber

1997). However, colonial behavior - at the level of species, population and/or colony - is the

result of foraging decisions at the individual level (Caraco et al. 1995, Uetz 1996), and selection

would therefore favor individuals that make risk-sensitive foraging choices based on their

current state (Real and Caraco 1986; Uetz 1988, 1996; Uetz and Hieber 1997).

As this prediction has not been tested experimentally, I conducted a series of

experiments designed to test the hypothesis that individuals forage in a risk sensitive manner –

i.e., they are sensitive to the starvation risk arising from variance in their food supply - by

exploring colony formation in M. spinipes. If these spiders exhibit risk sensitive foraging, well

fed individuals should be more likely to join colonies than poorly fed individuals. This

prediction was tested with field experiments manipulating food intake.

Methods

In August 2009, individual Metepeira spinipes were collected in equal numbers from

solitary webs and colonies and assigned at random to one of two treatment groups based on

their feeding regimen (N=105). One group was fed ad libitum with Musca domestica in order to increase feeding state and simulate an area with high prey availability, while a second group was starved in order to reduce feeding state and simulate an area with low prey availability.

After capture, dial calipers were used to take measurements of cephalothorax width and abdomen width, depth and length in order to estimate body condition. Spiders were then marked with non-toxic paint to indicate treatment and whether they were collected from a colony or a solitary web.

Each individual was housed for four days in a small plastic vial containing a strip of paper

towel to absorb excreta, and either fed or deprived of food. “Fed” treatment spiders received

live house flies ( Musca domestica ) ad libitum. Fly carcasses and soiled paper towels were

removed daily as necessary. “Starved” treatment spiders were maintained under identical

conditions but did not receive flies. After four days, each individual was measured a second

time to allow for verification of the effectiveness of the treatment, then released in the vicinity

of an existing colony of at least five spiders. Spiders were placed 20-30cm from the nearest silk

of the colony webbing. The following day spiders were scored as to whether or not they joined

the colony and nearest neighbor distance was recorded when possible using measuring tape.

Statistical Analysis

Body condition of spiders was calculated by dividing the volume of the abdomen, an ellipse (4π/3*depth*width*length), by the width of the cephalothorax. Body condition was non-normal and was log transformed, and one persisting outlier was eliminated prior to statistical analyses. Both the body condition at release and the change in body condition

(condition at release minus condition at capture) were then compared across feeding treatments using t-tests and an ANCOVA was performed examining the influence of cephalothorax width and feeding treatment on abdomen volume. The frequency of spiders that joined colonies was compared across both feeding treatments (Nfed = 55, Nstarved = 50) as

well as social experience (Nsolitary = 51, N colonial = 54 at capture) using chi square tests.

Results

Feeding treatment was effective in impacting body condition, as spiders which were fed

house flies ad libitum had a significantly greater ratio of abdomen volume (changes with current feeding state) over cephalothorax width (does not change with current feeding state) than starved spiders at release (Figure 3A; t102 =-3.754, p=0.005). Additional analyses of body

condition showed a significant difference in the change in body condition (Table 3; t102 =4.8875,

p<0.0001), and in an ANCOVA both treatment (F 1=16.1686, p=0.0001) and cephalothorax width

(F1=37.5942, p<0.0001) were significant covariates (Figure 3B). Treatment also had an effect on the likelihood that individual spiders would or would not join a colony, as well fed spiders

joined at a significantly higher rate than starved spiders (Table 2; = 4.086, p=0.043).

Experience (i.e., whether spiders were collected from colonies or solitary webs) was not a significant factor affecting whether spiders joined colonies (Table 2; = 1.088, p=0.2969).

Experiment 2 – test of influence of feeding state and aggressive behavior on decision to join a

colony or live solitarily.

The prediction tested in Exp. 1 assumed that any differences in joining frequency seen

between treatments would result from the decisions of well-fed or starved spiders. However,

behaviors of colony members might also cause the differences observed. In order to test this

assumption, I conducted a second set of experiments to tease apart the process of colony

formation further by more closely examining aggressive and territorial interactions between

introduced and resident spiders of a colony. If the likelihood of joining a colony is a function of

aggression, well fed spiders might be better at fighting their way into a colony, while poorly fed

spiders are more likely to be rebuffed. I predicted that well fed spiders should show a higher

escalation of aggression, more persistence and win more contests against residents than

starved spiders.

Methods

Experiments were conducted during August of 2010 at Half Moon Bay State Beach in

Half Moon Bay, California. As in the previous study (2009), Metepeira spinipes were collected in equal amounts from solitary webs and colonies and assigned at random to one of two

treatment groups based on their feeding regimen (N=52). After capture, dial calipers were used to measure the cephalothorax width and abdomen width, depth and length of each spider in order to estimate body condition. Spiders were housed in a small plastic vial with a strip of paper towel to absorb excreta, and spiders assigned to the “well fed” treatment group were offered 1/8 ” crickets ad libitum (flies were not available). Spiders were then marked with non- toxic paint to indicate treatment and whether they were captured from solitary webs or colonies. After four to five days, spiders were measured a second time and then released onto an existing colony just outside the prey catching orb of a resident spider.

After release into a colony, spider behavioral interactions were observed and recorded in real time using either a digital audio recorder or a digital video recorder. Outcome (retreat or takeover of the resident’s web/retreat) was recorded for each spider. Latency to enter the orb and latency to engage in aggressive behavior were quantified from replay of audio files, and all individual behaviors involved in the interaction were recorded. Behaviors observed during interaction were assigned a score indicating the relative intensity of escalation (see Table 1), based on the ethogram and aggression scale from an earlier study of M. spinipes (Hodge and

Uetz 1995). Subsequently, the highest escalation score was recorded for each individual as well as for the entire interaction. The following day, webs were re-checked and spiders were scored as to whether or not they joined the colony.

To test for the influence of the presence of a resident spider on the tendency to join a colony, a second experiment was conducted. Spiders were captured and assigned to treatments in the same manner as described above. These spiders, however, were released

after treatment into colonies adjacent to prey catching orbs and retreats from which the resident had been removed. Spiders were subsequently scored as to whether or not they took over the vacant retreat, and latency to enter the prey orb was recorded.

Statistical Analysis

As in the first experiment, body condition was calculated by dividing the abdomen volume by the cephalothorax width of each spider. As body condition was not normal, measurements were log transformed and one persistent outlier was removed prior to statistical analyses. Both body condition at release and the change in body condition were compared between fed (N=22) and starved (N=31) individuals using t-tests and an ANCOVA was performed examining the influence of cephalothorax width and feeding treatment on abdomen volume. T-tests were also used to compare fed and starved spiders with regard to measures of persistence such as the length of the interaction and the latency to retreat as well as the escalation score of residents, intruders and the entire interaction. Chi-squared tests were used to find the likelihood ratio that fed and starved treatments differ with regard to whether they overtook the resident’s web and whether or not they joined the colony. Chi-squared tests were also used to assess if starved and fed spiders overtook existing webs more often when that web was unoccupied and to determine whether level of aggression predicted whether or not an individual would join a colony.

Results

As in the previous year, treatment was effective in 2010 in impacting body condition.

Spiders with ad libitum access to crickets had significantly larger ratios of abdomen volume over cephalothorax width than starved spiders (Figure 4A; 49 =-4.3337, p<0.0001). Additional

analyses of body condition showed a significant difference in the change in body condition

(Table 3; t49 =-6.7036, p<0.0001), and in an ANCOVA both treatment (F 1=14.8206, p=0.0003) and

cephalothorax width (F 1=25.1402, p<0.0001) were significant covariates (Figure 4B). Feeding treatment did not significantly affect the escalation of aggression of either the intruder (t 51 =

0.2115, p=0.8334), the resident (t 51 =0.4248, p=0.6762) or the entire interaction (see Figure 5

and Table 3; t 51 =-0.2359, p=0.8145). Similarly, I did not find an effect of treatment on the outcome of the interaction, i.e. whether or not the intruding spider took over the resident’s web ( X2= 0.090, p=0.7637). There was no significant difference between well fed and starved spiders in measures of persistence (see Figure 6 and Table 3), such as length of interaction (t 32

=0.5454, p=0.5893), latency to retreat (t 23 =-0.0016, p=0.9987), and percentage of spiders who

returned after retreat ( X2= 3.118, p=0.0774). Additionally, spiders were significantly more likely

to overtake retreats from which the resident had been removed (N=76; = 15.603, p<0.0001).

Among spiders introduced to orbs with vacant retreats, there was a non-significant trend of

well fed spiders joining at a greater rate than starved spiders (N=25; = 2.915, p=0.0878, df =

1). The great majority of individuals of both groups remained in the colony as a member after

introduction, but well fed spiders were more likely to join colonies than starved spiders (Table

2; X2= 4.643, p<0.0312). Level of aggression, however, was not a predictor of which spiders

joined colonies ( X2= 0.3242, p<0.5691).

Discussion

Results from the first experiment are consistent with the predictions of a hypothesis of

risk sensitivity, as spiders that were well fed joined colonies at a significantly greater rate than

spiders that were starved. This is noteworthy, because the frequency of colonial spiders has

been shown to vary with food availability in populations across Mexico (Uetz et al 1982, Uetz

1988) and food abundance has been shown to affect the persistence and density of spider

colonies (Rypstra 1983, 1986; Uetz et al 1982). These findings are presumably the result of

individual foraging decisions and this is the first study to test the predictions of risk sensitivity

on the foraging decisions of individual spiders. However, this experiment was performed under

the assumption that any difference observed between treatments in the rate at which spiders

joined colonies would be the result of the decision of the focal spider under feeding treatment.

Alternatively, this difference could be explained if poorly fed spiders are being “evicted” by

residents at a greater rate than well fed spiders, which may be in better condition and

therefore more aggressive, persistent or better able to “bully” their way into a colony.

The results of the second study, however, do not support this alternative hypothesis.

Resident spiders appear to defend their individual webs, exhibiting aggression toward other spiders which enter their orb, and intruders overtook webs significantly more often when the resident had been removed. However, well-fed spiders placed on the periphery of a colony resident’s prey-catching orb showed nearly the same level of aggression (see Figure 5 and Table 23

3) and persistence (see Figure 6 and Table 3) and won an almost identical proportion of encounters as starved spiders. None of the behaviors that would be predicted by this hypothesis to differ between well fed and starved spiders showed any statistical difference.

However, despite the difference in methods between the first and second experiments, when spiders were placed directly into a colonial web instead of nearby, starved spiders still joined those colonies at a lower rate than their well fed counterparts (Table 2). Thus, individuals choose to join colonies differentially based on feeding state, suggesting that risk sensitive foraging explains facultative colonial foraging found in M. spinipes.

Chapter II – Natural History of M. spinipes on the California Coast

Habitat structure and colonial web-building

Features of habitat structure have been found to affect various aspects of social behavior in a number of animals, including level of competitive behavior in geckos (Petren and

Case 1998), group size in white-tailed deer (Hearth 1977), territory size in gulls (Ewald et al

1980) and social vigilance in squirrel monkeys (Boinski et al 2003) to name a few. As in other organisms, characteristics of a habitat such as access to food and protection from prey are important to spiders (Blamires et al 2007). However, because webs are central to most aspects of a spider’s life and web construction is limited by substrate in the surrounding habitat to build on, spiders are linked to their surrounding habitat structure in a more direct way than many other organisms. As such, a closer examination of the natural history of M. spinipes was warranted.

Because orb-weaving spiders utilize their web as both a place to live and a means to ensnare prey, web building decisions can be viewed as habitat selection in addition to foraging.

Many examples from the literature suggest that web placement is influenced by the composition and complexity of the surrounding vegetation (Uetz et al 1978, Rypstra 1983,

McNett and Rypstra 2000). Since it has been shown that environmental factors influence web site selection, and the decision to forage colonially in M. spinipes is essentially a choice of where to attach the structural threads of a web, it is possible that habitat structure impacts the likelihood of colony formation (Uetz and Burgess 1979).

The structure of the surrounding vegetation influences both the local abundance and diversity of web-building spiders (Rypstra 1983, Greenstone 1984, Robinson 1981, Gunnarsson

1990, Uetz 1991, Halaj et al 1998), because the construction of a web is structurally dependent upon the availability of stable attachment points for silk. In colonial spiders such as M. spinipes ,

the silk of neighboring webs, and not simply the surrounding vegetation, can serve as substrate

for the attachment points of structural threads. This may leave colonial spiders less dependent

on surrounding plant structure. Additionally, because colonial webs are larger than solitary

webs, a colony of spiders would most likely require a greater amount of space between

branches in which to construct their web than would solitary spiders. As a consequence,

colonial webs have different structural requirements than solitary webs, and it is possible that

characteristics of the surrounding vegetation may be associated with the frequency of colonial

foraging in spiders.

Features of the vegetation have been shown to affect web placement in orb weaving

spiders (Rypstra 1983, Greenstone 1984, McNett and Rypstra 2000), and the greater size of

colonial webs may require different habitat structure than solitary webs. As studies of M.

spinipes in central Mexico (see Uetz and Hodge 1990; Uetz and Hieber 1997) were conducted in

primarily desert riparian and grassland habitats, where spiders attach webs to cacti and

succulents, further attention to the natural history of M. spinipes and its habitat in coastal

California was necessary. I therefore collected data on microhabitat (e.g., vegetation structure,

cavity size, aerial cover, temperature, humidity) to explore potential role of environmental

factors in the location of colonial webs of M. spinipes .

Methods

Spider Census – In both 2009 and 2010 a census was taken of the M. spinipes population in

Roosevelt Beach, a stretch of coast that sits within Half Moon Bay State Beach. Along a north/south transect of coastal scrub following a bicycle path, all solitary and colonial webs were counted and the number of spiders in each web recorded. Observed levels of grouping were compared with levels of grouping expected by random chance using a zero truncated

Poisson analysis (Cohen 1960, 1971; Uetz and Burgess 1979).

Microhabitat Data - Observations were performed during June of 2011. Individual spider webs

and colonies of M. spinipes were located and flagged along a 500 meter transect of the same

coastal scrub habitat as described above in Half Moon Bay State Beach. I assessed the

vegetation surrounding each web for its density by placing a 1 meter dowel rod perpendicular

to the ground as near the web as possible without damaging it and counting all pieces of

vegetation contacting the rod (as in McNett and Rypstra 2000). In addition, in order to

measure “openness” within vegetation, I measured the distance from the rod to the nearest

branch that could be used for silk attachment at vertical intervals of 25cm, 50cm, 75cm and

100cm on the meter stick. I repeated these steps in five randomly selected spots around each

web on the transect. This entire process was replicated at 15 solitary webs, 15 colonies and 15

randomly selected locations without webs along this transect to serve as a control. The

Coefficient of Variation from these openness measures was used to calculate an estimate of

vegetation complexity (Roth “D s” index – Roth 1976).

I subsequently measured the size of the cavity within the vegetation that each web was occupying by measuring the largest distance between branches in both the north/south plane and the east/west plane at the height of the web. Identical measurements were taken at the

15 control sites by recording the largest distances between branches in the north/south and east/west planes at the mean height of all webs that had been measured prior to that point

(between 65cm and 70cm). Web volume was then recorded by measuring its largest span of silk in the north/south plane, the east/west plane and in the vertical plane. Web height was recorded as the mean between the height of the lowest and highest points of silk in the web.

Furthermore, I recorded the number of silk attachment discs of each web on the surrounding vegetation as well as the number of spiders in each colony. I then recorded the morpho- species of the plant which provided the substrate for the primary structural threads of all webs.

Non-normal data were log transformed.

At each of the aforementioned control, colonial and solitary web sites, temperature and humidity were recorded in the morning (between 8:30 and 10:30am) on two different days as well as in the afternoon (between 2:00 and 4:00pm) on the second day using an Oakton thermo hygrometer. The mean values were compared statistically between locations with colonies, solitary webs and no webs. Additionally, in an effort to assess visual cover from avian predators, I took photographs of the sky from the perspective of each spider’s retreat in the 15 solitary webs and 15 colonies locations mentioned above. I also took photos from the 15 control sites at the mean height of the webs that had been measured prior to that point (65cm-

70cm). The percentage of the sky covered by vegetation was estimated from the photographs

by means of transforming the photos into grayscale images (see Figure 1) and quantifying the amount of dark pixels in each one by obtaining a mean value on a 0-255 scale (0 = 100% black,

255 = 100% white) using ImageJ photo analysis software. These values were also compared statistically between locations with colonies, solitary webs and control sites.

Statistical Analysis

Observed colony size frequencies from both the 2009 and 2010 Roosevelt Beach censuses were compared with a Chi-Square test to a set of expected frequencies calculated using a zero-truncated Poisson distribution in order to test whether or not grouping was random. A dispersion index, which was derived by dividing the group size variance over the mean, was calculated in order to assess whether divergence from Poisson expected values was in the direction of overly even spacing or aggregation. In order to gather relevant natural history data, 45 microhabitats (15 with colonies, 15 with solitary webs and 15 randomly chosen control locations) were compared for each of the above measurements. Statistical comparisons were made using ANOVA for those tests comparing all three types of locations (i.e., vegetation density, vegetation complexity, web cavity size). T-tests were used to compare colonial and solitary webs for openness, web volume, silk attachments/spider, aerial cover, temperature, relative humidity and web volume/cavity area ratio (which was calculated by dividing the cube root of the web volume by the square root of the cavity area). Aerial cover, temperature and relative humidity were also compared between locations with webs (solitary and colonial) and locations without webs using t-tests. Complexity (Roth D s index) was determined as in Roth

(1976) by calculating the coefficient of variation of the distance from the dowel rod to the

nearest branch. A 95% confidence interval for each of these values was found using the following equation from Woolf (1968): L 1 = C.V. – 1.96 (C.V./√2n), L 2 = C.V. + 1.96 (C.V./√2n).

Results :

Spider Census:

Analyses of 2009 and 2010 censuses of Roosevelt Beach confirmed that grouping in this population is not random, with more spiders in aggregations. Colony size frequencies from both the 2009 and 2010 Roosevelt Beach census (Figure 2) were significantly different from those expected by a zero-truncated Poisson distribution (2009: X2=134.66, p<0.001; 2010:

X2=1990.64, p<0.001), indicating that grouping is not random. In general over both years, observed frequencies of solitary spiders and large colonies were greater than expected by a

Poisson distribution, while frequencies of small colonies were smaller than expected. An Index of Dispersion (ID) showed high values for each year, which are indicative of aggregated distributions (2009 - 488.727; 2010 – 399.548). Group size distributions for 2010 showed more large groups than 2009, which fits previously observed patterns, as 2010 was an El Niño year and spider populations have been shown to change with El Niño cycles (Hieber & Uetz 1990;

Polis et al 1998).

Microhabitat Data: assessing differences between colonies and solitary webs:

Many aspects of the structural profile of microhabitat vegetation differed between habitats supporting colonial webs and habitats supporting solitary webs (see Figure 7, A-C).

Vegetation density, as measured by the number of branches touching a vertical meter long

dowel rod placed as near the focal web as possible, was significantly lower near colonies than near solitary webs and control microhabitats (ANOVA F 2=4.7601, p=0.0137). Vegetation openness, as measured by distance to the closest branch at four different heights along the dowel rod, however, did not differ significantly between treatments, although a t-test showed a non-significant trend in which microhabitats near colonies showed marginally greater distances

(log transformed for normality) than those near solitary webs (t 28 = -1.98229, p=0.0573).

Vegetation complexity was assessed by calculating Roth’s D s from the Coefficient of Variation

for point distance measures of vegetation openness discussed above (see Figure 8). Using

methods from Woolf (1968), 95% confidence intervals for each CV value were calculated. The

Ds index for colonies was lower than that of solitaries, and the 95% confidence intervals did not overlap, indicating that the difference is statistically significant. The cavities within the vegetation that housed colonial webs were significantly larger in area than those cavities with no webs (control) or solitary webs (ANOVA F 2=13.365, p<0.0001).

Characteristics of colonial web structure also differed in many important aspects from

solitary web structure (see Figures 9 and 10). As one would expect, overall web volume is

greater in colonial webs than in solitary webs (Figure 9A; t28 =-4.6456, p<0.0001). Colonies also have a greater web size to cavity size ratio (cube root of web volume over square root of cavity area) than solitary webs are to theirs (Figure 9B; t27 =-3.20506, p=0.0035). As colonial webs are much larger than solitary ones, they included a greater number of silk attachment disks to anchor webs to surrounding vegetation (Figure 11A; t28 =-6.76334, p<0.0001). However, the ratio of attachment disks per spider was much smaller in colonial webs than in solitary webs

(Figure 11B; t28 =8.780206, p<0.0001). The ratio of attachment disks to web volume was also significantly smaller in colonial webs than in solitary webs (Figure 11C; t28 =2.5318, p=0.0172).

The distribution of webs at each height category (25-50cm, 50-75cm, 75-100cm and >100cm) is

shown in Figure 10. The greatest percentage of solitary webs was found from in the 25-50cm

height cohort, although solitary webs were generally spread throughout all height cohorts.

Colonial webs were not found above 100cm and the greatest percentage of colonial webs was

found in the 50-75cm height. The vast majority (87%) of colonies were found between 40 and

80cm. In contrast, 41% of solitary webs are over 80cm. The 0-25cm cohort is not shown in

Figure 10, as no webs were found at that height.

There was no statistical difference between solitary and colonial webs in terms of aerial

cover over a spider as estimated by level of dark pixilation on a grey-scale photo taken from the

retreat (Figure 12A; t28 =-0.45662, p=0.6515), however locations with spider webs (solitary or colonial) had significantly less cover than those randomly-chosen areas without them (Figure

12B; t43 =3.2929, p=0.0020). Similarly, microclimate data showed that while there was no

statistical difference between solitary and colonial webs (temp – t27 =0.4587, p=0.6501; RH –

t28 =-0.8723, p=0.3905), microhabitats that contained webs (solitary or colonial) had significantly higher morning temperatures (t 42 =2.0456, p=0.0471) and lower morning relative humidity (t 43 =-

2.1222, p=0.0396) than the control locations which did not (see Figure 13).

Discussion

While evidence from this and prior studies supports risk sensitivity as a factor in the

decision of M. spinipes to join a colony or build a solitary web, behavioral and ecological 32

interactions tend to be complex, and I thought it may be productive to explore other possible factors involved in colony formation. Web building spiders like M. spinipes are highly dependent on their web in many aspects of their life history, and the construction of the web is highly dependent on habitat structure. Composition and complexity of the surrounding vegetation have been shown to affect web placement in spiders (Uetz et al 1978, Rypstra 1983,

McNett and Rypstra 2000), and joining or not joining a colony is essentially a decision of where to attach one’s web. With this strong dependence on the structural characteristics of their habitat, it seems possible that the density and level of complexity of the surrounding vegetation could be another component in the formation of spider colonies.

The data from this pilot study are consistent with this possibility and suggest that the role of habitat structure in colony formation merits further research. Microhabitats in areas where spiders had built colonies were less dense than those where solitary webs or no webs had been built, and colonies also occupied habitats that showed less complexity in vegetation structure. Similarly, colonies were built in larger cavities within the vegetation than those containing solitary webs or those without webs measured at a similar height in the brush at control locations. This is intuitive, as colonial webs were much larger than individual webs and therefore may require more space, although colonial webs tended to be larger than the cavity they occupy (i.e., colonial webs often extend into additional cavities), whereas solitary webs did not tend to occupy the entire cavity. Colonial spiders may be better able to exploit these areas with lower density and larger cavities within the vegetation because they are able to attach

their structural threads to the webs of neighboring spiders and, as such, are not as reliant on dense, complex vegetation structure support their webs.

This is supported by the data, as colonies have a significantly smaller ratio of silk-to- vegetation attachments per spider and a smaller ratio of silk attachments to web size than solitary webs. Thus, colonial foraging may not only provide silk savings, as has previously been shown (Uetz & Hieber 1997), but may also reduce the dependence on dense surrounding vegetation structure for silk attachment and structural support. These larger cavities did not seem to cause a tradeoff in total areal cover between colonies and solitary webs, but that tradeoff may be present for all spiders as control areas had significantly more cover than those with webs. Taken together, these field measurements suggest the possibility that colonial webs require different habitat features and further research may show that the characteristics of the surrounding vegetation play a role in the frequency of spiders living in colonies.

Conclusions and Future Directions

These studies provide new insight into the drivers of colony formation in this species in two ways. This is the first study to explore the effect of individual feeding state on the social decision-making of individual spiders, and as such it addresses gaps in the extensive body of research providing evidence for risk sensitivity as an explanation for existing patterns of group foraging. Additionally, this study creates a clearer picture of the California coastal habitat in which these M. spinipes populations live and suggests that habitat structure not only impacts the location of web placement, as has been shown previously (Uetz et al 1978, Rypstra 1983,

McNett and Rypstra 2000), but may also play a role in whether spiders forage colonially or in solitary webs.

As is the case with much scientific work, this research has resulted in as many questions as it answers and future work exploring the process of colony formation and the ecological factors affecting fitness and behavior would be valuable. While this study showed that, consistent with risk sensitivity, spiders with consistent access to food are more likely to join colonies, this experimental design was not able to establish that feeding state is the specific mechanism for assessing food availability. Alternatively, spiders could have detected levels of prey availability via chemical, vibratory or tactile cues from the prey provided to them. These factors were not distinguished in this study, but could be separated experimentally in order to establish the mechanism by which M. spinipes assesses food availability.

Additionally, because the natural history data from this study suggest that colonial and solitary spiders may exploit different microhabitats, future research exploring the role of habitat structure in the decision to build colonies may prove interesting. While these pilot observational data provide some evidence that solitary spiders build webs in denser, more complex vegetation than colonial spiders, manipulative experimentation is necessary to establish a link between structural features of the habitat and social foraging. Field and laboratory studies manipulating both the amount and complexity of structure would be helpful in hashing out the role that surrounding vegetation plays in the web dynamics of M. spinipes.

Kin selection is a highly researched area of sociobiology, and while it has been proposed as a driver of social evolution in many organisms, its possible role in the social foraging behavior 35

of M. spinipes has not been established. While prior attempts to examine genetic relatedness within and between colonies have not succeeded, technological advances may open the door for testing the predictions of kin selection in M. spinipes using genetic methods in the future. In the mean time, studies of the behavioral predictions of kin selection are possible. For instance, kin selection would predict that if siblings recognize one another as such, rates of cannibalism should be lower in groups of related spiderlings compared to groups of unrelated spiderlings.

Additionally, kin selection may favor a reduction of nearest neighbor distance and behaviors such as prey stealing or inter-individual aggression in groups that are made up of siblings.

Further field data on dispersal of spiderlings may also shed light on the likelihood that kin selection is acting in this system. Further exploration of spiderling dispersal would also be intriguing beyond questions of kin selection. Because spiderlings emerge from egg sacs together into what remains of the mother’s web, spiders which become solitary may essentially be making the choice to depart a colony of their siblings. Are spiderlings born from egg sacs within a colonial web more likely to remain social than those born from egg sacs in solitary webs? Is dispersal reduced or slowed as a result of the social experience of the mother, or is dispersal simply a function of prey availability after emergence?

By virtue of their behavioral flexibility in building both solitary and colonial webs and

their phylogenetic position as a group-living species within a primarily solitary taxon, M.

spinipes is a useful model animal to explore conditions which may favor sociality. While the last

thirty years have seen much research exploring factors influencing the evolution of social

behavior in spiders, many important questions still remain. Data from this study are consistent

with prior research indicating that risk sensitive foraging is one major factor explaining the patterns we see today. Additionally, pilot observational data suggest that other ecological factors such as habitat structure may also play a role in the observed frequency and size of spider colonies, opening the door to future research.

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Figures and Tables

Table 1: Ethogram describing common behaviors associated with interactions between spiders. Each behavior is assigned a score indicating the degree of escalation of aggression shown.

Behavior Description Score Ignore Spider does not respond to opponent 0 Freeze Spider stops movement in response to opponent moveme nt 0 Retreat Spider moves out of web after aggression from opponent 0 Pluck Spider pulls the web toward the body as if locating prey that has 1 entered the orb Approach Spider moves toward opponent 2 Shake Spider uses both front legs to move the web up and down robustly 3 Bounce Spider shakes the web by bouncing the entire body violently and 4 repeatedly Chase/contact Spider chases or contacts its opponent 5

Table 2: Effects of feeding treatment (A) and social experience (B) on the decision to join a colony or not in 2009 and 2010.

No. fed No.starved No.starved chi- spiders that No.fed spiders spiders that spiders that square year joined that didn’t join joined didn’t join (df = 1) p-value 2009 43 12 30 20 4.111 0.0426 2010 22 0 27 4 4.521 0.0335

No. of chi- No. of colonial colonial No. of solitary No. of solitary square spiders that spiders that spiders that spiders that (df = year joined didn’t join joined didn’t join 1) p-value 2009 40 14 33 18 1.088 0.2969 2010 20 1 29 3 0.409 0.5223

Table 3: Effects of feeding treatment on body condition, aggression and measures of persistence.

Mean (± S. E.) for Mean (± S. E.) for Dependent fed treatment starved treatment Variable df spiders spiders t-value p-value 2009 Change in Condition 102 1.3711(±.0269) 1.1796(±.0285) 4.8875 < 0.0001 2010 Change in Condition 49 0.0392(±.0192) -0.1273 (±.0157) -6.7036 < 0.0001 Escalation of Aggression (intruder) 50 0.545 0.6 0.1443 0.8858 Escalation of Aggression (resident) 50 2.36 2.17 -0.3646 0.717 Escalation of Aggression (entire interaction) 50 2.36 2.27 -0.1796 0.8582 Length of Interaction 32 180.571 239.464 0.6416 0.5243 Latency to Retreat 38 30.125 62.625 1.2627 0.2144

Figure 1: Sample grayscale image created from a photograph of the sky from the perspective of a spider’s retreat; used to estimate vegetative cover from aerial predators.

Figure 2: Results of 2009 and 2010 surveys of Metepeira spinipes in Roosevelt Beach (Half Moon Bay State Beach) showing the colony size distribution.

Figure 3: 2009 Body Condition and Feeding Treatment 2009

A) Mean estimate of body condition of fed and starved spiders ( ± S. E.). Asterisks indicate significant difference between groups as determined by a two-sample T-test; * P = 0.005 (Nfed

= 55, N starved = 49)

B) Regression plot of ln abdomen volume by cephalothorax width of fed vs. starved spiders

Figure 4: 2010 Body condition and feeding treatment in 2010

A) Mean estimate of body condition of fed and starved spiders ( ± S. E.). Asterisks indicate significant difference between groups as determined by by a two-sample T-

test; * P < 0.0001 (N fed = 21, N starved = 29).

B) B) Regression plot of ln abdomen volume by cephalothorax with of fed vs starved spiders

B) Regression Plot

Figure 5: Graph of the percentage of spiders that displayed each category of maximum level of aggression escalation score (i.e. the highest level of aggression reached in an interaction - see table 1) for (A) fed and starved spiders, or (B) intruder and resident spiders. Frequency of spiders exhibiting scores for Bounce (4) and chase/contact (5) were combined, as every spider who exhibited a web bounce went on to chase/contact its opponent.

Figure 6: (A) Mean length of interaction (± S.E.) from first to last agonistic behavior; and (B), latency to retreat after first agonistic behavior.

Figure 7: Structural profiles of the vegetation at Half Moon Bay State Beach used as habitat by solitary and colonial spiders and control (random) locations . (A) Mean number of plant structures touching a vertical 1 m dowel rod. (B) Mean area (± S.E.) of the cavity in the vegetation where the web is located. (C) Mean distance from the rod to the closest vegetation at four different heights (25cm, 50cm, 75cm, 100cm). Letters above bars indicate statistically significant differences between web types at p < 0.05 based on Tukey HSD post- hoc test; bars with the same letter are not statistically different.

A) Vegetation Density B) Cavity Area A B A A

B B

C) Vegetation Openness

Figure 8: Estimated vegetation complexity (Roth’s “D s” index; see text for details) for (A) web types and control locations, and (B) height categories. Error bars represent the 95% confidence interval of CV calculated using methods from Woolf (1968). Different letters above categories in (A) indicate that the 95% confidence intervals do not overlap.

A) Vegetation Complexity (Roth’s D s) by Web Type

B AB A

B) Vegetation Complexity (Roth’s Ds) by Height Category (grouped by web type)

Figure 9: Mean web volume (A) and web/cavity ratio (B) for colonial and solitary spiders. Significant differences between means, as determined by two-sample t-tests, are indicated by an asterisk (*P ≤ 0.05)

A) Web Volume (cm 3)

B) Web Volume/Cavity Area ratio

Figure 10: Distribution of webs within 4 height categories (25cm-50cm, 50cm-75cm, 75cm- 100cm and >100cm)

Figure 11: Web attachment characteristics for solitary and colonial spiders. (A) Mean (± S.E.) number of silk attachment disks in each web. (B) Mean (± S.E.) ratio of attachment disks per spider in each web. (C) Mean (± S.E.) ratio of attachment disks to total web volume (cm 3) Significant differences between means, as determined by two-sample t-tests, are indicated by an asterisk (*P ≤ 0.05)

A) B)

* *

Figure 12: Estimation of Aerial Vegetation Cover using grayscale images of photos taken of the sky from the perspective of each spider’s retreat (see text for details). (A) Mean (± S.E.) cover score for colonial and solitary webs. (B) Mean (± S.E.) cover score for areas with webs and randomly chosen control areas without webs. Significant differences between means, as determined by two-sample t-tests, are indicated by an asterisk (*P ≤ 0.05)

A) B)

* *

Figure 13: Microclimate data from the Half Moon Bay State Beach vegetation used as habitat by solitary and colonial spiders. (A) Mean (± S.E.) temperature for solitary and colonial webs; (B) Mean (± S.E.) temperature of areas with webs of both types and randomly selected areas without webs. (C) Mean (± S.E.) relative humidity for solitary and colonial web sites; (D) Mean (± S.E.) relative humidity for areas with webs of either type and randomly selected areas without webs. Significant differences between means, as determined by two-sample t- tests, are indicated by an asterisk (*P ≤ 0.05)

A) B)

* *

C) D)

* *