1
2
Preface
I had the clear view that I was done with physiology in my career as a student. In my Master I wanted to focus on ecology and behaviour. I therefore contacted Trine Bilde to find a suitable project for my Master after end maternity leave. Trine referred me to Michelle Greve and told that Michelle had thoughts about an exciting project about social spiders and their metabolic rate and desiccation tolerance. So before I knew I had myself deep involved in a behavioural /physiological Master Thesis. So once again I said “hallo” to my old friend physiology hidden behind a spider web. I ended up with two supervisors, Trine, the spider woman and Johannes Overgaard, a hardcore zoo-physiologist.
This thesis is the result of the work I conducted during my Master study. It has the form of a manuscript, but with modifications to fit a Mater Thesis. Hopefully one day my results of my Master will be published, so other can enjoy the story of energy and water conservation in social spider Stegodyphus dumicola.
Acknowledgements
I will start by thanking my two supervisors:
Trine, for good support during my Master and for useful advice and comments.
Johannes, for always having a few minutes, that often turn into half and whole hours, when I think I am coming to him with a simple question. But also for his sometimes crazy comparisons, to make me understand basic physiological and physical laws.
In addition, I would like to thank:
Michelle, for good advice and support throughout my Master. And for collecting spiders for me.
Marie, Linda, Kirsten and my good friend, Helene for help in the lab, when a lot of spiders needed to be weighed or pulverized.
Signe and Malene my sweet office mates and the whole Spiderlab for help, support and friendly advice.
And last but not least a big and warm thanks to my family, my parents and sister for always believing in me. To my husband, Jacob for his unconditional love and support. To my little son, Thor, that keeps things real!
Thank you!!!
3
Group size effects on energy and water conservation in the social spider Stegodyphus dumicola
Anne Vestergaard Druedahl Bruun
Department of Bioscience, Aarhus University, Ny Munkegade 116, 8000 Aarhus C, Denmark
Abstract
Animals that live in groups may acquire physiological benefits. Several physiological hypotheses have been proposed that predict benefits in the form of reduced energy expenditure or water conservation with increasing group size. Here I investigated the presence of physiological “group size effects” in the social spider Stegodyphus dumicola. This was done by examining the standard metabolic rate (SMR), the lipid and protein content during starvation, and the production of silk at two temperatures at different group sizes. Furthermore desiccation tolerance and survival rate under a relative humidity of < 5% was examined at different group sizes. The prediction was that individuals in groups either achieve greater benefits or costs or no difference than single
individuals. I found that larger group sizes did not have a lower SMR compared with single individuals. Group sizes of five, 10 and 20 experienced a lower weight loss rate during desiccation than single individuals, a trait that is ecologically relevant since the spiders occupy arid habitat.
However, this effect did not translate into significant differences in survival rate among group sizes. Individual lipid and protein content was under the influence of temperature and starvation duration but not of group size. Single individuals produced less silk than individuals in groups of five, with more silk produced under higher temperature in both group sizes. Spiders in groups may experience improved desiccation resistance, which may represent a major water conserving benefit. I found little evidence for other physiological benefits of group living. It is possible that other types of benefits, for example better predation protection or security in mate finding, favour group living in S. dumicola.
4
Introduction
An important question in evolutionary ecology relates to how group living and cooperation have evolved and is maintained. Social living in animals can occur at many levels, from the formation of groups with little or no social interaction between group members (Stensland et al. 2003), to complex cooperative societies with division of labour, morphological casts that specialize in separate tasks, and extreme reproductive skew, as for example in ants (Wilson 1975). The
selective forces that drive and maintain group living and cooperation are intriguing, and there is, in particular, continued interest in understanding the selective advantages that outweigh the
individual costs that are inevitably associated with group living. Groups can form because they provide direct benefits such as protection against predation or increased foraging efficiency, benefits that do not necessarily involve cooperation. Cooperation in animals is associated with sharing of tasks such as caring of brood, nest building, foraging and anti-predator behaviours. In addition, members of cooperative groups are often closely related. Group living and cooperation may ensure higher rates of survival through a lowered risk of predation (Elgar 1989, Fitzgibbon 1990, Unglaub et al. 2013), and may increase inclusive fitness through the increased reproduction of close relatives (Hamilton 1964). Despite direct or indirect reproductive benefits for individuals, group living is also associated with costs. Competition is one of the main disadvantages of group living, as the group members compete for food and mating opportunities (Elgar 1986). In addition, an increased risk of diseases due to close contact with conspecifics can be a disadvantage (Krebs and Davies 1993). On the other hand, group living could improve overall fitness if cooperation lowers energy requirements and/or improves the success of energy gain (Gallé 1978, Muradian et al. 1999).
The term “group effect” was introduced by Grassé and Chauvin (1944) and describe the phenomenon where group living confers physiological advantages compared with solitary living.
Grassé and Chauvin (1944) found that the survival rate of certain social insects was bound to group size and that individuals in larger groups exhibit extended lifespan compared with individuals from smaller groups or single individuals. Assessing fitness benefits attributed to
“group effect” can be done in numerous ways. Studies have been conducted to test for group size effect on the standard metabolic rate (SMR) i.e. the consumption of oxygen or the production of carbon dioxide. A decrease in SMR with increasing group size indicates benefits in terms of
5 conserving an individual’s energy consumption. Most studies done using SMR on arthropods have been inconclusive showing both benefits (lower SMR) (Gallé 1978, Anderson 1993, Tojo et al.
2005) and no differences in the SMR (Brian 1973, Lighton and Bartholomew 1988, Schoombie et al. 2013) of increasing group size. Furthermore, increased group size might reduce susceptibility to desiccation. Several studies on water balance in relation to group size or clustering behaviour have been conducted in arthropods (Abushama 1974, Sigal and Arlian 1982, Yoder and Barcelona 1995, Yoder and Grojean 1997, Glass et al. 1998, Benoit et al. 2005, Benoit et al. 2007). These studies revealed that, confronted with low relative humidity or dehydration stress, larger group sizes had a significant lower water loss rate and often a better rate of survival during desiccation compared to single individuals. In contrast, a recent study showed that aggregation in caterpillars was a disadvantageous for water conservation, which could be due to higher growth rate or increased sensory stimuli between individuals in groups (Schoombie et al. 2013). The content of lipid and protein of individuals in groups or solitaire individuals could also give an indication of their
condition in terms of surviving starvation and reproduction abilities (Wilder 2011). Starving spiders have been found to have a low content of lipids compared to their content of proteins, suggesting that lipids are the primary energy source under starvation (Collatz and Mommsen 1975, Knudsen 2011). For example, Santos et al. (2007) found in termites, that groups compared to individuals had decreased lipase activity, which could be interpreted as reduced use of lipid for energy in group living individuals. Therefore, the content of the two macronutrients could be used to assess the benefits and costs of group living. When examining the effect of group size on performance (e.g. per capita energy consumption, desiccation tolerance, survival rate), three outcomes are possible 1) Group living has no influence on per capita energy consumption or other parameters.
2) Group living is a more energetically optimal strategy and will reduce per capita energy
consumption and optimizes other parameters (optimization hypothesis). 3) Group living leads to increased energy consumption or other costs per capita as social life increases complexity (thermodynamic hypothesis) (Fonck and Jaffé 1996, Muradian et al. 1999).
Here I investigate group size effects in a social spider, Stegodyphus dumicola (Eresidae). The spider occurs in Southern Africa where they inhabit dry thorn bush savannahs and form communal nests with up to several hundred individuals. The nests are built around twigs or branches of the bushes and consist of a central sponge-like mass of silk used as a refuge surrounded by sheets of
6 capture web to trap prey in (Avilés 1997). Within nests the spiders cooperate in prey capture, feeding, web building and maintenance, and maternal care of the brood (Seibt and Wickler 1988).
Since S. dumicola occupy hot and arid environments (Majer et al. 2013), group living may therefore provide better protection against thermal extremes due to larger and thicker nests.
Furthermore, cooperation in capture web and nest building may provide direct benefits such as reduced individual production of silk, thereby saving energy.
The aim of this study was to test the hypothesis that group living is associated with physiological benefits in the social spider species Stegodyphus dumicola. Specifically I tested (1) if group size and temperature affect the standard metabolic rate of S. dumicola. If there are any beneficial group size effect on the SMR, I would expect a reduced SMR in groups compared to solitaire individuals (Gallé 1978, Lighton and Bartholomew 1988, Anderson 1993, Zimmermann 2007). Furthermore, spiders acclimated to a low temperature would have a higher SMR than spiders acclimated to a high temperature, because at low temperature spiders must increase their SMR to be able to perform necessary tasks. (2) The spiders’ desiccation tolerance when kept at different group sizes was investigated and their survival rate under desiccation was examined. Here I expected that spiders in larger groups would tolerate desiccation better and have a longer lifespan (Abushama 1974, Sigal and Arlian 1982, Glass et al. 1998, Benoit et al. 2007). (3)I also tested whether the amount of silk made per individual was lower when living in groups compared to solitary living, and how the production was affected by temperature. This could be expected, as earlier studies have proposed that group living spiders build less web per spider than solitarily living spiders and that maintenance costs are higher for smaller than for larger groups (Riechert 1985, Tietjen 1986, Lloyd and Elgar 1997). Finally, (4) I examined the content of lipid and protein under starvation of spiders kept at different group sizes and at different temperatures. I expected that lipid content would decline compared to protein content in single individuals, and that individuals in groups would lose less lipid than single individuals and thereby have a higher lipid to protein ratio (Santos et al. 2007, Jensen et al. 2010). Furthermore, differences in L:P ratio between group sizes should be smaller in spiders kept at higher temperatures than for spiders kept at lower temperatures, because at lower temperatures solitary spiders need more energy to increase their metabolic rate to perform a range of tasks.
7
Materials and methods
Study animals and housing
The social spider Stegodyphus dumicola have a completely permanent social lifestyle and form colonies with up to several hundred individuals, that survives for a few years with discrete generations of an annual lifecycle (Seibt and Wickler 1988). The sex ratio in the colonies of S.
dumicola is female-biased. Avilés et al. (1999) found a mean sex ratio of 17% males embryos in S.
dumicola, but as the colonies mature it has been shown that the sex ratio becomes less skrewed.
Colonies are highly inbred as mating occurs within the nest among siblings (Bilde and Lubin 2011).
Colonies of S. dumicola from three different populations were collected three times at three different localities. In May 2012, 12 colonies of juvenile S. dumicola were collected from a population near Weenen (S 28.8386, E 29.9816) in the KwaZulu-Natal Province. In January 2013, 14 colonies of S. dumicola were collected from a population near Colenso (S 28.7091, E 29.8091) in the KwaZulu-Natal Province and in July 2013, 17 colonies were collected from a population near Sodoma (S 23.5483, E 28.7303) in the Limpopo Province. The spiders were transported by air to Aarhus, Denmark, where they were kept in a laboratory at Aarhus University. The colonies collected in May 2012 where held at room temperature (22-23°C) under natural photoperiod (from May to October). Colonies collected in January 2013 were held in a climate room at 25°C with an approximate 10L:14D photoperiod. Colonies collected in July 2013 were held in a climate chamber at 25°C with a 12L:12D photoperiod. In the lab, the spiders were kept in clear square plastic containers, 11 cm high and 17 cm times 17 cm at the base. A hole was cut in the lid of the container and covered with mesh to allow airflow. The spiders were fed to satiety three times a week with houseflies (Musca domestica) or crickets (Gryllus bimarculatus), and sprayed with water twice a week.
Only female spiders were used for experiments, but they differed slightly in developmental status.
Spiders collected in May 2012 were all adults or sub adults that had not laid egg sacs (weighing between 23 - 154 mg wet mass at the onset of experiments). Spiders collected in January 2013 were all adult females and some had egg sacs (weighing between 50-322 mg wet mass at the onset of experiments). Finally spiders collected in July 2013, were juvenile females (weighing between 4-18 mg wet mass at the onset of experiments).
8 Experimental protocol
The experiments were designed to investigate whether group size affects important characteristics of energy and water balance in social spiders. This was done by examining the effect of group size on 1) the standard metabolic rate, 2) the water loss rate and the survival rate, 3) the production of web and 4) the content of lipid and protein. The experiments were performed in three rounds on the three groups of spiders (see above). Spiders collected near Weenen were used for the first round of measuring the effect of group size on metabolic rates, its effect on lipid and protein content, and on web building activity. Spiders collected near Colenso were used to measure group size effects on water balance, and spiders collected near Sodoma were used for a second round of metabolic rate experiments.
Standard Metabolic rate experiment (SMR)
The standard metabolic rate experiment was run in two parts. The first experiment was conducted in October 2012, with spiders collected in May 2012 and the second was conducted in September 2013 with spiders collected in July 2013. The first experiment (using 9 colonies) was designed to assess the effect of temperature acclimation (22 and 30°C) and group size (one and five individuals per metabolic chamber) on the standard metabolic rate. Spiders were fed to satiety immediately before onset of the experiments after which they were weighed to the nearest 0.01 mg using a Sartorius Laboratory Balance (type 1712; Göttingen, Germany) and separated out into clear cylindrical metabolic glass chambers (70 mm high and 20 mm in diameter) with a mesh at both ends. A blocked design was used, with one group of one and one group of five spiders allocated to each temperature in cabinets set at 22 and 30˚C. In total 55 groups of five spiders and 55 groups of one spider were acclimated to high temperature (30°C) and 51 groups of five and 51 groups of one were acclimated to low temperature (22°C). Spiders remained in the temperature chambers for 12 days without food, but were sprayed with water every second day.
After 12 days of thermal acclimation I measured metabolic rate from the rate of CO2 production (VCO2) over a total experimental period of six days. 12 groups were measured daily (six replicates with five and six replicates with one spider) making a total of 72 groups (18x1, 18x5) from both acclimation temperatures.
9 The VCO2of S. dumicola was measured using intermittent closed respirometry in an experimental setup that sequentially measures the
CO2
V in 16 metabolic chambers (12 metabolic glass chambers with spiders and four empty chambers). The experimental setup was similar to the one described in Jensen et al. (in press). Briefly, two parallel 8-channels-multiplexers (RM Gas Flow Multiplexer, Sable Systems, Las Vegas, Nevada, USA) control the sequentially flushing and closing of the metabolic chambers such that the stop-flow respirometry system enabled me to obtain repeated measures of VCO2in 16 parallel metabolic chambers over a 21-hour period. During the flush phase the metabolic chambers were perfused with CO2 stripped air (air passing through a soda lime column (MERCK Millipore, Darmstadt, Germany) at a fixed rate of 200 ml min-1). Airflow was controlled by an adjustable mass flow meter (Side-Trak, Sierra Instruments, Monterey, California, USA) controlled by a flow controller (MFC 2-channel v. 1.0, Sable Systems, Las Vegas, Nevada, USA). After the flush phase the metabolic chamber was closed while the remaining 15 chambers were flushed sequentially in a similar manner such that the duration of the closed phase was 15 times the duration of the flush phase. The air leaving the metabolic chambers passed a calcium chloride column (AppliChem, Darmstadt, Germany) to remove water before entering a CO2
analyzer (Li-6251 CO2 Analyzer, LI-COR Environmental, Lincoln, Nebraska, USA). To optimize the signal-to-noise ratio the opening time was set at five minutes at 22°C and three minutes at 30°C (giving a closing time for 22°C at 15 x five minutes and for 30°C 15 x three minutes). Using these flush times, I obtained 15 and 26 independent measurements of
CO2
V at 22°C and 30°C,
respectively. The first five measurements were always discarded due to the possible confounding effects of handling. The standard metabolic rate was then estimated using the average of the three lowest remaining measurements during the experimental period for 22°C, and the two lowest measurements for 30°C. Experiments were run at two experimental temperatures 22°C and 30°C such that a sample with one and five spiders from each colony and each acclimation
temperature was measured. After the experiment the spiders were re-weighed, placed in Eppendorf tubes and frozen for later analysis.
The second round of measurements of standard metabolic rate also included 9 colonies. The experiments were conducted using the same approach as described above, but here all spiders were only measured and acclimated at 22 °C and the opening time was set at seven and a half
10 minutes (closing time for each chamber was 15x7.5 minutes). I obtained 10 independent
measurements of VCO2 at 22°C. The main objective of the second round of experiments was to examine the effect of larger group sizes, so in this round of experiments I examined group sizes of one, five and 20. Briefly, spiders were fed to satiety, weighed and placed in metabolic glass chambers in group sizes of one, five and 20. Spiders were then exposed to 12 days of thermal acclimation at constant 22 °C. From each colony we used 27 spiders (One group of 20 individuals, one group of five individuals and two replicates of individual spiders) for metabolic rate
experiments.
Analysis of metabolic data
The raw data of fractional CO2 content from the air flushing the metabolic chambers was
processed by a script in Mathematica (version 7.0, Wolfram Research, Champaign, Illinois, USA), which automatically identified the start, found the baseline value of each CO2 top and integrated the area between the graph and baseline. The signal from all measurements was examined individually and all recordings that were abnormal discarded (if the CO2 curves were cut before they peaked). All measurements were corrected by subtracting the average value of CO2
production found in the empty chambers (resulting from a minor CO2 diffusion into the system).
The CO2 productionoutput from Mathematica was transformed into micro litres per gram spider per hour (VCO2 μL/g/h) and the standard metabolic rate (SMR) was estimated using the average of the two lowest measures of CO2 at 22°C and the three lowest measures at 30°C for every day of measurements at the given temperature.
Water balance
To access the effect of group size on water balance and survival rate under high desiccation stress I used spiders from 14 colonies that were fed to satiety with crickets one day before onset of the experiments. Spiders from each of the 14 colonies were randomly separated out and placed in 7 squared containers (115 x 115x 6mm). Two holes were cut in each container and the holes were covered with gauze thereby ensuring that “container humidity” would be similar to the
surrounding air. From each colony the spiders were divided so four containers held one spider, one container held five spiders, one container held 10 spiders and one container held 20 spiders.
11 However, due to insufficient spider numbers in two colonies these were not represented with containers with 20 spiders. In total 506 spiders and 110 containers were used. At the onset of the experiment four spiders from each colony were sampled to establish baseline values of water content. They were kept at -18°C until further analysis.
Each empty container was weighed to the nearest 0.001 g with a Mettler Toledo balance (type PJ360 Delta Range; Greifensee, Switzerland). Spiders were then placed in the containers which were reweighed to obtain the weight of the spiders. Containers with the spiders were then placed in desiccation chambers and reweighed daily over a 44 day period to assess the desiccation- induced loss of mass. 19 empty containers were used as control to correct for any mass loss related to the container itself. The containers were randomly distributed into 6 hermetically closed tanks with glass lids. These tanks had 2-3 cm silica gel on the bottom to ensure a relative humidity (RH) < 5% and all tanks were placed in the climate room with constant 25 °C. In each tank a humidity data logger (iButton® Dataloggers, Maxim, Sunnydale, California, USA) was inserted and data from the loggers was extracted by OneWireViewer (Maxim, Sunnydale, California, USA).
The humidity in the tanks was checked every second day. Throughout the experiment, the humidity was kept <5%.
During the experiments, all containers containing spiders were weighed every day and therefore taken out of the desiccation tanks in a room with a relative humidity around 25% and a room temperature around 25°C for roughly 45 minutes per day. Spiders that died during the experiment were collected during the daily weighing. Dead spiders were weighed and frozen at -18 °C until further analysis of water content. After 44 days all spiders were dead and the water content of all spiders was examined by measuring mass of the spiders before and after being dried in an oven at 60°C for eight days.
To find the LT50 values (lethal time for 50% of the spiders) for the survival curve, a dose-response curve with variable slope was fitted to the survival data and IC50 values (LT50 values) were performed. This was done using the Graphpad Prism 6.0 program.
Web building activity
To assess the effect of temperature and group size on web-building activity we used spiders from 12 colonies. Spiders from each colony where randomly allocated to group sizes of one or five. The
12 spiders were weighed to the nearest 0.01 mg with a Sartorius Laboratory Balance (type 1712;
Göttingen, Germany). Spiders were placed in clear plastic cylindrical containers (113 mm high and 240 mm in diameter) and small holes were pierced into the containers to give the animals
attachment points for web building. The containers were closed with a mesh cover and
subsequently allocated to temperature cabinets set at 22 and 30˚C, respectively. The spiders were kept for three days without access to food or water after which all web was removed and web building activity was assessed by weighing the web produced to the nearest 0,001 mg (Sartorius Laboratory Balance (type MC 5; Göttingen, Germany). The production of web was measured as mg web spider-1. Only web not containing exoskeleton parts from moulting was weighed and used for statistical analysis.
Lipid and protein content
To assess the effect of temperature acclimation and group size on lipid and protein content I used spiders from 12 colonies that were fed to satiety immediately before onset of the experiments.
Spiders were housed individually or in groups of five in clear plastic cylindrical containers, 113 mm high and 240 mm in diameter closed with a piece of foam rubber and weighed to the nearest 0.01 mg with a Sartorius Laboratory Balance (type 1712; Göttingen, Germany). At the onset of the experiment, one spider from each of the 12 colonies were sampled (and placed at -20 before subsequent analysis) to establish baseline values of lipid and protein content. The remaining groups of one and five were subsequently allocated to temperature cabinets set at 22 and 30˚C.
Spiders for lipid and protein content were subsequently starved and collected after 13 and 26 days, respectively. In total 96 samples were made. Twelve samples from day 0 had one spider in each sample (control group). Thirty-six samples were collected after 13 days of starvation, 16 with groups of five spiders per sample and 16 had one spider per sample. Forty-eight samples were collected after 26 days of starvation, half of them had groups of five spiders per sample and the other half had one spider per sample. Half of the spiders from day 13 and half of the spiders from day 26 in mixed group sizes had been kept at 22°C. The remaining groups of one and five had been kept at 30°C for 13 and 26 days.
Spiders from all the sampling days were frozen at -20°C for several weeks before lipid and protein content analyses commenced. Prior to analyses, spiders were dried in an oven for approximately
13 90 hours at 60°C remaining in groups of one or five. Their dry weights were measured and the spiders were placed in desiccators to keep dry.
Determination of lipid contents
The analysis of lipid content was carried out using a Soxhlet apparatus. A round-bottomed boiling flask containing petroleum ether (bp 40-60°C) was heated in an oil bath and connected to an extraction chamber with a bypass sidearm conducting solvent vapour and a siphon arm refluxing the organic solvent. A condenser with water as coolant was placed on the top. Each spider sample was placed in a small tin container and transferred to the extraction chamber (20 containers at a time) and refluxed for approximately 48 hours (> 40 rounds of reflux). Afterwards the samples were left for one day in the fume hood to allow most of the remaining petroleum ether to vaporise, before samples were placed in the oven at 60°C to dry for three to four days. After the drying period the samples were weighed to the nearest 0.01 mg with a Sartorius Laboratory Balance (type 1712; Göttingen, Germany) and lipid content was calculated from the difference in mass before and after removal of the total lipid store.
Determination of protein contents
Samples used for protein analysis (CN analysis) were those where lipid had previously been
extracted (See above). The dried samples were pulverized with metal balls (50 Hz in 2 min) using a TissueLyzer LT (Qiagen, Copenhagen, Denmark). Afterwards ~ 2 mg of the tissue powder was weighed out to the nearest 0,001 mg (Mettler Toledo AX26; Greifensee, Switzerland) and packed in tinfoil. The samples were combusted using He in NA 2000 N-Protein (Fisons Instruments, Italy) and N content was calculated relative to standards with known C:N ratio (Cysteine, Atropina, and Sulfanilamid). Protein content was calculated using a protein:nitrogen factor of 6.25 (AOAC 2000).
Data analysis and statistics
All statistical analysis was performed in JMP 10.0.0 (SAS Institut Inc., Cary, NC, USA). Residual plots were inspected for normal distribution, and data were tested for homogeneity of variance. In some cases data were analysed with parametric models despite marginal non-homogeneity of
14 variance. If normality of residuals was not obtained data was transformed. Effect of independent variables (group size, temperature and day ) on dependent variables (web weight, lipid content, protein content, energy content, energy rate, CO2 production and % mass loss) were analysed with Mixed model ANOVA or ONEWAY ANOVA. In all analyses a random effect of colony ID was
included, to control for colony determined variance of the model.
Results
Standard metabolic rate (SMR)
In the first SMR experiment I found a significant effect of experimental temperature on
CO2
V . Thus,
regardless of group size the standard metabolic rate was much greater at 30°C than at 22°C. At 30°C the mean
CO2
V across groups was 154.9±12.31 µl h-1 g-1 compared to the mean
CO2
V at 22°C at 81.5±6.45 µl h-1 g-1, thus a difference of more than 52% (mixed model ANOVA, F=1,49;P<0.0001, n=72, figure 1a and 1b), equivalent to a Q10 value of 2.2. Q10 is an indicator of the temperature effect on metabolic rate when temperature increases with 10°C. There was no interaction
Figure 1. The standard metabolic rate of spiders kept at two group sizes (one=yellow; five=blue) and two acclimation temperatures (22°C=clear and 30°C=shaded), measured at a) 22 degrees and b) 30 degrees. Whiskers represent standard errors.
a) b)
)
15 Figure 2. The standard metabolic rate at 22°C of spiders that had been kept at different group sizes. Whiskers
represent standard errors. Group sizes are represented in colors, one=yellow, five=blue and 20=red.
between measurement temperature and acclimation temperature (mixed model ANOVA, F=1,59; P=0.348, n=72), while I found an almost significant effect of acclimation temperature (comparing spiders acclimated to 22 and 30°C) on SMR, indicative of a slightly elevated VCO2in the cold acclimated spiders (mixed model ANOVA, F=1,56; P=0.055, n=72). There was no significant effect of group size on the SMR when comparing groups sizes of one and five (mixed model ANOVA, F=1,59; P=0.431, n=72). Colony ID and respiratory chamber number were included in the statistical model and although these factors explained 17% and 8% of the variance, respectively, I found no
significant difference between colonies or metabolic chambers when using a Tukey post-hoc comparison test.
In the second experiment examining VCO2on sub adult females, I compared the effect of group sizes including one, five and 20 individuals on SMR. Here I examined the standard metabolic rate at 22°C and found that a mean SMR was 77.4±5.2 µl CO2 h-1 g-1 for single individuals and 73.5±5.3 µlCO2 h-1 g-1 and 69.8±5.0 µl CO2 h-1 g-1 for group sizes of five and 20 respectively.
As previous I found that there was no significant effect of group size on the standard
metabolic rate (mixed model ANOVA, F=2,38; P=0.476, n=48, figure 2). Colony ID and respirometry chamber explained 35% and 0% of the variance respectively, but a Tukey post-hoc comparison test showed no significant difference between colonies.
16 Water balance
Survival
The first spider died after 24 hours of the desiccation treatment, this was a spider that had been kept solitarily. The last spider died after 44 days, and had originated from an experimental group of 20 spiders. However, I did not find a significant difference between group size and survival rate (Wilcoxon test, χ2 = 1.3647, DF = 3, P=0.714, n=556) (figure 3). The LT50 is shown in Table 1. It shows that group size one has the lowest LT50 and group size five the highest LT50, but from figure 3 it is apparent that there are no significantly differences in LT50s among group sizes.
Figure 3. The survival rate of four group sizes (one=yellow, five=blue, 10=green and 20=red) over 44 days enclosed in chambers with a relative humidity of < 5% and under fasting condition.
Mass loss rate
Mass loss rate and water content for the control group and the four group sizes are presented in Table 1.
17 It is apparent from Table 1 and figure 4 that the mass loss rate for solitary individuals was higher than for any of the other group sizes (mixed model ANOVA, F=3,101; P<0.0001, n=1516).
Figure 4. The mean (±SE) mass loss rate of four group sizes (one=yellow, five=blue, 10=green and 20=red) under desiccation between day four and day 20. Capital letters in the legend indicates significant differences between group sizes.
There were no differences in water content in the spiders from the four group sizes (Mixed model ANOVA, F=3,148;P=0.396, n=503, data log transformed, Table 1). However, all four group sizes had a significantly lower water content compared with the control group (mixed model ANOVA, F=4,539; P<0.0001, n=556, data log transformed). There was significant effect of colonies on spider water content explaining 35% of the variance of the model. Selecting data from between day four and day 20 when measurements had stabilized (see below), the time span showed a significant mass loss rate (Mixed model ANOVA, F=16,119; P<0.0001, n=1516). The colony that spiders originated from explained 9% of the variance, with spider from some colonies losing water more rapidly than others.
The tank that the spiders had been held in during the experiment also explained some variation (9%).
18 Table 1. The lethal time for 50% of spiders (LT50), the mean(±SE) mass loss rate over 17 days and mean(±SE) water content for the control and the four group sizes (one, five, 10 and 20). Values (mean ±SE) followed by the same letter within a column are not significantly different (P>0.05).
Colony size # of colonies
Mean (±SE) initial weight per spider (mg)
LT 50 (±SE) Mean(±SE) Mass loss rate (% day-1)
Mean(±SE) Water content
(mg water mg dw-1)
Control 14 126.43±0.02ab 1.99±0.04a
1 14 136.46±0.02a 27.51±1.02 -1.72±0.08a 1.59±0.05b
5 14 128.11±0.03ab 31.97±1.05 -1.23±0.05b 1.64±0.04b
10 14 126.29±0.03ab 29.97±1.02 -1.20±0.02b 1.66±0.03b
20 12 116.71±0.03b 29.14±1.02 -1.20±0.02b 1.63±0.02b
Statistical analysis on mass loss rate data was performed on data from the forth to the twentieth day of the experiment’s 44 days duration time. This was decided due to a large drop in the total mass of spiders and containers from day one until day three and because of a low survival rate after day 20.
Web building activity
Spiders produced more silk per individual at the higher experimental temperature of 30°C
compared to 22°C (mixed-model ANOVA F=1,134; P=0.0007, n=147, data log transformed) (Table 2).
Furthermore, solitary spiders demonstrated a tendency towards increasing silk production per individual compared with spiders kept in groups of five, although this effect was marginally non- significant (mixed-model ANOVA, F=1,132; P=0.062, n=147, data log transformed). From Table 2 it is visible that groups of five at 30°C produce less silk per individual than single individuals do. The interaction between experimental temperature and group size was not significant (mixed model ANOVA, F=1,133; P=0.477, n=147, data log transformed), but a Tukey post-hoc comparison showed that there was a significant difference effects of group size and temperature, showing that the
19 differences in silk deposition per individual was smaller in animals kept at higher temperatures than for animals at lower temperatures. The colony that the spiders originated from explained 31% of the variance and a Tukey post-hoc comparison showed that there was significant difference in individual silk production among some of the colonies.
Table 2. The mean (±SE) weight per spider and mean silk deposition per spider kept at two different group sizes (one and five) and two different temperatures (22°C and 30°C).
Lipid and protein content
I analyzed the content of lipid and protein at two group sizes (one and five), at two temperatures (22°C and 30°C) and at 13 and 26 days of starvation respectively (Table 3).
During starvation, spiders in both group sizes lost weight, although the spiders in both group sizes gained higher water content. I found a significant effect of starvation days on weight loss (mixed model ANOVA, F=2,81; P<0.0001, n=96, data log transformed)( Table 3). In addition, solitary spiders lost significantly less weight than individuals in groups of five did (mixed model ANOVA, F=1,69; P<0.023, n=84, data log transformed). Temperature likewise affected weight loss: more weight was lost at the higher temperature than weight lost at lower temperatures (mixed model ANOVA, F=1,69; P<0.003, n=84, data log transformed) for both group sizes. The content of lipid and protein were not influenced by group size (lipid: mixed model ANOVA, F=1,79; P=0.283, n=93, protein:
mixed model ANOVA, F=1,79; P=0.972, n=93). There was a tendency for spiders to lose lipids during starvation rather than protein. For lipid content (mixed model ANOVA, F=2,81; P=0.0595, n=93). A Tukey post hoc comparison test showed significant difference between day zero and day 26. For
Temperature (°C) 22 30
# of colonies 12 12
Group size 1 5 1 5
Mean(±SE) weight (mg ww spider-1)
79.7±0.036 80.0±0.036 72.3±0.035 78.4±0.032
Mean(±SE) silk deposition (mg spider-1)
0.205±0.14 0.242±0.10 0.310±0.22 0.293±0.11
20 protein content no significant difference was found (mixed model ANOVA, F=2,81; P=0.094, n=93).
The acclimation temperature influenced the content of protein.
Table 3. The mean values (±SE) of start wet weight (ww), end wet weight (ww) and the percentage lipid and protein content per group size, and the water content, energy content per individual and energy rate for control spiders collected prior to the start of the experiment, and groups of spiders kept in two different group sizes (one or five) at two temperatures regimes (22°C and 30°C) for 13 or 26 days of starvation.
Day Control 13 26
Temperature
(°C) 22°C 30°C 22°C 30°C
Group size 1 5 1 5 1 5 1 5
Start weight (mg ww ind-
1)
77.10±0.01 71.20±0.01 74.05±0.01 76.74±0.01 78.64±0.01 71.82±0.01 80.41±0.01 67.01±0.01 81.82±0.01
End weight (mg ww ind-
1)
77.10±0.01 62.77±0.01 64.29±0.01 59.36±0.01 29.69±0.01 57.16±0.01 63.28±0.01 44.78±0.01 50.00±0.01
Water content (%
ww)
69.13±0.59 73.18±0.84 73.92±0.58 75.49±0.92 76.21±0.54 75.21±0.60 76.23±0.76 79.25±0.87 80.27±0.93
Lipid content (% dw)
14.87±1.53 11.05±2.1 12.52±1.1 11.34±2.2 13.31±1.1 14.83±1.6 11.43±1.0 7.49±2.0 11.63±1.4
Protein content (%
dw)
70.69±1.64 73.49±2.5 71.63±1.7 76.73±2.4 73.81±1.6 69.93±1.7 75.02±1.3 78.68±2.4 76.80±1.6
Energy content (kcal g-1)
1.35±0.05 1.24±0.06 1.27±0.05 1.36±0.07 1.37±0.04 1.33±0.06 1.26±0.06 1.26±0.09 1.44±0.06
Energy rate (cal day-1 g-1)
19.04±4.46 17.14±3.09 22.68±6.02 26.57±2.47 10.54±1.72 13.69±1.69 21.22±2.33 24.33±2.73
Note. The energy rate (cal/g/day) was calculated by subtracting the finial energy content from the calculated initial energy content divided by the fasting time and by the mass (In the calculation it was assumed that the body mass was intermittent to the initial and finial body mass). The finial energy content (kcal) was calculated as (end dry body mass (g)*fraction of dry mass* fraction of lipid*9.4 kcal/g) + (end dry body mass (g)*fraction of dry mass* fraction of protein*4.25 kcal/g). The estimated initial energy content was calculated as ((mean finial energy content for the control spiders*start wet body mass)/start wet body mass). Lipid contains 9.4 kcal/g and protein contains 4.25 kcal/g.
Protein content was higher when the temperature was high (mixed model ANOVA, F=1,67; P=0.0006, n=81)(Table 3), while there was no difference in lipid content between the two temperatures (22°C and 30°C) (mixed model ANOVA, F=1,67; P=0.074, n=81). Table 3 shows a
tendency for spiders during starvation to experience a decline in their energy content compared to the control, but this decline was not significant (mixed model ANOVA, F=2,82; P=0.623, n=94, data log transformed). Furthermore, group size had no significant effect on energy content (mixed model ANOVA, F=1,79; P=0.232, n=94, data log transformed). Temperature showed a tendency for
21 affecting the energy content, so that at higher temperature, energy content was higher (mixed model ANOVA, F=1,68; P=0.0502, n=82, data log transformed). How much energy spiders lost per day (energy rate) was under the influence of acclimation temperature. Spiders kept at 22°C had a lower energy rate than spiders kept at 30°C regardless of group size (mixed model ANOVA, F=1,71; P=0.0001, n=84). Energy rate was lower after 26 days than after 13 days of starvation regardless of group size and temperature (mixed model ANOVA, F=1,77; P=0.026, n=84).
Lipid protein ratio (L:P ratio)
To evaluate the relative impact of starvation I calculated the ratio between the lipid and protein content in percent of the dry weight at each of the two group sizes. When kept at 22°C the two group sizes had a higher L:P ratio than kept at 30°C. (mixed model ANOVA, F=1,65; P=0.023, n=79, data
Figure 5. The lipid protein ratio for control spiders collected before the onset of the experiment, and of spiders after 13 and 26 days of starvation for group sizes one and five, which had be acclimated at temperatures of 22°C and 30°C. Whiskers represent standard errors. Clear bars = 22°C and dashed bars= 30°C.
square root transformed, figure 5). Group size had a marginally non-significant effect on the L:P ratio (mixed model ANOVA, F=1,65; P=0.085, n=79, data square root transformed). The interaction between temperature and group size showed that in treatments tested after 26 days of starvation
22°C
30°C
22 groups responded differently depending on whether they were acclimated at 22 or 30°C ( mixed model ANOVA, F=1,65; P=0.0213, n=79, data square root transformed, Figure 5). Individuals from group size one did not differ in their L:P ratio at any day of starvation (13 or 26 days) in
comparison to the control group (mixed model ANOVA, F=2,41; P=0.215, n=52).
Discussion
The standard metabolic rate
I examined the effect of group sizes on the standard metabolic rate (SMR) of the social spider S.
dumicola using groups of one, five and 20 individuals, testing the hypothesis that group living is a more energetically optimal strategy and task sharing would lower energy consumption, i.e.
lowering individuals’ SMR. The result shows that there was no effect of group size in S. dumicola on SMR, although there was a tendency for a reduced SMR in groups compared to single
individuals. As expected there was a clear effect of temperature on SMR no matter the size of the group. This supports the generally theory of higher enzymatic activity at higher temperature and therefore a higher metabolic rate in ectoterm animals as spiders (Lighton and Bartholomew 1988, Randall et al. 2002). The Q10 is normally in the range of two-three, so the effect of temperature in the experiment was within the expected range. My results showed that acclimation had an almost significant effect, so that individuals acclimated at 22°C regardless of group size showed a
tendency for higher SMR. The explanation for this could be that at lower temperatures the spiders need a higher SMR to perform tasks (Addo-Bediako et al. 2002).
The metabolic rate of spiders has generally been found to be lower compared to other
arthropods (Anderson 1970, Greenstone and Bennett 1980). However, the low metabolic rate of spiders have been questioned in recent years, because it could be an artefact of experiments performed on starved spiders (Lighton and Fielden 1995). Nonetheless, the low metabolic rates of spiders have been associated with their sit-and-wait lifestyle where they rely on stochastic food availability. In my study an average spider had a body mass of 77.6 mg (Table 2) (first round of SMR experiment). Following the relationship between body mass and metabolic rate described in Overgaard and Wang (2012), the average spider would have a metabolic rate at 22°C of 158.9 µl CO2 h-1 g-1. However, this does not correspond with my findings. Here the average metabolic rate
23 at 22°C was approximately half of the expected from Overgaard and Wang (2012) (81.5 µl CO2 h-1 g-1). It is therefore possible that social spiders have a lower SMR compared to other non social spider species as well as other arthropods, possible suggesting a benefit of group living. This can however only be clarified which more comparative studies on social and solitary spider species.
The effect of group size on the standard metabolic rate or field metabolic rate (average rate of energy utilization under normal daily activities) has been well studied in ants. Gallé (1978)
investigated the group effect on SMR on four species of ants and found a significant group effect in each species. Here workers in groups of 10 of the species Formica cunicularia showed a significant lower SMR than single workers, this was also similar for the other three species in the study. However, it has been suggested that this group size effect simply reflect lower levels of activity in groups than in single individuals, because an isolated ant would never be at rest
(Bartholomew et al. 1988). In most other studies group size had no effect on the metabolic rate of ants (Brian 1973, Lighton and Bartholomew 1988, Lighton 1989). In contrast, studies of other arthropods taxa have generally shown a reduction in metabolic rate when group size increased (Muradian et al. 1999, Tojo et al. 2005, Santos et al. 2007). In study cases conducted on group living species in the class Arachnida, the standard metabolic rates were often found to be lower for grouped individuals than single individuals (Anderson 1993, Zimmermann 2007). But generally few studies have focused on the standard metabolic rate of subsocial or social spiders.
Stegodyphus dumicola normally lives in colonies with up to several hundred individuals, and although I found no group effect, it is possible that group effects are only found within very large group sizes. Furthermore, it could be argued that it would be more relevant to examine spider’s field metabolic rate, as SMR represents a measurement of the basal energy consumption
(supporting cellular and subcellular processes occurring in the tissue) (Randall et al. 2002), and not a situation where cooperative efforts unfold. But for logistical reasons the field metabolic rate is difficult to measure as many other factors (including temperature) fluctuate considerably under these conditions. An alternative would be to focus on the routine metabolic rate, as this rate includes the overall energy utilization of the spider. In this way normal daily activities such as prey capture and web building which are shared tasks (Seibt and Wickler 1988) could be estimated.
Although I did not measure the field routine metabolic rate, I have indirectly assessed this rate in single individuals and in groups of five individuals over 13 days. This was done by measuring the
24 spiders’ energy content (i.e. lipid and protein) after 13 days of starvation and subtracting this from an estimated initial energy content (Table 3). This estimation relies on the assumption that the loss of energy is principally associated with respiration and not with loss of biomass through web production, shedding of exoskeleton etc. My estimate of the metabolic rate based on the energy content was 19.04 cal g-1 day-1 for individual spiders at 22°C and for groups of five 22.68 cal g-1 day-
1. This generally confirms the rates of 11.73 and 14.18 cal g-1 day-1 I obtained by respirometry for individuals and groups of five respectively at 22°C. At 30°C the estimated metabolic rate from the energy content was 22.68 cal g-1 day-1 for individuals and 26.57 cal g-1 day-1 for groups of five again confirming the rates obtained by respirometry that for individuals and group of five which were 22.16 and 22.53 cal g-1 day-1 respectively. Thus, concluding that both respirometry (that can be considered as a momentarily picture of the spiders energy use) and energy content over time (giving a more balanced picture) can be considered suitable ways of estimating SMR
independently in this spider species.
Water balance
I tested the optimization hypothesis on desiccation tolerance, predicting that larger group sizes would tolerate desiccation better and have a higher survival rate. I tested this by exposing S.
dumicola to a relative humidity of 0-5% in groups of one, five, 10 and 20 individuals. I found a significant positive effect of group size on the spiders’ ability to tolerate desiccation. Individuals had a higher mass loss rate compared to groups of five, 10 and 20 individuals, however, I observed no significant differences in survival rates among different group sizes.
Most arthropods consist of approximately 70% water and can resist a water loss up to 30% of their body weight (Edney 1977). Earlier desiccation tolerance studies performed on other social
arthropods found that group living decreases the vulnerability of individuals against desiccation and thereby increases their survival rate (Sen-Sarma 1964, Abushama 1974, Sigal and Arlian 1982).
The tendency observed was that the desiccation tolerance of the social ant species Formica exsectoides was higher when living in groups. The net water loss was more than 50% reduced for group workers (9-15 individuals) than for the isolated workers held at a RH of 45%. Furthermore, for ants held at low RH group individuals had a 40% higher survivorship compared to single ants held at low RH. The same tendency was shown for termites (Sen-Sarma 1964, Abushama 1974).
25 This conforms to a certain extent with my findings, as groups had a lower mass loss rate, but not a higher survival rate.
The reason that desiccation tolerance is so low for individuals could be due to “social” stress as isolation promotes higher activity levels (Sigal and Arlian 1982). This could trigger higher metabolic rate that increase ventilation and give rise to higher water loss to the surroundings. In addition, isolation could prevent trophallactic exchanges, which have been suggested to be of high importance for social animals’ communication with each other (Blum 1970). It has further been proposed that the ability to lower water loss in groups could be due to the formation of a ‘super organism’ that lowers the overall surface area volume ratio. In addition, the formation of a
‘microcosm’ among the individuals within the group, could raise the RH to a higher degree than that of the surroundings (Glass et al. 1998) and likewise reduce water loss.
The fact that groups in my experiment tolerate desiccation better than single individuals is ecologically relevant, and likely represents an important benefit of group living. Stegodyphus dumicola occupies relative hot and arid environments, which selects for high water conservation adaptations and desiccation resistance, and spiders are expected to be adapted to low RH.
Improved desiccation tolerance would be a major benefit of group living. Clustering could also decrease oxygen consumption rate and in response decrease respiratory water loss (Lighton and Bartholomew 1988, Hadley 1994). I observed during my experiment that spiders had a tendency to cluster together covered with silk, a behaviour that was not possible for the isolated individuals, supporting the ‘super organism’ or ‘microcosm’ theory, even though it had no significant effect on the survival rate. Although this suggests that spiders kept individually experienced higher
desiccation upon death than spiders from larger groups, there was no significant difference in water content among the four group sizes at the time of death as all groups had lost similar proportion of water compared to the control group.
In the starvation experiment, spiders starved for 26 days and only few individuals died during this time period, suggesting that these spiders are tolerant to starvation and that spiders in the desiccation experiment probably did not die from starvation. Overall this result implies that spiders during the experiment lost water more than macronutrients (i.e. lipid and protein).
Macronutrients are the reserves to overcome starvation, which would have been important in this experiment during which the spiders were not fed. It could also be the web itself that protected
26 the spiders against desiccation as S.dumicola occupy very arid environments. Seibt and Wickler (1990) found that the nest of S. dumicola had no effect in protection against desiccation, however this result needs further study since the thick silk layer may have small but important
thermoregulating function.
Web building activity
In my study of S. dumicola, I measured the silk production per individual in single individuals and in groups of five individuals under two temperatures. I hypothesized that single individuals produced more silk per individual than individuals in groups, and that both group sizes produced more silk at the higher temperature. I found that there was a tendency for single individuals to produce less silk than individuals in groups of five did. However, from Table 2 it is also apparent that at 30°C individuals in groups of five showed a tendency to produce less silk than single individuals, making it difficult to conclude any form for group effect on silk production.
My finding is not in direct contradiction to the hypothesis and previous findings. It has been proposed that one advantage of group living is lower web building activities per individual and thus saving of energy. Riechert (1985) found in the social spider Agelena consociata that web area was not a linear function of numbers of individuals in a colony, and that significantly smaller web areas per individual were made in larger nests. The decreased energy investment in web building with increasing group size would also entail lower investment in web maintenance per individual.
In accordance with this observation, Riechert (1985) found that the consumption level of prey decreased with group size. Furthermore, Riechert (1985) estimated that energy acquired for web construction was 366.8±2.8J per day for a single layer of web, and that a spider on average spends 366.5±1.6J per day in the capture of prey, biomass accumulation and metabolism. This study concluded that web construction is a cost to the single individual as it doubled the daily energy expenditure. Majer (2012) found similar effects of group size in S. dumicola in the field. Here it was observed that the web size does not increase proportionally with group size and web production per individual was smaller in groups compared with single individuals. In addition, it was found that the per individual energy saving associated with decreased silk production in groups was not outweighed by the ability of spiders to catch prey. The cooperative foraging behaviour increased the ability to catch larger prey, but failed to catch sufficiently more prey to prevent decreased
27 biomass intake per individual with increasing group size. That my result on silk production does not exactly match previous findings could perhaps be explained by too small group size
comparisons. In comparison Tietjen (1986) found less web production per individual in quite small group sizes (two, five, 10 and 20 individuals) compared to single individuals in the social spider Mallos gregalis. Often studies have suggested that the benefits of larger groups, and the increase in web and nest area in larger groups is better protection against predators and/ or environment conditions such as rain and wind (Riechert 1985, Seibt and Wickler 1990, Henschel 1998, Bilde et al. 2007, Majer 2012). It may be that such benefits are prevailing, rather than an increased ability to catch larger prey items, although this has been suggested to be one advantage of group living.
Finally, I found that at higher temperature spiders from both group sizes produced more silk than they did at a lower temperature, which could be explained by an increase in activity because of higher enzymatic activity yielding hungrier spiders, so they build more web or produce web faster.
Lipid and protein content
I measured the content of lipid and protein of S. dumicola to test the hypothesis that during starvation, groups of five should consume less lipid and protein than isolated individuals and that both group sizes would use more lipid and protein at a higher temperature. I found that groups of five lost more weight during starvation than single individuals, but that group size did not
influence the content of lipid and protein. Temperature influenced the content of both
macronutrients in the spiders, so more lipid and protein was used at higher temperature. There was a tendency for spiders to lose more lipid than protein during starvation, which corresponds well with earlier findings in spiders (Collatz and Mommsen 1975, Knudsen 2011). During the starvation period of 26 days the spiders in both group sizes had a similar decrease in energy content and energy rates. Both the energy content and the energy rate were under the influence of temperature, showing that there was a higher energy loss when temperature was high, which is in good standing with the general theory (Randall et al. 2002). I also found that water content increased during starvation, with is consistent with earlier studies (Stewart and Martin 1970, Collatz and Mommsen 1975, Knudsen 2011). This increase in water content has been suggested to be a result of the fact that lipid cells contains less water than other cell types, so fewer lipid cells will give a higher overall water content. It could also simply be, that water is taken up to fill the
28
“empty” volume that spiders experience when they use up energy reserve stores (Overgaard and Wang 2012). Water is also a by-product of lipid catabolism, so during starvation this will also add to the water content.
Lipid protein ratio (L:P ratio)
In my study, the lipid protein ratio did not differ significantly between the two group sizes, but temperature had an effect on the L:P ratio, showing lower ratios at higher temperature, which could be explained by faster running enzymatic processes at higher temperatures. The interaction effect between acclimation temperatures and group size was significant, so the two group sizes behaved differently under different acclimation temperatures and differently between days of starvation. The biological implication for this could be a mix of factors. Spiders that live solitarily and at a higher temperature could be consuming more lipids to overcome daily activities
compared to groups. Furthermore at a lower temperature the enzymatic activity is lower and this could increase lipid savings. In addition, in groups individuals share tasks and can therefore save energy (i.e. lipid).
Starving spiders have been found to have a lower content of lipids than content protein,
suggesting that lipids comprise the primary energy source under starvation (Collatz and Mommsen 1975, Knudsen 2011). Jensen et al. (2010) studied different nutritional status in the spider
Pardosa prativaga and found that during starvation the spiders that had previously been fed with lipid-rich prey survived longer than spiders earlier fed with protein-rich prey. Therefore, the content of lipids and protein could offer an indication of spiders’ starvation tolerance, with spiders low in lipid content presumably having a lower starvation tolerance. The contents of lipid and protein could thus be a way of assessing the benefits and costs of group living. For example, Santos et al. (2007) found that groups of third instar of the termite species Cornitermes cumulans showed a decrease in the activity of lipase, which could be interpreted as a reduced use of lipid as energy source when living in groups.
Only few studies have investigated the physiological benefits of group living, such as energy and water conservation. Recently Schoombie et al. (2013) examined the possibility of physiological benefits in aggregations of caterpillars from South Africa. They showed in alignment to my results
29 that group living did not reduce the SMR when group size increased. But in contrast to my results, they found that water loss increased with group size. In my study, in consistency with my
prediction, I found a significant positive effect of group size in desiccation tolerance. This could represent a significant benefit of group living, as S.dumicola occupy hot and arid environments (Majer 2012), conservation of water may directly influence survival rates. Other explanations for my result could be that a “group effect” modifies the cuticle structure of the spiders making it more waterproof, or that there exists some kind of physical or chemical communication between individuals that tells them how to act under desiccation. Furthermore, my results showed that group size did not significantly affect the spiders’ ability to save energy (i.e. lipid and protein) and little indication for silk saving benefits of group living. Silk production in groups could be
depending on temperature, as I found a tendency for group effects when spiders were acclimated to 30°C but not to 22°C. However, lack of group effects could also be due to relative small group size comparisons as previous studies indeed proposed a group effect on energy and silk
conservation when living in groups (Riechert 1985, Tietjen 1986).
In addition, colonies explained a great part of the variance in many of the statistical models (second round of SMR, mass loss rate, silk production and water content). This could seem odd, as the colonies in the experiments were collected in relative short distance from each other.This could be explained with intraspecific adaption to the same environmental conditions, genetic difference or weight difference among colonies. But most likely it is an expression for the large variance of the colonies sampled.
In conclusion, I found that group effect improve desiccation resistance thereby confirming the optimization hypothesis, that group living do confer advantage. Other physiological benefits of group living were not profound in my experiments. Mainly my results confirmed the hypothesis that group living do not have any influence on energy consumption or other parameters such as energy savings in form of reduced lipid and protein consumption or silk production. Although, I found a tendency towards a lowering of the SMR at larger group sizes, and therefore at tendency towards a confirmation of the optimization hypothesis, other types of benefits for group living in this species in addition to water conservation may exist to maintain benefits of group living. These benefits could be the ability to catch larger prey, better protecting against predators and higher
