Spiders in the genus Tetragnatha feed on emerging aquatic insects, including mosquitoes and midges, but there is little known about the foraging behavior of these spiders. We hypothesized that female spiders actively developing egg sacs would increase food consumption to provide more energy to produce and provision their eggs. We tested this hypothesis by measuring foraging rates of Tetragnatha spiders kept in jars and provisioned with different levels of midges. We then tested for a difference in the functional response of spiders that did or did not lay egg sacs in their jars. Egg-laying and non-egg-laying spiders showed significantly different functional responses, indicating that Tetragnatha spiders can change their behavior or web structure to increase their foraging rate, presumably to accommodate increased energy demand for reproduction

Boswell, Mary E.

DeLong, John P.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications in the Biological Sciences Papers in the Biological Sciences 2019 Gravid Tetragnathid Spiders Show an Increased Functional Response Mary E. Boswell University of Nebraska - Lincoln, mary@theboswells.org John P. DeLong University of Nebraska - Lincoln, jpdelong@unl.edu Follow this and additional works at: https://digitalcommons.unl.edu/bioscifacpub  Part of the Biology Commons, Other Animal Sciences Commons, and the Terrestrial and Aquatic Ecology Commons Boswell, Mary E. and DeLong, John P., "Gravid Tetragnathid Spiders Show an Increased Functional Response" (2019). Faculty Publications in the Biological Sciences. 824. https://digitalcommons.unl.edu/bioscifacpub/824 This Article is brought to you for free and open access by the Papers in the Biological Sciences at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Boswell and DeLong in Food Webs (2019) 21: e00122. Copyright 2019, Elsevier. Used by permission. 1 Gravid Tetragnathid Spiders Show an Increased Functional Response [Short communication]  Mary E. Boswell and John P. DeLong  Abstract Spiders in the genus Tetragnatha feed on emerging aquatic insects, including mosquitoes and midges, but there is little known about the foraging behavior of these spiders. We hypothesized that female spiders actively developing egg sacs would increase food consumption to provide more energy to produce and provision their eggs. We tested this hypoth-esis by measuring foraging rates of Tetragnatha spiders kept in jars and provisioned with different levels of midges. We then tested for a difference in the functional response of spiders that did or did not lay egg sacs in their jars. Egg-laying and non-egg-laying spiders showed significantly different functional responses, indicating that Tetragnatha spiders can change their behavior or web structure to increase their foraging rate, presumably to accommodate increased energy demand for reproduction.  Keywords: Foraging, Prey selection, Spider webs, Energy needs, Tetragnatha, Midges  Predator-prey interactions are crucial to our under-standing of ecological systems. Predation plays a key role in energy and nutrient movement through food webs, directly affecting the abundance of prey species (Pimm et al., 1991), the structure of food webs (Schmitz et al., 1997), and the func-tioning of ecosystems (Schmitz, 2008). Globally, spiders play an outsized role as predators of invertebrates, contributing both to the control of insect pests and the overall structure of terrestrial food webs (Nyffeler and Birkhofer, 2017). Thus, un-derstanding sources of variation in spider foraging is critical to understanding the structure and function of terrestrial food webs. The functional response, or foraging rate with respect to prey density, provides a standardized way of assessing variation in foraging rates. The functional response typically is written as a saturating, type II function using the ‘disc’ equation:  y =  ax1+ahx             (1)  where a is the space clearance rate of the predator, h is the handling time for one prey item, x is the number of prey items available, and y is the per capita rate of prey consumption (Holling, 1959). The handling time and space clearance rate of a predator are affected by several factors, including the body size of the predator and the prey, the body condition of the predator, temperature, and the predator’s foraging strategy (Lyon et al., 2018; Pawar et al., 2012; Rall et al., 2011; Uiterwaal et al., 2017). Foraging strategy may be a fixed aspect of a pred-ator’s behavior, such as sit-and-wait versus active pursuit predation, but foraging behaviors also may be influenced by the energetic needs of the organism. Some organisms, for ex-ample, increase their food intake (hyperphagy) in preparation for energy-intensive activities such as migration (Odum, 1960) or reproduction (Gurney and Nisbet, 2004). Higher foraging rates and diet quality are also correlated with increased qual-ity of egg production in arachnids (Rahman et al., 2012; Wil- der and Rypstra, 2008). It is less clear, however, whether sit-and-wait predators can increase prey intake in response to the increased energy demand associated with reproduction. Long-jawed orb-weavers vary foraging strategies in re-sponse to prey density and type (Gillespie and Caraco, 1987; Tahir et al., 2009; Yoshida, 1987). In particular, Tetragnatha spi-ders may adjust their foraging location in response to prey density (Gillespie and Caraco, 1987), presumably with the outcome of increasing prey capture. Such differences in for-aging behavior should translate to a steeper functional re-sponse with either a greater space clearance rate, lower han-dling time, or both, for those individuals with higher energy demands, but this hypothesis has not been tested. We compared the functional response of wild-caught female Tetragnatha laboriosa spiders that did and did not lay eggs while they were kept in the lab. While previous studies have controlled for the ef-fects of gravidity on foraging rates by using only gravid females, we sought to use both gravid and non-gravid females to compare the two (Ali et al., 2011; Rahman et al., 2012). We hypothesized that fe-male spiders that had laid egg sacs in their containers would show a steeper functional response (greater rates of foraging at all prey den-sities) than those that did not, since higher consumption rates are presumably needed to produce and provision the eggs. We conducted our experiment during June of 2017 and 2018 on the south shore of Lake Ogallala, Nebraska, at Cedar Point Biological Station. We collected Tetragnatha spiders in the morning by sweep-netting grassy roadsides and mead-ows. Each spider was individually housed in a cylindrical 250 mL plastic jar, with a height of 7.5 cm and diameter of 6.5 cm. We placed a damp piece of paper towel in each jar to provide moisture and rewetted them as needed to maintain moisture. We left the spiders alone during the day to allow time for each spider to settle and build its web. We assumed that all spiders were female, since male Tetragnatha spiders do not build webs. We collected a pre-determined random number of midges (Chironomidae) directly into the spider habitats from the windows of the Cedar Point Biological Station lab after sunset using a jar lid fitted with an aspirator. We chose midges   Boswell: School of Natural Resources, mary@theboswells.org (corresponding author) DeLong: School of Biological Sciences, jpdelong@unl.edu University of Nebraska-Lincoln, Nebraska, USA2 Figure 1. The functional response of Tetragnatha spiders forag-ing on midges. Spiders that laid eggs had a steeper functional response with a higher asymptote than spiders that did not.  as prey because they make up the bulk of Tetragnathid spi-ders’ diets (Yoshida, 1987) and are a dominant aerial insect in the area. We left spiders to hunt overnight for 10 hours, and at 7:30 A.M. the following morning, we counted the uneaten midges to determine the number of midges that were eaten. Throughout the experiment, several of the spi-ders laid egg sacs in their containers. We waited 3–4 days after the foraging trial ended to allow gravid spiders to lay egg sacs. We took care to not damage the egg sacs while changing lids and remoistening the jars. Temperature in the lab was approxi-mately 21°C. We did not replace prey as they were eaten through the night, and therefore the prey density within the jars declined with each prey capture. We therefore fit our data to the Rogers random predator equation, which accounts for this within-ex-periment prey depletion (Bolker, 2008; Rogers, 1972), where parameters are the same as in Equation (1) and W is the Lam-bert W function:  y = x −  𝑊(ahx  exp(−a(0.42−hx)))ah          (2)  We used ordinary non-linear least squares regression to esti-mate the mean and 95% confidence intervals of the parame-ters of Equation (2) (space clearance rate a and handling time h). We fit the data for egg-laying and non-egg-laying spiders separately. We used a bootstrap approach to determine if a and h were significantly different between egg-sac-laying and non-egg-sac-laying spiders. We randomly sampled with re-placement each original data set to create 100 bootstrapped datasets, and we fit Equation (2) to each dataset. We then cal-culated the difference between the parameters for each possi-ble pair of curves that differed in egg-laying status. This pro-cess generated a distribution of differences between the two parameters, and we assessed significant differences between the parameters by determining whether the 95% confidence interval of the difference between parameters included zero. Egg-sac-laying and non-egg-sac-laying spiders each showed Type II functional responses (Figure 1; alaying = 8.14 arenas per predator per day, 95% CIs: 5.67 to 27.54; hlaying = 0.003 days, 95% CIs: ~0.00 to 0.008, R2 = 0.90; anon-laying = 7.53 arenas per predator per day, 95% CIs: 4.57 to 15.72; hnon-laying = 0.01 days, 95% CIs: 0.007 to 0.013, R2 = 0.65). The confidence intervals for the differences in a and h between the two data sets did not include zero, so the two parameters were consid-ered significantly different from each other, with a being higher and h lower for egg-sac-laying spiders (Figure 2; 95% CIs difference in a: 0.09 to 21; 95% CIs difference in h: 0.0016 to 0.012). Much of what we know about variation in the func-tional response comes from across-species comparisons (e.g. Uiterwaal and DeLong, 2018). There is clear variation in both   Difference in a       Difference in h  Figure 2. The distribution of differences between A) space clearance rate, a and B) handling time, h. The 95% confidence intervals on these distributions did not include zero, indicating that both parameters were significantly different between egg-laying and non-egg-laying spiders. The leftmost bins include only positive differences. In A) the inset shows detail for the lower bins.  3 space clearance rate and handling time across species associ-ated with body mass, temperature, predator satiation, forag-ing dimensionality, and the taxonomic identity of both pred-ator and prey (DeLong et al., 2015; Kalinoski and DeLong, 2016; Li et al., 2018; Pawar et al., 2012; Rall et al., 2011, 2008; Uiterwaal and DeLong, 2018). Additional variation in the functional responses within predator-prey pairs is associated with body mass, prey defenses, temperature, energetic his-tory, and individual identity (Barnhisel and Kerfoot, 2004; Hammill et al., 2010; Lyon et al., 2018; Schrer et al., 2016; Spitze, 1985). However, it is less clear that particular energy-demanding activities drive individual-variation in the func-tional response. Our experiment clarifies that the shift to be-ing gravid increases both the steepness and height of the func-tional response in long-jawed orb-weavers. This shift could have implications for the strength of inter-actions linking these spiders to their prey and consequently for the predator-prey dynamics.  Tetragnatha spiders in this study that laid egg sacs ate more midges at all prey densities than those that did not. Although it was clear that there was not complete separa-tion of foraging behavior between egg-sac-laying and non-laying spiders, suggesting some possible miscategoriza-tion of individual spiders, our results show that reproduc-tive status should be considered when determining the functional response of a predator, as individuals provi-sioning offspring need higher prey intake. How a sit-and-wait predator achieves an increase in prey consumption, however, is not clear. The most likely candidate for the in-creased foraging is alterations to spider web structure. A previous study showed that starved Octonoba sybotides built webs with tighter spacing than usual, likely to target smaller but more numerous prey items (Watanabe, 2001). The egg-laying spiders in this experiment may have mod-ified their webs in a similar manner to increase their intake of midges. More investigation into differences in web structure is necessary to determine how these sit-and-wait spiders are able to alter their prey consumption rate.  Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest None. References Ali, M., Naif, A.A., Huang, D., 2011. Prey consumption and functional response of a phytoseiid predator, Neoseiulus womersleyi, feeding on spider mite, Tetranychus macfarlanei. Journal of Insect Science 11, 167. https://doi.org/10.1093/jis/11.1.167. Barnhisel, D.R., Kerfoot, W.C., 2004. Fitting into food webs: behavioral and functional response of young lake trout (Salvelinus namaycush) to an introduced prey, the spiny cla-doceran (Bythotrephes cederstroemi). Journal of Great Lakes Research, Exploring Superior 30, 300–314. https://doi.org/10.1016/S0380-1330(04)70393-7. Bolker, B.M., 2008. Ecological Models and Data in R. Princeton University Press.  DeLong, J.P., Gilbert, B., Shurin, J.B., Savage, V.M., Bar-ton, B.T., Clements, C.F., Dell, A.I., Greig, H.S., Harley, C.D.G., Kratina, P., McCann, K.S., Tunney, T.D., Vasseur, D.A., O’Connor, M.I., 2015. The body size dep-endence of trophic cascades. American Naturalist 185, 354–366. https://doi.org/10.1086/679735. Gillespie, R.G., Caraco, T., 1987. Risk-sensitive foraging strat-egies of two spider populations. Ecology 68, 887–899. https://doi.org/10.2307/1938360. Gurney, W.S.C., Nisbet, R.M., 2004. Resource allocation, hy-perphagia and compensatory growth. Bulletin of Mathemat-ical Biology 66, 1,731–1,753. https://doi.org/10.1016/j.bulm.2004.03.008. Hammill, E., Petchey, O.L., Anholt, B.R., 2010. Predator func-tional response changed by induced defenses in prey. American Naturalist 176, 723–731. https://doi.org/10.1086/657040. Holling, C.S., 1959. Some characteristics of simple types of predation and parasitism. Canadian Entomologist 91, 385–398. https://doi.org/10.4039/Ent91385-7. Kalinoski, R.M., DeLong, J.P., 2016. Beyond body mass: how prey traits improve predictions of functional response pa-rameters. Oecologia 180, 543–550. https://doi.org/10.1007/s00442-015-3487-z. Lesar, C.D., Unzicker, J.D., 1978. Life history, habits, and prey preferences of Tetragnatha laboriosa (Araneae: Tetrag-nathidae). Environmental Entomology 7, 879–884. https://doi.org/10.1093/ee/7.6.879. Li, Y., Rall, B.C., Kalinkat, G., 2018. Experimental duration and predator satiation levels systematically affect func-tional response parameters. Oikos 127, 590–598. https://doi.org/10.1111/oik.04479. Lyon, S.R., Sjulin, C.A., Sullivan, K.M., DeLong, J.P., 2018. Condition-dependent foraging in the wolf spider Hogna baltimoriana. Food Webs 14, 5–8. https://doi.org/10.1016/j.fooweb.2017.12.003. Nyffeler, M., Birkhofer, K., 2017. An estimated 400–800 mil-lion tons of prey are annually killed by the global spider community. Science of Nature 104, 30. https://doi.org/10.1007/s00114-017-1440-1. Odum, E.P., 1960. Premigratory hyperphagia in birds. Ameri-can Journal of Clinical Nutrition 8, 621–629. https://doi.org/10.1093/ajcn/8.5.621. Pawar, S., Dell, A.I., Savage, Van M., 2012. Dimensionality of consumer search space drives trophic interaction strengths. Nature 486, 485–489. https://doi.org/10.1038/nature11131. Pimm, S.L., Lawton, J.H., Cohen, J.E., 1991. Food web pat-terns and their consequences. Nature 350, 669. https://doi.org/10.1038/350669a0. Rahman, V.J., Babu, A., Roobakkumar, A., Perumalsamy, K., 2012. Functional and numerical responses of the predatory mite, Neoseiulus longispinosus, to the red spider mite, Oligo-nychus coffeae, infesting tea. Journal of Insect Science 12, 1–12. https://doi.org/10.1673/031.012.12501. Rall, B.C., Guill, C., Brose, U., 2008. Food-web connectance and predator interference dampen the paradox of enrichment. Oikos 117, 202–213. https://doi.org/10.1111/j.2007.0030-1299.15491.x. Rall, B.C., Kalinkat, G., Ott, D., Vucic-Pestic, O., Brose, U., 2011. Taxonomic versus allometric constraints on non-lin-ear interaction strengths. Oikos 120, 483–492. https://doi.org/10.1111/j.1600-0706.2010.18860.x. Rogers, D., 1972. Random search and insect population mod-els. Journal of Animal Ecology 41, 369–383. https://doi.org/10.2307/3474. 4 Schmitz, O.J., 2008. Effects of predator hunting mode on grassland ecosystem function. Science 319, 952–954. https://doi.org/10.1126/science.1152355. Schmitz, O.J., Beckerman, A.P., O’Brien, K.M., 1997. Behavior-ally mediated trophic cas-cades: effects of predation risk on food web interactions. Ecology 78, 1,388–1,399. https://doi.org/10.1890/0012-9658(1997)078[1388:BMTCEO]2.0.CO;2. Schröder, A., Kalinkat, G., Arlinghaus, R., 2016. Individual variation in functional response parameters is explained by body size but not by behavioural types in a poeciliid fish. Oecologia 182, 1129–1140. https://doi.org/10.1007/s00442-016-3701-7. Spitze, K., 1985. Functional response of an ambush predator: Chaoborus americanus predation on Daphnia pulex. Ecology 66, 938–949. https://doi.org/10.2307/1940556. Tahir, H.M., Butt, A., Sherawat, S.M., 2009. Foraging strategies and diet composition of two orb web spiders in rice ecosystems. 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Gravid Tetragnathid Spiders Show an Increased Functional Response

UNL | Libraries

University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Faculty Publications in the Biological Sciences Papers in the Biological Sciences 
2019 
Gravid Tetragnathid Spiders Show an Increased Functional 
Response 
Mary E. Boswell 
University of Nebraska - Lincoln, mary@theboswells.org 
John P. DeLong 
University of Nebraska - Lincoln, jpdelong@unl.edu 
Follow this and additional works at: https://digitalcommons.unl.edu/bioscifacpub 
 Part of the Biology Commons, Other Animal Sciences Commons, and the Terrestrial and Aquatic 
Ecology Commons 
Boswell, Mary E. and DeLong, John P., "Gravid Tetragnathid Spiders Show an Increased Functional 
Response" (2019). Faculty Publications in the Biological Sciences. 824. 
https://digitalcommons.unl.edu/bioscifacpub/824 
This Article is brought to you for free and open access by the Papers in the Biological Sciences at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications in the 
Biological Sciences by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Boswell and DeLong in Food Webs (2019) 21: e00122. 
Copyright 2019, Elsevier. Used by permission. 
1 
Gravid Tetragnathid Spiders Show an Increased Functional Response 
[Short communication] 
 
Mary E. Boswell and John P. DeLong 
 
Abstract 
Spiders in the genus Tetragnatha feed on emerging aquatic insects, including mosquitoes and midges, but there is little 
known about the foraging behavior of these spiders. We hypothesized that female spiders actively developing egg sacs 
would increase food consumption to provide more energy to produce and provision their eggs. We tested this hypoth-
esis by measuring foraging rates of Tetragnatha spiders kept in jars and provisioned with different levels of midges. We 
then tested for a difference in the functional response of spiders that did or did not lay egg sacs in their jars. Egg-laying 
and non-egg-laying spiders showed significantly different functional responses, indicating that Tetragnatha spiders can 
change their behavior or web structure to increase their foraging rate, presumably to accommodate increased energy 
demand for reproduction. 
 
Keywords: Foraging, Prey selection, Spider webs, Energy needs, Tetragnatha, Midges 
 
Predator-prey interactions are crucial to our under-
standing of ecological systems. Predation plays a key role in 
energy and nutrient movement through food webs, directly 
affecting the abundance of prey species (Pimm et al., 1991), 
the structure of food webs (Schmitz et al., 1997), and the func-
tioning of ecosystems (Schmitz, 2008). Globally, spiders play 
an outsized role as predators of invertebrates, contributing 
both to the control of insect pests and the overall structure of 
terrestrial food webs (Nyffeler and Birkhofer, 2017). Thus, un-
derstanding sources of variation in spider foraging is critical 
to understanding the structure and function of terrestrial food 
webs. 
The functional response, or foraging rate with respect 
to prey density, provides a standardized way of assessing 
variation in foraging rates. The functional response typically 
is written as a saturating, type II function using the ‘disc’ 
equation: 
 
y =  
ax
1+ahx
             (1) 
 
where a is the space clearance rate of the predator, h is the 
handling time for one prey item, x is the number of prey items 
available, and y is the per capita rate of prey consumption 
(Holling, 1959). The handling time and space clearance rate of 
a predator are affected by several factors, including the body 
size of the predator and the prey, the body condition of the 
predator, temperature, and the predator’s foraging strategy 
(Lyon et al., 2018; Pawar et al., 2012; Rall et al., 2011; Uiterwaal 
et al., 2017). Foraging strategy may be a fixed aspect of a pred-
ator’s behavior, such as sit-and-wait versus active pursuit 
predation, but foraging behaviors also may be influenced by 
the energetic needs of the organism. Some organisms, for ex-
ample, increase their food intake (hyperphagy) in preparation 
for energy-intensive activities such as migration (Odum, 1960) 
or reproduction (Gurney and Nisbet, 2004). Higher foraging 
rates and diet quality are also correlated with increased qual-
ity of egg production in arachnids (Rahman et al., 2012; Wil- 
der and Rypstra, 2008). It is less clear, however, whether sit-
and-wait predators can increase prey intake in response to the 
increased energy demand associated with reproduction. 
Long-jawed orb-weavers vary foraging strategies in re-
sponse to prey density and type (Gillespie and Caraco, 1987; 
Tahir et al., 2009; Yoshida, 1987). In particular, Tetragnatha spi-
ders may adjust their foraging location in response to prey 
density (Gillespie and Caraco, 1987), presumably with the 
outcome of increasing prey capture. Such differences in for-
aging behavior should translate to a steeper functional re-
sponse with either a greater space clearance rate, lower han-
dling time, or both, for those individuals with higher energy 
demands, but this hypothesis has not been tested. 
We compared the functional response of wild-caught female 
Tetragnatha laboriosa spiders that did and did not lay eggs while they 
were kept in the lab. While previous studies have controlled for the 
ef-fects of gravidity on foraging rates by using only gravid females, 
we sought to use both gravid and non-gravid females to compare the 
two (Ali et al., 2011; Rahman et al., 2012). We hypothesized that fe-
male spiders that had laid egg sacs in their containers would show a 
steeper functional response (greater rates of foraging at all prey den-
sities) than those that did not, since higher consumption rates are 
presumably needed to produce and provision the eggs. 
We conducted our experiment during June of 2017 and 
2018 on the south shore of Lake Ogallala, Nebraska, at Cedar 
Point Biological Station. We collected Tetragnatha spiders in 
the morning by sweep-netting grassy roadsides and mead-
ows. Each spider was individually housed in a cylindrical 250 
mL plastic jar, with a height of 7.5 cm and diameter of 6.5 cm. 
We placed a damp piece of paper towel in each jar to provide 
moisture and rewetted them as needed to maintain moisture. 
We left the spiders alone during the day to allow time for each 
spider to settle and build its web. We assumed that all spiders 
were female, since male Tetragnatha spiders do not build 
webs. We collected a pre-determined random number of 
midges (Chironomidae) directly into the spider habitats from 
the windows of the Cedar Point Biological Station lab after 
sunset using a jar lid fitted with an aspirator. We chose midges 
 
 
Boswell: School of Natural Resources, mary@theboswells.org (corresponding author) 
DeLong: School of Biological Sciences, jpdelong@unl.edu 
University of Nebraska-Lincoln, Nebraska, USA
2 
Figure 1. The functional response of Tetragnatha spiders forag-
ing on midges. Spiders that laid eggs had a steeper functional 
response with a higher asymptote than spiders that did not. 
 
as prey because they make up the bulk of Tetragnathid spi-
ders’ diets (Yoshida, 1987) and are a dominant aerial insect in 
the area. We left spiders to hunt overnight for 10 hours, and 
at 7:30 A.M. the following morning, we counted the uneaten 
midges to determine the number of midges that were eaten. 
Throughout the experiment, several of the spi-ders laid egg 
sacs in their containers. We waited 3–4 days after the foraging 
trial ended to allow gravid spiders to lay egg sacs. We took 
care to not damage the egg sacs while changing lids and 
remoistening the jars. Temperature in the lab was approxi-
mately 21°C. 
We did not replace prey as they were eaten through the 
night, and therefore the prey density within the jars declined 
with each prey capture. We therefore fit our data to the Rogers 
random predator equation, which accounts for this within-ex-
periment prey depletion (Bolker, 2008; Rogers, 1972), where 
parameters are the same as in Equation (1) and W is the Lam-
bert W function: 
 
y = x −  
𝑊(ahx  exp(−a(0.42−hx)))
ah
          (2) 
 
We used ordinary non-linear least squares regression to esti-
mate the mean and 95% confidence intervals of the parame-
ters of Equation (2) (space clearance rate a and handling time 
h). We fit the data for egg-laying and non-egg-laying spiders 
separately. We used a bootstrap approach to determine if a 
and h were significantly different between egg-sac-laying and 
non-egg-sac-laying spiders. We randomly sampled with re-
placement each original data set to create 100 bootstrapped 
datasets, and we fit Equation (2) to each dataset. We then cal-
culated the difference between the parameters for each possi-
ble pair of curves that differed in egg-laying status. This pro-
cess generated a distribution of differences between the two 
parameters, and we assessed significant differences between 
the parameters by determining whether the 95% confidence 
interval of the difference between parameters included zero. 
Egg-sac-laying and non-egg-sac-laying spiders each 
showed Type II functional responses (Figure 1; alaying = 8.14 
arenas per predator per day, 95% CIs: 5.67 to 27.54; hlaying = 
0.003 days, 95% CIs: ~0.00 to 0.008, R2 = 0.90; anon-laying = 7.53 
arenas per predator per day, 95% CIs: 4.57 to 15.72; hnon-laying = 
0.01 days, 95% CIs: 0.007 to 0.013, R2 = 0.65). The confidence 
intervals for the differences in a and h between the two data 
sets did not include zero, so the two parameters were consid-
ered significantly different from each other, with a being 
higher and h lower for egg-sac-laying spiders (Figure 2; 95% 
CIs difference in a: 0.09 to 21; 95% CIs difference in h: 0.0016 
to 0.012). 
Much of what we know about variation in the func-
tional response comes from across-species comparisons (e.g. 
Uiterwaal and DeLong, 2018). There is clear variation in both
 
 
 Difference in a       Difference in h 
 
Figure 2. The distribution of differences between A) space clearance rate, a and B) handling time, h. The 95% confidence intervals on 
these distributions did not include zero, indicating that both parameters were significantly different between egg-laying and non-
egg-laying spiders. The leftmost bins include only positive differences. In A) the inset shows detail for the lower bins. 
 
3 
space clearance rate and handling time across species associ-
ated with body mass, temperature, predator satiation, forag-
ing dimensionality, and the taxonomic identity of both pred-
ator and prey (DeLong et al., 2015; Kalinoski and DeLong, 
2016; Li et al., 2018; Pawar et al., 2012; Rall et al., 2011, 2008; 
Uiterwaal and DeLong, 2018). Additional variation in the 
functional responses within predator-prey pairs is associated 
with body mass, prey defenses, temperature, energetic his-
tory, and individual identity (Barnhisel and Kerfoot, 2004; 
Hammill et al., 2010; Lyon et al., 2018; Schrer et al., 2016; 
Spitze, 1985). However, it is less clear that particular energy-
demanding activities drive individual-variation in the func-
tional response. Our experiment clarifies that the shift to be-
ing gravid increases both the steepness and height of the func-
tional response in long-jawed orb-weavers. This shift could 
have implications for the strength of inter-actions linking 
these spiders to their prey and consequently for the predator-
prey dynamics.  
Tetragnatha spiders in this study that laid egg sacs ate 
more midges at all prey densities than those that did not. 
Although it was clear that there was not complete separa-
tion of foraging behavior between egg-sac-laying and non-
laying spiders, suggesting some possible miscategoriza-
tion of individual spiders, our results show that reproduc-
tive status should be considered when determining the 
functional response of a predator, as individuals provi-
sioning offspring need higher prey intake. How a sit-and-
wait predator achieves an increase in prey consumption, 
however, is not clear. The most likely candidate for the in-
creased foraging is alterations to spider web structure. A 
previous study showed that starved Octonoba sybotides 
built webs with tighter spacing than usual, likely to target 
smaller but more numerous prey items (Watanabe, 2001). 
The egg-laying spiders in this experiment may have mod-
ified their webs in a similar manner to increase their intake 
of midges. More investigation into differences in web 
structure is necessary to determine how these sit-and-wait 
spiders are able to alter their prey consumption rate.  
Funding sources 
This research did not receive any specific grant from 
funding agencies in the public, commercial, or not-for-profit 
sectors. 
Declaration of competing interest 
None. 
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Gravid Tetragnathid Spiders Show an Increased Functional Response

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