Direct Evidence for Trade-Offs between Foraging and Growth in a Juvenile Spider

7 Pages·1995·7.6 MB·English

by  Linden E Higgins

1995. TheJournal ofArachnology 23:37-43
DIRECT EVIDENCE FOR TRADE-OFFS BETWEEN FORAGING
AND GROWTH IN A JUVENILE SPIDER
Linden E. Higgins: Dept, ofZoology, University ofTexas at Austin, Austin, Texas
78712 USA
ABSTRACT. A simple modification ofclassical optimal foraging models yields explicit predictions ofthe
allocation ofresources to orb-web synthesis and weight gain in a spider. Because the spiders may be trying to
avoid weight loss, weight gain and web size are predicted to be be negatively correlated at higher food levels
and positively correlated at lower food levels. These predictions were upheld qualitatively by experiments
involvingjuvenile Nephilaclavipesand N. maculata.
RESUMEN. Una sincilla modificacion a los modelos clasicos del forrajeo optimo dapredicciones explicitas
sobre la repartition de recursos entre el sintesis de la tela orbiculary el aumento de peso en una arana. Dado
quearanas posiblemente estan ententando evitarlaperdidade peso, la interaction entre el aumento de peso y
el tamano de la tela deberia estar negativamente correlacionada bajo majores niveles de alimentacion, y posi-
tivamente correlacionada bajo de condiciones de alimentacion reducida. Estas predicciones fueron appoyadas
experimentalmente utilizandojuvenilesde Nephila clavipesyN. maculata.
Classical foragingmodels are based on the as- al. 1991).Thus,constructionofeachorbrequires
sumption that optimal allocation oflimiting re- decisions concerning the allocation ofmetabol-
sourcesamongvariousexpendituresincreasesthe icallyimportantcompounds;suchdecisionscould
fitness ofthe organism either directly (through directly affect growth.
increased reproductive success) or indirectly In some large araneoid spiders, fitness is in-
(throughincreasedgrowth)(e.g.,Schoener 1971; creasedby rapiddevelopmentand weightlossis
Pykeetal. 1977; Mangel &Clark 1986). Despite predicted to reduce the fitness of the females
thekeyroleofthisassumption, fewstudieshave (Higgins & Rankin, in press). The existence of
described the interaction between foraging in- resources common to both foraging and growth
vestment and growth at different levels offood and the fitness penalties associated with weight
availability. This appears in part due to the dif- loss imply that when resources are limiting, for-
ficulty of finding common units of measure agingbehaviormightbe different from that pre-
(Pianka 1981; Stephens & Krebs 1986). Ifcom- dicted by foraging models calculating maximi-
monresourcesareused forbothgrowthand for- zationoftherateofenergyintake.Here,Ipresent
aging,thenexplicitpredictionscanbemadecon- a simple model that predicts patterns ofalloca-
cerningtheirallocationatdifferentlevelsofprey tion of resources between foraging and weight
availability. gainatfoodlevelsvaryingfrom verylowtovery
Thestudyofforagingin orb-webbuildingspi- high. I experimentally tested this model using
ders allows observation of resource allocation juvenile Nephila clavipes (L.) and N. maculata
because foraging investment is measurable as (Fabr.) (Araneae: Tetragnathidae) that were fed
materialinvestmentinto theweb, andthe orbis at very low, intermediate and high levels.
composed of physiologically important com-
THE MODEL
pounds. The orb web is renewed regularly, and
changes in orb-web size reflect the response of Ifit is true that the functions ofweight gain
the spiderto foragingconditions (Buskirk 1975; and web building are competing for common
Gillespie 1987; Higgins & Buskirk 1992). The resourcesand ifit is important to the individual
viscid silk that forms the spiral ofthe orb is a to avoid weight loss, a simple modification of
protein thread coated with a mixture oforganic theclassicforaginginvestmentmodels(Schoener
compounds that also have physiological func- 1971) is necessary to predict behavior at very
tions (Tillinghast 1984; Dadd 1985; Townley et lowlevelsofpreyavailability. The model ispre-
37
:
38 THEJOURNALOFARACHNOLOGY
vestment, the spiders should reduce their for-
agingeffort.Thus,therelationshipbetweenweight
gain and web investment in spiders is predicted
to be negativelycorrelatedat higherfoodlevels,
and positively correlated lower food levels.
METHODS
Inordertotestthepredictionthattherelative
investmentintotheorbandweightgainwilldif-
fer between the shift from very low to interme-
low high diate foraging success and the shift from inter-
foodlevel mediate to high foraging success, I controlled
prey availability in juveniles of two species of
Nephila lab-reared N. davipes in Panama and
N
field-collected maculata in Madang, Papua
.
New Guinea. In both cases the investment into
weightgain and orb area were recorded as func-
ro tions ofprey availability, measured as the daily
0
prey weight/initial spider weight.
1 0
O The basic methods were the same in the two
experiments. To control the structural environ-
ment used in web construction, I placed indi-
vidualson 30cmdiametersphericalframescon-
mm mm
sistingoftwohoopsof3 x 6 fiberglass,
residualoforbsize fixedatrightanglesandsuspendedfromtheceil-
Figure L—The model ofresource allocation by an ingorbracesinopen-airinsectaries. Ifedspiders
orb-weavingspider. A. As food resources increase for cloocamlmlyonavapirleayblietesmtinogfletshsesbeeesspi(dTerrsig(oHniaggsipn.)s,&a
agivenindividual,orbsizeincreasesthendeclinesand
weightgainincreases. B.Amongspiders,theresiduals Buskirk 1992;Higgins,pers.obs.).Beeswerecol-
oforbsizeandweightgainregressedagainstspidersize lected from the same nests throughout the ex-
arecompared(eliminatingthe variationdueto spider periment.
size). Atlow food levels(square), orb size and weight Foreachanimal,Irecordeddailyorb-webarea
gain are lower than average; at medium food levels andrate ofweightgain. (Mesh size, measuredas
(circle, diamond), orb size is greater and weight gain the number ofspiral strands per 2 cm (Higgins
islowerthanaverage;andathighfoodlevels(triangle), &Buskirk 1992), didnotvarywith foodlevel in
orbsizeislowerandweightgainishigherthanaverage. either species. N davipes: F = 3.16, ns; N
. (1j34) .
maculata: F(2>13) = 0.34, ns). Orbs builtby these
seated graphically in Fig. 1. At high to inter- spidersare asymmetrical, and orb-web areawas
mediatelevelsoffood,thespidershavesufficient estimated as a halfellipse using measurements
resources to avoid weight loss and increase in- ofthe vertical and horizontal radii. While many
vestment into foraging as food levels decrease. orb weavers replace the orb each day, these spi-
Thisisconsistentwithclassicalforagingmodels: derssometimesonlypartiallyrenewtheorb(Lu-
as average foraging success decreases, the in- bin 1973). When a web was partially new, the
vestment into hunting (e. g., searching time per areawasestimatedby calculatingthe area ofthe
patch) is predicted to increase (maximizing en- orbthen reducingtotheestimatedareathatwas
ergygain/effort;Stephens&Krebs 1986).Ifcom- new (<V3, <V2, <%, <%). Orbs are not ofcon-
mon resources are being allocated into growth stant size during the intermolt interval (Higgins
and foragingthen the rate ofweightgain will be 1990), therefore, foraging investment was mea-
slower, due to both decreased prey capture and sured as the mean area ofall orbs spun during
increased foraging investment. However, this the intermolt period. If a spider did not molt
prediction will not hold at lower food levels if duringtheexperiment, themeanareaduringthe
the spiders must avoid weight loss. When re- experiment was used.
sourcesare solimitingthatweightlosswouldbe Websizeandrateofweightgainarebothfunc-
the price ofcontinued increases in foraging in- tions ofspidersize. Spidersize was measuredas
HIGGINS-TRADE-OFFSBETWEEN FORAGINGANDGROWTH 39
web cm
residual of radius,
mean ±
1 s.d.
Figure2.—Therelationshipbetweenmeanorb-websizeandmeandailyweightgaininNephilaclavipesunder
threedifferentfood levels(% initial spiderweight), twotrials. Trial 1 (filledsymbols): diamond-20%, triangle
- 35%. Trial 2 (open symbols): square- 10%, circle- 20%.
the length ofthe leg I tibia + patella length, cm gravid females ofeach generation. The first two
(TPL), which does not vary between molts. Spi- cohorts of spiderlings hatched in January, and
der weight, which increases between molts, can the second twocohortshatchedin Mayof1986,
be estimated from TPL and abdomen volume resulting in two separate trials. Each cohort was
(Higgins 1992a; the same equation was used for kept in a fine-screened cage (15 cm w. x 30 cm
both species). The rate ofweight gain in mgwas 1. x 60 cm h.) within an open-air, screened in-
calculated for all spiders as <5wt mg (= wtFma! - sectary, and large numbers offield-caught Dro-
Swtinraai/no. days observed). To eliminate the in- sophila spp. were released into the cages twice
fluenceofsize,theresidualsofwebsizeandweight daily. As each spider reached the third instar, it
gain regressed against spider size were the de- was removed from the common cage, marked
pendentvariablesinthestatisticalanalyses.Mul- and placed onto a spherical frame. During this
tivariate analysis of variance (MANOVA) al- instar, each spider was offered one stingless bee
lowed simultaneous comparison ofthe effects of daily (mean weight 3.5 mg, mean body length 4
prey level on the rate of weight gain and web mm).
size. WhentheMANOVAwas significant, as in- The experimental treatment was initiated in
ANOVA
dicated by Wilks’ lambda, were used eitherthe fourth (firsttrial) orfifth (second trial)
toexaminethevariationineachdependentvari- instar. Within each trial, the spiders were fed
able. either two or one stingless bees daily, the mean
Nephilaclavipes.—Forthisexperiment,Iraised number ofprey captured per day in the field in
spidersfrombothgenerationsofabivoltinepop- the rainy and dry seasons, respectively (Higgins
ulationonBarroColoradoIsland, Panama. First- & Buskirk 1992). As each spider molted, it was
generationfemalesmatureintheearlyrainysea- measured(TPL,abdomenlengthandwidth,cm),
son (May-June), and second generation females individuallymarkedwithTestors®enamelpaint,
mature as the rains end (December-January) and randomly assigned to receive one or two
(Higgins 1992a). Eggs were collected from two bees/day. Penultimate-instar males may parti-
40 THEJOURNALOFARACHNOLOGY
05
E
sz.
05
05
5
ro
B
IB
go
'w
0)
-100 -50 0 50 100
web cm
residual of radius,
mean ±
1 s.d.
Figure 3.—The relationship between mean orb-web size and mean daily weight gain in Nephila macuiata
under three different relative food levels (% initial spider weight), square: <17%; triangle: <22%; diamond:
<45%.
tion resources differently (Higgins 1993); there- The regressionsoforb areaand rate ofweight
fore, males in this instar were not used in the gainagainstspidersizeweresignificant(orbarea
experiments. Treatment lasted 14 days, slightly = 166.6 + 521.6(TPL);R2=0.57,F(U6) =48.81,
longer than the mean intermolt interval ofthe P < 0.001; ln(dwt) = -1.82 + 2.80 (TPL), R2 =
fourthinstar.If,atanytime,anindividualmoved 0.09, F(ij34) = 3.53, ns). Insufficient spiders sur-
offitsframe, the supportstrandsofthenewweb vived to allow examination ofvariation among
were carefully cut and the spider with its web clutches, but the spiders ofboth clutches were
was placed back onto the frame, after measure- assigned to each treatment within a trial, con-
ments(afterM. Robinson, pers, comm.). Move- trolling for this factor.
ment offofthe frames occurred with equivalent Nephila macuiata.—In February and March
frequency in all groups (test for full indepen- of 1993, I conducted a similar experiment with
dence: G= 6.61,df= 4, ns;Sokal&Rohlf1981). N. macuiata. Small groups (no more than 10 at
Theexperimentaltreatmentwasstartedonav- a time) ofsecond - fifth instar spiders from the
erageoneinstarlaterinthesecondtrial. Because forests ofBaitabag village, west ofMadang, Pa-
ofthe differencein instarbetweenthetwo trials, pua New Guinea, were held indoors overnight
the spiders differed in initial size and weight. toencouragebuildingontheframes.Afterwards,
Therefore, the two treatments (1 vs. 2 prey/day) the spiders were moved to a screened, roofed
yielded three levels of prey weight relative to insectary, and randomly assigned to a diet (low
initial spider weight: high (35%) and medium orhigh). Thenumberofpreyitemswasadjusted
(20%) in the first trial, medium (20%) and low accordingtospiderinstar. Ifedsecondandthird
(10%) in the second. One individual’s relative InstarspidersDrosophilaspp.(0.67 mgeach; low
food weight was 26%; preliminary analyses in- = 2/day, high = 4/day (second instar) or 6/day
dicated no difference in the statistical outcome (thirdinstar)), andlargerspiders Trigonabees(5
when it was included in the medium or high mg each; low = 1/day, high = 2/day). The ex-
group, so it was assigned to the former. perimentaltreatmentwascontinuedfor 10days,
HIGGINS—TRADE-OFFSBETWEEN FORAGINGANDGROWTH 41
afterwhichthespiderswerereturnedtotheforest A
and newindividualswere collected. The shorter
interval reflects both the shorter intermolt du-
ration ofthis species, and the need to maximize
the number ofanimals used in the experiment.
Although I had randomly assigned prey avail-
ability,Icouldnotmakethespiderseatandprey
consumptionwashighlyvariable. Therefore,the
independentvariableusedinthestatisticalanal-
yses is the relative mean weight ofprey actually
eaten (presentedas % initial spiderweight). The
continuous distribution of prey consumed was
divided into three approximately equal groups
ofincreasing relative prey weight.
N. maculata were not as likely as N. clavipes
to build an orb each day, perhaps due to the
insectary being roofed. Therefore, the foraging
investmentreported isthetotalarea oforbbuilt
duringtheexperimentdividedbythenumberof
dayshunting(determinedasthenumberofdays
with a neworb plusthe numberofdaysthatthe
spiders accepted prey although a new orb had
not been built). The size range ofspiders used
was larger than in the previous experiment, so
the data had to be transformed to correct for
heteroscedasticity prior to statistical analysis of
the residuals (In (Swt) = -0.997 + 2.698 (TPL),
R2=0.33,F -7.398,P < 0.025;(web/hunt)*
- 8.506 + 1(01>.156)7 (TPL), R2 = 0.34, F = 8.26,
{1 15)
P <
0.025). spidersize,TPL1,cm
RESULTS Figure4.—Therelationshipbetween mean orb-web
sizeand meandailyweightgain inindividualNephila
Nephila clavipes.—Both rate of weight gain
maculata under three different food levels (% initial
(5wt) and mean orb size varied with food level spiderweight). Square - <17%, triangle - <22%, di-
(Fig. 2). From 10%to 20%relativefood(second amond- <45%. A.There is noeffectofthe sexofthe
trial),dwtandorbsizeincreased,whilefrom20% individual on resource allocation. Male - filled sym-
to 35% relative food (first trial), 5wt increased bols, female = open symbols. B. There is no effect of
MANOVA
and orb size decreased. Three com- initialspidersizeonresourceallocation.Filledsymbols
pared the responses ofthe spiders to food level = residual oforb-web size, open symbols = residual
within each trial, and compared between trials of5wl.
atthe20%foodlevel;thesignificancelevelswere
adjusted for multiple, non-independenttests. In is probably due to the larger mean initial spider
the second trial, there was a significant effect of size in trial 2.
food level on <5wt but not on web size (Wilks’ X Reduced rate ofweight gain had a significant
= 0.356, P < 0.01; ANOVA: 6wt: F = 23.5, negative effect on the development rate of the
P < 0.001; web size: F = 0.20,(1>n1s3)). There spiders, observed as an increase in the number
(1 13)
was a significant effect in the first trial on both of days between molts. Eighteen spiders from
web size and <5wt (Wilks’ X = 0.568, P < 0.01; trial 1 were held on the assigned diet until they
ANOVA: §wt: F = 8.09, P = 0.01; web size: molted and the number ofdays between molts
F(h 19) = 5.41, P(1j=19)0.03). Between trials, there was correlated with the rate ofweight gain and
was a significant difference in <5wt but not web initialspidersize(In(daysbetweenmolts)= 1.96
size (Wilks’ X = 0.514, P < 0.01; ANOVA 5wt: - 37.7 (5wt) + 4.81 (TPL); R2 = 0.46, F =
F(1 17) = 16.05, P < 0.01; web size: F(1 17) = 0.03, 6.25, P = 0.011; only 5 spiders molted i(n2>1t5r)ial
ns). The effect oftrial in the medium food level 2).
42 THEJOURNALOFARACHNOLOGY
Nephila maculata —Although the pattern of 1974;Wise 1975, 1979).Inweb-buildingspiders,
.
variation in orb investment and rate ofweight thewebmustbebuiltbeforetheforagingquality
gain among the three groups is similar to that of a site can be assessed. Perhaps due to this
observed forN. clavipes, the differences are not initial investment, or due to temporal variation
significant(Fig. 3;Wilks’X-0.78,P=0.5).What in prey availability, many web-building spiders
is most striking is the variation among individ- arehighlysite-tenacious(Eberhard 1971;Enders
uals in response to the same level offood intake 1976;Tanaka 1989;however,seeTurnbull 1964).
(Fig. 4). Often, different individuals made op- Ratherthanabandoningpoorpatches,astrategy
positeresponses:onespiderbuildinglarger-than- oftenpredictedbyforagingtheory(e.g.,Chamov
average orbs and gaining little weight, another 1971; Pyke et al. 1977), spiders show other be-
building smaller-than-average orbs and gaining havioral and physiological responses to nutri-
weightrapidly. Thevariationwasnotafunction tional stress (Riechert & Luczak 1982; Sherman
of the sex nor of the initial size (TPL) of the 1994). However,theseresponsescanslowgrowth
individual (Fig. 4). and affect both survival and female fecundity
(Higgins 1992a, 1992b, pers. obs.). The results
CONCLUSIONS
ofthesestudiessupportthehypothesisthatthere
The two experiments uphold the predictions are potentially limiting resources necessary for
ofthe model. Juvenile Nephila ofboth species both orb silk synthesisandgrowth, predictedby
tendedtoincreaseorb-websizeanddecreasethe the chemical analyses oforb-silk by Tillinghast
rate of weight gain with slightly reduced prey and Townley (Tillinghast 1984; Townley et al.
capture; they decreased both orb-web size and 1991). The spiders must invest in the orb web
rate ofweightgain in response to greater reduc- in order to capture prey, but unless critical re-
tions in prey capture. This response resulted in sources are in abundance, such investment nec-
the counter-intuitive pattern of decreased for- essarilydelaysgrowthanddevelopmentandpo-
aginginvestmentwith lowfood availability, up- tentially compromises individual fitness. Pre-
holdingthe initial assumptions that resource al- dation rate declines with increasing spider size
locationisconstrainedbyaneedtoavoidweight and fecundity increases with early maturation
lossandthat common resourcesare used in for- (Higgins 1992b, pers. obs.), so lengthening time
aging and growth in these spiders. At very low between molts can have a detrimental effect on
food levels, webs were small and few spiders fitness. To avoid reduction in weight during pe-
gained weight but none lost weight. This food riods of low food availability, the interactions
leveliswithintherangecapturedinPanama,but betweenforaginginvestmentandgrowthmaybe
much lower than median prey capture rates ob- more complex than predicted by classical opti-
served in Madang (4 prey/12 h diurnal obser- mal foraging models.
&
vation) (Higgins Buskirk 1992; Higgins, pers.
obs.). ACKNOWLEDGMENTS
Nephila maculata juveniles varied greatly in
their response to the experimental treatment. J. Wright’s comments and logistical support
Such differences were not found in the experi- from the Smithsonian Tropical Research Insti-
ment with N. clavipes, when the spiders were tute staffwere essential in the development of
rearedunderhomogeneousconditions. Thisdif- this project. M. Townley and E. Tillinghastpro-
ferencemightrepresentsomeintrinsicdifference videdvaluableinformationfortheinterpretation
betweenspecies,butprobablyreflectsdifferences oftheexperimentandthey,R.Buskirk,W.Eber-
in experience prior to the experiment. N. ma- hard, M. A. Rankin, C. Macias, G. Miller, C.
culatajuveniles were collected in the field, and Pease, G. Perry, M. Singer, D. Wise and anon-
undoubtedly differed in their foraging histories. ymous reviewers commented upon earlier ver-
The foraginghistoryofan individual maycause sions of the manuscript. This work was sup-
UT
it to respond in a particular fashion to changes ported by Sigma-Xi, the Graduate Fellow-
in prey capture rates. This impliesdifferencesin ship program, an NSF Dissertation Improve-
the response to short vs. long term variation in mentGrant(BSR-8413831),aSTRIshort-term
prey capture, which cannot be examined by the fellowship,andtheChristensenFundGrant.This
short-term experiments presented here. is contribution No. 138 from the Christensen
There is evidence that many spiders evolved Research Institute (P.O. Box 305, Madang, Pa-
under conditions of food limitation (Anderson pua New Guinea).
HIGGINS TRADE OFFS BETWEEN FORAGINGAND GROWTH 43
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