EFFECTS OF MATERNAL BODY SIZE ON CLUTCH SIZE AND EGG WEIGHT IN A PHOLCID SPIDER (HOLOCNEMUS PLUCHEI)

4 Pages·2003·4.4 MB·English

by  Christa D Skow

2003. The Journal of Arachnology 31:305-308
SHORT COMMUNICATION
EFFECTS OF MATERNAL BODY SIZE ON CLUTCH SIZE AND
EGG WEIGHT IN A PHOLCID SPIDER
{HOLOCNEMUS PLUCHEI)
Christa D« Skow: Neuroscience and Behavior Program, Tobin Hall, University of
Massachusetts, Amherst, Massachusetts 01002 USA
Elizabeth M. Jakob: Department of Psychology, Tobin Hall, University of
Massachusetts, Amherst, Massachusetts 01002 USA
ABSTRACT. The pholcid spider Holocnemus pluchei (Scopoli 1763) competes for food with coespe-
cifics, and spiders reared on high food levels are generally larger. In this study, we examined whether
largerfemale body size (as estimatedby tibia=patella length) translated into increasedreproductivesuccess
in the form ofincreased clutch size, clutch weight, and average egg weight. Larger spiders had more eggs
and thus heavier clutches, but there was no relationship between maternal size and average egg weight.
We also looked for a tradeoff between average egg weight and egg number, and we found a weak rela=
tionship in which average egg weight was highest for intermediate-sized clutches. Larger female body
size thus translates into increased reproductive success in terms ofegg number and clutch weight, but not
weight of individual eggs.
Keywords: Pholcidae, fecundity, egg size, clutch size, fitness
Number and size ofeggs produced are important 1997). Because H. pluchei shifts webs frequently
components of spider fitness. Clutch size in most and it is difficult to follow marked individuals for
arthropod species, including spiders (Jann & Ward extended periods, lifespan and total number of
1999; reviews in Marshall & Gittlemae 1994; clutches in the field is unknown. In the laboratory,
Simpson 1995) increases with female body size. In spiders often live for over a year, and we have oc-
some arthropod species (Fox & Czesak 2000), in- casionally observed individuals to lay multiple
cludingAgelena limbata Thorell (Tanaka 1995), fe- clutches (Jakob pers. obs.).
males can alter investment in individual eggs based We were interested in linking more directly the
on maternal body size and condition, whereas in behaviors of group living and fighting with their
other arthropod species egg size is invariant (re- fitness consequences by establishing whether an in-
viewed in Fox & Czesak 2000), Life history theory crease in maternal body size translates into more
predicts tradeoffs between clutch size and offspring eggs orheavierclutches. We were also interestedin
size (Smith & Fretwell 1974), although these are whether there were relationships among body size,
frequently not apparent (Fox & Czesak 2000). clutch size and mean egg size. In other species, egg
We examined the effect ofmaternal body size on and offspring size has numerous fitness conse-
clutch size and average egg weight in the pholcid quences, with larger generally being better (re-
spider Holocnemus pluchei. (Scopoli 1763). The viewed in Tanaka 1995). In H. pluchei, variation in
behavior and life history of this species is well offspring size or condition may have an additional
known. Spiders often share webs and fight vigor- consequence, because whether spiderlings join
ously over prey and larger spiders generally win group webs or build their own webs depends sig-
(Blanchong et al. 1995; Jakob 1991, 1994; Jakob et nificantly on their condition, which is in turn influ-
al. 2000). Spiders reared on higherprey levels grow enced by their recent feeding success (Jakob un-
to a larger size than spiders reared on lower prey publ. data). Any variation in the resources allocated
levels, except for those on extremely limited food to individual eggs that affects the condition of the
regimes that may add an additional instar before spiderlings mightpredispose newly independent in-
maturation (Jakob & Dingle 1990). Females in the dividuals toward or away from group living.
laboratory readily mate multiply, and second male During August of 1998, 57 female Holocnemus
sperm priority predominates (Kaster & Jakob pluchei with egg sacs were collected on the campus
305
.
306 THE JOURNAL OF ARACHNOLOGY
Mother's Tibia-Patella Length (mm)
—
Figure 1. Clutch size (number of eggs) regressed against mother’s tibia-patella length.
of the University of California at Davis. Upon col- weighed only 34 clutches of eggs and 22 clutches
lection, 35 randomly chosen females were imme- of hatchlings.
diately anesthetized with carbon dioxide and pre- To estimate the size of the mother, we used fe-
served with their eggs in 95% ethanol in screw cap male dry weight and tibia-patella lengths of legs 1
vials. The remaining 22 females were housed in- and 2. Tibia-patella length ofleg 1 was highly cor-
dividually with their eggs as in Jakob (1991), and related with leg 2 (Fi.6o = 892.76, ^2 = 0.968, P <
kept in growth chambers on a 15:9 L:D schedule 0.0001), and with weight (F,go = 80.391, =
with daytime temperatures of 32 °C and nighttime 0.573, P < 0.0001), so for subsequent calculations
temperatures of 22 °C. All clutches hatched within we used only the tibia-patella length of leg 1
four days ofcapture. Females normally do not feed We compared clutch size and clutch weight of
while guarding and were not supplemented with offspring measured as eggs versus those measured
food or water while in the chamber. When a clutch as hatchlings and found no differences (clutch size:
hatched, the female and her hatchlings were anes- eggs 48 ± 2.8, hatchlings 53 ± 3.7, Fi^^o = 1.300,
thetized with carbon dioxide and placed in screw P > 0.2; clutch weight: eggs 6.00 ± 0.002 mg,
cap vials (along with any unhatched eggs) with hatchlings 6.00 ± 0.002 mg, = 0.08, P > 0.7).
95% ethanol for preservation. All samples were Nonetheless, because we were concerned that
transported back to the University ofMassachusetts clutch weight might be affected by hatching, for all
at Amherst, where we measuredfemaletibia-patella weight analyses we examined the two separate
lengths of legs 1 and 2 with calipers, teased apart treatment groups as well as the pooled data. We
egg sacs and counted eggs, and counted hatchlings. found no qualitative differences in these analyses
After counting, we dried the females and their off- and present only the pooled data for brevity’s sake.
spring in an oven at 128 °C for a minimum of 6 We found that the mother’s tibia-patellalength of
hours until they reached aconstant weight. Samples leg 1 significantly predicted both the number of
were weighed on a microbalance within 24 hours eggs (Fi,6o = 36.574, = 0.379, F < 0.0001, Fig.
of drying. Dried samples were kept in loosely 1) and the total clutch weight (Fi^54 = 30.838, R- =
capped vials in a sealed plastic containerwith silica 0.362, P < 0.0001). The R2 values indicate that a
gel (to prevent absorption of water) until being moderate amount of the variance is explained by
weighed. We weighed clutches rather than individ- maternal size, and is higher than or similar to that
ual eggs or hatchlings because ofthe limits ofscale reported for other spiders (e.g., Buddie 2000; Ta-
accuracy. Some samples were inadvertently de- naka 1995; Uetz and Hieber 1997).
stroyed after counting and before weighing, so we These data provide a snapshot ofa population in
SKOW & JAKOB—MATERNAL SIZE AND REPRODUCTION IN PHOLCIDS 307
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Figure 2. Mean egg weightregressed against clutch size. There is no evidence ofaclassic egg weight/
egg number tradeoff; average egg weight was highest for intermediate-sized clutches.
the field. Some of the variance probably results weight. However, we believe that a tradeoff is un-
from the fact that our spiders were field caught. We likely to be apparent under normal field conditions.
could not control for many conditions likely to in- We looked for a tradeoff between egg size and
fluenceclutch size and weight, such as foraginghis- number by regressing mean egg weight against
tory, temperature, female ageandfemaleparity (see clutch size. There was no significant linearrelation-
Buddie 2000 for additional discussion). For exam- ship (Fi55 = 2.905, P > 0.09). The data were best
ple, spiders of all sizes are present year round, so fit by a second order regression (F355 = 3.455,
it is likely that at the time of our collection in Au- = 0.166, P < 0.01, Fig. 2): intermediate clutch siz-
gust there was a range of spider ages. In spite of es had thehighestmeanegg weight. This isdifficult
this, the relationship between body size and clutch to interpret, especially as differentmechanisms may
size is robust (Fig. 1), suggesting a fitness advan- determine the shape of the curve at either end (for
tage of larger females under field conditions. How- example, females in poor condition may only be
ever, iffemale size is negatively correlated with to- able to lay few small eggs, whereas females laying
tal number of clutches produced over a lifetime large clutches may be constrained by abdominal
(e.g., iflarge females die younger) then this fitness volume). Only a small percent of the variance in
benefit would be reduced or eliminated. This seems mean egg weight is explained by clutch size. Other
unlikely: in several cases wherearthropodlongevity potential sources ofvariation include female age: in
andbody sizehave been measured inthefield, larg- most arthropods, progeny size decreases with ma-
er size correlates with longer lifespan, especially ternal age (Fox & Czesak 2000). Relationships be-
overwintering success (e.g., Ohgushi 1996). tween egg size and numbercan be difficulttodetect
We found no significantrelationship betweentib- because the total quantity of resources allocated to
0ia.-4p6a4t,ella l—eng0t.h009a,ndPav>er0a.g4e).eIgng mwaeingyhtar(tFhjr5o4po=d trheipsroidsucutniloinkemlyusttobbeeatsrsuuemeudndteorbfeiecldonsctoanndti,tiaonnds
populations, larger females lay larger eggs, but ex- (Fox & Czesak 2000). Further experiments will be
ceptions abound, and where there is a relationship needed to establish the robustness of this pattern
it tends to be weak (reviewed in Fox & Czesak and its underlying processes. In addition, the avail-
2000). Given that our spiders were field caught and able microbalance did not allow us to measure in-
other important environmental variables were not dividual eggs, and thus we cannot draw any con-
controlled, we cannot definitively exclude a rela- clusions about variation of egg size within female
tionship between female body size and mean egg clutches.
308 THE JOURNAL OF ARACHNOLOGY
In general, our findings are in line with the con- Jakob, E., J. Blanchong, M. Popson, K. Sedey ,&
clusions of Marshall & Gittleman (1994) that there M. Summerfield. 2000. Ontogenetic shifts in the
is more flexibility in clutch size ratherthan egg size cost ofliving in large groups: focal observations
in spiders. Taken with previous studies ofH. pluch- ofa pholcid spider {Holocnemuspluchei). Amer-
ei behavior and life history (Jakob & Dingle 1990), ican Midland Naturalist 143:405-413.
we conclude that there is a direct relationship be- Jakob, E.M. & H. Dingle. 1990. Food level and life
tween food intake, body size at maturity and repro- history characteristics in apholcid spider. Psyche
ductive success as measured by numberofeggs and 97:95-110.
clutch weight. We have argued that spiders that are Jann, P. & P.L Ward. 1999. Maternal effects and
able to compete successfully for food have a selec- their consequences for offspring fitness in the
tive advantage (Jakob 1991, 1994; Jakob et al. Yellow Dung Fly. Functional Ecology 13:51-58.
2000), and this study provides additional support Raster, J. and E.M. Jakob. 1997. Last-male sperm
for this assumption for females. priority in a haplogyne spider: correlations be-
tween female morphology and patterns ofsperm
ACKNOWLEDGMENTS
usage. Annals of the American Entomological
We thank B. Curtis for valuable assistance in the Society 90:25&4-259.
field. H. Dingle generously provided space in his lab- Marshall, S.D. J.L. Gittleman. 1994. Clutch size
oratory. C. Yin kindly allowed us to use his balance, in spiders: is more better? Functional Ecology 8:
and D. Ferro loaned us his drying oven. We thank L. 118-124.
Baker, E. Wells, J. Houser, C. Hoefler, and A. Porter Ohgushi, T. 1996. Consequences of adult size for
for comments on the manuscript, and M. Baker for survival and reproductive performance in a her-
insightful discussion. This study was supported by bivorous ladybird beetle. Ecological Entomology
NSF IBN 95-07417 to EMJ. 21:47-55.
Simpson, M.R. 1995. Covariation ofspideregg and
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