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Body Condition and Mate Choice in Tetragnatha Elongata (Araneae, Tetragnathidae)

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Body Condition and Mate Choice in Tetragnatha Elongata (Araneae, Tetragnathidae) 2002. The Journal of Arachnology 30:20±30 BODY CONDITION AND MATE CHOICE IN TETRAGNATHA ELONGATA (ARANEAE, TETRAGNATHIDAE) Anne Danielson-FrancËois1: Dept. of Biology, Swarthmore College, Swarthmore, PA 19081. E-mail: [email protected] Christine A. Fetterer and Peter D. Smallwood: Dept. of Biology, University of Richmond, Richmond, VA 23173 ABSTRACT. The mate preference characteristics of adult Tetragnatha elongata were assessed with respect to measures of female mass, linear size (length), and condition (mass scaled by length: body condition). Males preferred longer, heavier females and females with higher body condition indices. When mass is partially controlled, males still preferred females of higher body condition, but reversed their preference for length and chose smaller females. We present evidence that female body condition and mass are associated with the volume of her egg load and the proximity of oviposition, whereas female body length is not associated with either. Females displayed no clear preference among males for mass or linear size, but were reluctant to mate in female-choice trials. The small sample size obtained may have obscured the detection of female mate preferences if they exist. This may be the ®rst evidence that mate choice is in¯uenced by body condition rather than mass or linear size among spiders. Keywords: Mate choice, size, body condition, oviposition, Tetragnatha elongata Body size is a signal used by many animals 1983; Rubenstein 1987; Watson 1990; Hack to assess the quality of a mate. Measurements et al. 1997). If this pattern holds true, where of mate quality, including overall energy in- mate competition exists females should prefer take, genetic quality, and other components of larger males as mates when given a choice. ®tness, are associated with larger body size in Similarly, males should prefer heavier females both sexes (Andersson 1994). Larger males as mates when female fecundity is associated often achieve higher mating success by win- with body mass. Female body mass has been ning competitions for mates or through pre- associated with higher fecundity in spiders or post-copulatory mate guarding (Parker (Wise 1979; BricenÄo 1987; Vollrath 1987; 1970). Females may bene®t by choosing larg- Morse 1988; Uetz 1992; Head 1995) and other er males because these males have higher life- arthropods (Honek 1993; Spence et al. 1996). time mating success, higher genetic quality or Males may also prefer heavier females be- more vigorous copulatory courtship (Eberhard cause their greater mass may indicate that they 1991, 1996). Males may bene®t by choosing are closer to oviposition. Mating with females larger females when larger females have high- immediately prior to oviposition may give er quality offspring or greater fecundity (An- males a reproductive advantage in species dersson 1994). with sperm mixing or last-male sperm prece- For spider species, reproductive success for dence (Parker 1970; Waage 1979; Miller both sexes also seems to be in¯uenced by 1984; Siva-Jothy & Tsubaki 1989; Parker & body size (as measured by linear dimensions) Simmons 1991; Birkhead & Moller 1998). and mass. Male body size has been associated Last-male sperm precedence is predicted for with higher reproductive success in several the orb-weaving spider, Tetragnatha elongata spider species via larger males winning com- Walckenaer 1805 based on female reproduc- petitions for mates (Riechert 1978; Christen- tive morphology (Austad 1984; West & Toft son & Goist 1979; Vollrath 1980; Austad 1999). Regardless of sperm precedence, mat- ing closer to oviposition might also reduce the 1 Current Address: Dept. of Ecology & Evolution- likelihood that a male will lose his gametic ary Biology, University of Arizona, Tucson, AZ investment due to female mortality before ovi- 85721. position. 20 DANIELSON-FRANCOIS ET AL.ÐMATE CHOICE IN TETRAGNATHA 21 Recently, it has been argued that body con- males live for several months and mate dition is a better estimate of an animal's phys- repeatedly. Females lay multiple egg sacs be- iological state (and by extenstion, its ®tness) ginning two weeks after the ®nal molt and than either linear size or mass (Jakob et al. continuing until death (Danielson-FrancËois, 1996 and references therein). Body condition pers. obs.). Several generations may overlap indices are a function of the mass of the ani- during the warmer months. mal, adjusted for its linear size, and may be The 105 adult T. elongata for the initial particularly useful in measuring female spi- mating study and the oviposition study were ders, as they cyclically mature and oviposit collected along the banks of the Crum Creek their eggs. While the female's linear size re- in eastern Pennsylvania, Delaware County mains constant, her mass and body condition (398379 N; 75879 W) from June to September vary considerably over the course of this cy- 1993 and 1994. An additional 144 adults were cle, as her unsclerotized abdomen expands or collected from the banks of Westhampton contracts with feeding and starvation, yolk de- Lake in Richmond, Virginia (328309 N; position or oviposition. Jakob et al. (1996) 7738329 W) from June to September 1998. evaluated three different body condition in- These spiders were used to continue mating dices using two spider species. They found studies. The study sites were repeatedly sam- that a condition index based on the residual pled for spiders and as new adults matured index performed the best, and had the most they were collected. However, given the high straightforward biological interpretation (see frequency of mating these adults were unlike- Kotiaho 1999; Marshall et al. 1999 for further ly to be virgins and, after collection, several discussion). However, we are not aware of females laid egg sacs that later hatched. While data to support the hypothesis that body con- it would be ideal to use virgins in the study, dition per se (c.f., mass) correlates with fe- at least in T. elongata, males and females do cundity in female spiders or with the timing mate multiple times and males reinduct sperm of oviposition. It is also not known if body (Danielson-FrancËois, pers. obs.). Separate sets condition directly in¯uences mate choice (in- of animals were used in the mating and ovi- dependent of mass and/or linear size). position studies. Here we examine the in¯uence of linear Individual spiders were marked with bands size, mass, and body condition on mate choice of Testor's model paint and housed in clear in T. elongata. We also examine oviposition acrylic aquaria (ranging from 40 L 3 20 W in female T. elongata to see how linear size, 3 15 H to 100 3 50 3 62 cm). Spiders were mass, and body condition correlate with mea- fed ad libitum (Drosophila spp. and Tipulidae sures of fecundity and the timing of oviposi- spp.) for the mate choice study and for the tion. oviposition study were kept on a standardized diet (eight D. virilis daily). Voucher speci- METHODS mens for the study are deposited in the col- Natural history and collection.ÐLittle is lection curated by the Department of Ento- known about the natural history and mating mology, University of Arizona, Tucson, behavior of T. elongata (Levi 1981; Gillespie Arizona, USA. 1987; Smallwood 1993). Individuals build orb Linear size, mass and condition.ÐWe webs over or near water throughout the east- measured the right femur of the ®rst leg to the ern and southern United States (Levi 1981). nearest 0.1 mm (as a measure of linear size) They typically rebuild their webs daily and, in and weighed each adult collected to the near- some circumstances, relocate almost as often est 0.001 g. We assigned each female a body (Gillespie & Caraco 1987; Smallwood 1993). condition score (BCS hereafter) by generating Individuals exhibit aggregative behavior at a residual index. The residual index was con- high prey densities and often share silk lines, structed by regressing log-transformed mass although not prey, in these groups (Gillespie against log-transformed femur length follow- 1987). They are nocturnally active spiders. ing the methods of Jakob et al. (1996). BCS During daylight hours, females rest beside for an individual spider is its residual (positive their webs while adult males often move be- or negative) from the regression. Residuals tween aggregations of females (Danielson- were analyzed and variance in the index was FrancËois, pers. obs.). Both adult males and fe- homogeneous across different linear sizes as 22 THE JOURNAL OF ARACHNOLOGY determined by visual inspection. Females with cantly longer than small males in femur length higher BCS are heavier for their linear size. (n 5 11, xÅ 5 0.06 6 0.02 mm, xÅ 5 0.02 6 Mate choice trials.ÐMale preferences 0.01 mm, respectively; paired t-test; t 5 5.27, were assessed by introducing a male into an df 5 10, P 5 0.0004). Heavy males weighed aquarium with two females of signi®cantly signi®cantly more than lighter males (n 5 11, different linear size and mass and observing xÅ 5 0.06 6 0.02 g, xÅ 5 0.02 6 0.01 g, re- his behavior. Collected females were divided spectively; paired t-test; t 5 4.97, df 5 10, P visually into large and small linear size clas- 5 0.0006). Each female was placed in an ses. The spiders were kept on a 12:12 light- aquarium and allowed to adjust to her enclo- dark cycle at room temperature. We conducted sure and spin a web at least a day before the mate choice trials between 13:00±16:00 (dark males were introduced. Two males were si- period of the cycle) and observed the spiders multaneously placed on opposite sides of her under a dim red light. web. Female choice was determined by ob- We recorded both body length and mass for serving until the female copulated with one of both female spiders in each trial.
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