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Permanent Link: http://ufdc.ufl.edu/UF00091523/00635
 Material Information
Title: Integrating Environmental Quality and Sexual Selection in the Cactus Bug Chelinidea vittiger aequoris McAtee (Hemiptera: Coreidae)
Series Title: Journal of Undergraduate Research
Physical Description: Serial
Language: English
Creator: Thickman, Jake
Miller, Christine
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
 Subjects
Subjects / Keywords: Ecology
Sexual selection
Evolutionary ecology
Mating systems
Genre: serial   ( sobekcm )
 Notes
Abstract: Knowledge of the effects of variable environments on sexual selection remains incomplete, especially in ecologically relevant settings. This research examines the effects of a spatially and temporally heterogeneous environment on a natural population of the cactus bug Chelinidea vittiger aequoris to determine associations between resource quality, male size, and social groupings. Field sampling of C. vittiger took place at the Ordway-Swisher Biological Station, located in North-Central Florida, from June 8 to August 12, 2011. Multiple transects were conducted throughout the station, with information recorded on social behavior, adult size, and cactus quality. Males found in close proximity to females showed no significant difference in size, an indicator of male attractiveness in related species, compared to those males found alone, but were located on larger cactus patches. This result suggests that size may not be a reliable indicator of male quality in this population, possibly due to the effects of environmental variation on the processes of sexual selection.
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Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: sobekcm - UF00091523_00602
System ID: UF00091523:00660


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University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 1 Integrating Environmental Quality and Sexual Selection in the Cactus Bug Chelinidea vittiger aequoris McAtee (Hemiptera: Coreidae) Jake Thickman and Dr. Christine Miller College of Agricultural and Life Sciences University of Florida Knowledge of the effects of variable environments on sexual selection remains incomplete, especially in ecologically re levant settings. This research examines the effects of a spatially and temporally heterogeneous environment on a natural population of the cactus bug Chelinidea vittiger aequoris to determine associations between resource quality, male size, and social groupings. Field sampling of C. vittiger t ook place at the Ordway Swisher Biological Station, located in N orth Central Florida, from June 8 to August 12, 2011. M ultiple transects were conducted t hroughout the station, with information recorded on social behavior, adult size, and cactus qual ity. Ma les found in close proximity to females showed no significant difference in size, an indicator of male attractiveness in related species, compared to those males found alo ne but were located on larger cactus patches This result suggests that s ize may not be a reliable indicator of male quality in this population, possibly due to the effects of environmental variation on th e processes of sexual selection. INTRODUCTION Sexual selection, or the selection of traits driven specifically by competition for mates, has long been a subject of scientific study (Andersson, 1994) Sexual selection is both intersexual and intrasexual Intrasexual selection involves members of the same sex competing for access to members of the opp osite sex, and usually takes the form of male competition for females. Intersexual selection typically takes place when females select males based on male traits that are reliable indicators of male quality. The combined effects of these sexual selection m echanisms help shape the mating system displayed within a population Sexual selection plays a particularly important role in mating systems where one sex, often males, attempts to control access to the opposite sex. This can occur both directly, by physic ally defending members of the opposite sex, and indirectly, by monopolizing critical resources (Emlen & Ori ng, 1977) Variation in the availability and quality of resources is known to influence the process of sexual selection in many ways (Emlen & Oring 1977; Ingleby, Hunt, & Hosken, 2010; Mills et al., 2007) Patterns of resource distribution can influence the evolution of mating systems, as more clumped distribution patterns provide greater potential for monopolization of critical resources. Enviro nmental variation can also affect the reliability of sexual signals within a population. Spatial or temporal variation in a traits involved in sexual selection, possibly resulting in these traits becoming unreliable signals of mate quality. If such eff ects persist, this may alter the mechanisms of sexual selection, resulting in a decline in preference for a specific trait. Although environmental variation may strongly influence sexual selection, t he effects of environmental variation on sexual sele ction and mating systems remain unclear Purely g ene focused exp eriments are unable to adequately address these questions, particularly in artificial laboratory settings. T hus in order to advance sexual selection theory, research needs to integrate development, ecology, and evolution i n ecologically relevant environments (Cornwallis & Uller, 2010) To address this gap in knowledge we examine a natural population of cactus bugs, Chelinidea vittiger aequoris McAtee (Hemiptera: Coreidae), to determine associations between resource quality, male size, and social groupings. Study Species Chelinidea vittiger is found throughout the United States, extending n orthward to Montana and as far south as Florida and northern Mexico. It feeds on the cladodes (cactus pads) of the prickly pear cactus Opuntia spp Females lay eggs on the underside of cactus spines. Once hatched, juveniles, or nymphs progress through five instars, often remaining on the same cactus patch due to limited mobility (Hamlin, 1924) Adult C. vittiger vary in size, a trait that has been shown to be predictive of ma le attractiveness as well as mating success in related species and so male size is used here as a metric of male quality (Gillespie, Tudor, Moore, & Miller, n.d.; Partridge & Farquhar, 1983; Procter, Miller, & Moore, 2009)

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JAKE THICKMAN & DR CHRISTINE M ILLER University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 2 In this stud y we examined a population of C. vittiger (subspecies aequoris) found at the Ordway Swisher Biol ogical Station. This insect feeds on Opuntia humifusa cactus in North Central Florida, and cactus patch size is highly variable. Cactus plants produce areas of new growth only once a year in spring, and cladodes gradually mature through summer. Throughout this time cactus undergoes herbivory from animals such as deer and tortoises. Together, seasonal growth and herbivory add spatial and temporal variation to a fu ndamental resource for C. vittiger populations. Such resource variation has been found to be important for sexual selection in laboratory studies of related species, as variable resource quality in developmental environments can affect the expression of traits important to sexual selection processes as adults ( Miller & Emlen, 2010; Nageon de Lestang, 2010) Male proximity to females wa s us ed as a proxy for male attractiveness (Shelly & Kennelly, 2007) Those males found paired with females on the same patch were assumed to be more attractive than solitary males. This assumption was based on the relatively low population density of C. vittiger within the station as well as observations that uninhabited cactus patches are often located nearby male female pairings. Low population density reduces the likelihood of close proximity of males to females due to chance encounters, while the p resence of uninhabited pat ches close by suggests attraction to a specifi c patch. We predicted that 1) larger C. vittiger males would be found more frequently on larger cactus patches, 2) males found paired with females would be larger than solitary males, and 3) males found paired with females would be found on larger cactus patches than solitary males. METHODS Transect Point Generation We conducted field sampling at the Ordway Swisher Biological Station, located in North Central Florida, from June 8 to August 12, 2011. Prior to the field season, transect points throughout the biological station were generated using GIS. These points were l imited to those areas of the station designated as ruderal habitat, as the majority of prickly pear cactus is found in this habitat type within the station. To ensure an adequate range of environments and coverage of habitat area, 100 transect points with a minimum of 50m between points were randomly generated within ruderal habitat for use in field sampling. Because of the large and varied coverage area, some transects were found to contain no cactus and were removed from subsequent sampling. Transect Type s We conducted two types of transects during the field season, insect sampling transects and vegetation sampling transects. Insect sampling transects were designed to provide information on the habitat quality and social interactions of Chelinidea vittiger adults and nymphs. Vegetation sampling transects were designed to provide data on cactus patches regardless of insect presence, yielding a baseline for vegetation throughout the station. Insect Sampling Transects We conducted insect sampling tr ansects each week throughout the course of the field season. To reduce bias in field sampling for insects, a different set of transects was selected from the total pool each week using a random number function. To increase the speed of sampling, transects were placed into eight groups according to area, allowing multiple transects to be completed within the same area before moving onto the next. To reduce any effect time of day had on insect activity, the first area of transects visited was rotated each wee k. T he remaining areas were then visited in ascending numerical order each week. As an ex ample, in week one of sampling we visited areas in the order of 1,2,3,4,5,6,7,8; order was th en 2,3,4,5,6,7,8,1, and so on. We visited transects b etween the hours of 8:30am and 2:30pm. T ransect points were located using a handheld GPS unit, with the GPS coordinates used as the midpoint of transects. Once a midpoint was located a coin was flipped twice to determine one of four directions the resulting transect would follow, Northeast Southwest, Southwest Northeas t, North South, or East West. A flag was then anchored into the ground to secure the two sets of transect tape used by my field partner a nd myself. We wo uld then proceed 25m in opposite directions, resulting in a total transect distance of 50m. Transects were 1m in width. When a patch was located it was checked in its entirety even if only partially located w ithin the transect. A distance of 20cm designated separate patches If a nymph or adult C. vittiger was found o n a patch, data on the patch were recorded. These data included the number of cladodes contained in a patch, the number and instar of nymphs and the nu mber, sex, and size of a dults. I nsect size was measurd in terms of pronotum width using digital calipers. Vegetation Sampling Transects For vegetation sampling, four randomly selected transects were constructed in the same manner as insect sampling transects. Unlike trans ects used for insect sampling, we collected data on every cactus patch located within vegetation transects despite insect presence or absence. Vegetation transects remained at the same points and in the same direction throughout the course of the field season i n order to provide consistent baseline information

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INTEGRATING ENVIRONM ENTAL QUALITY AND SE XUAL SELECTION IN TH E CACTUS BUG University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 3 on cactus in t he biological station. A ll four vegetation transects were checked once every two weeks in rota ting order. To ensure accuracy each cactus patch within the transects was flagged and numbered. Statistical Analyses To determine relationships that should be relevant to sexual selection each cactus patch was assigned a classification based on the number of male and female C. vittiger present. The classes are as follows, solitary: one male found wi th no females, paired: one male found with one or more females, multiple male: more than one male found with no females, mixed: more than one male found with one or more females. To avoid pseudoreplication, male size was taken as the average size of all ma les measured on a patch. We used generalized liner models to compare solitary and paired males in terms of size as well as number of cladodes per patch. A generalized liner model was also used to analyze changes in male and female size over the course of t he field season. A n additional linear regression analysis was performed to determine the relationship between number of cladodes present in a patch and male size. SPSS 19 was used for all statistical analyses RESULTS Male Size and Cladode Number We found 196 adult Chelinidea vittiger 97 of which were ma le, on cactus patches of varying size. Of the 97 males found, 12 escaped before size was recorded. Cactus patches containing males showed high variation i n cladode number, ranging from 2 to 225 clado des Male size was found to have no association with the number of cladodes present in a patch (F 65 =0.421, p =0.519 ) Social Behavior C. vittiger males were found on 85 total patches, with nymphs also present in 33% of these patches. Of the patches containing both a male and nymphs, a female was also present 46% of the time. Females were found on 84 total patches, with nymphs also present in 20% of these patches. Of the patches containing both females and nymphs, a male was also present 76% of the time. Males and females were found together on 29 total patches, with nymphs also present in 45% of these patches (Fig. 1 ). Only four males were found mat ing. Figure 1 Social Distribution of cactus patches containing C. vittiger adults. Comparison of Solitary and Paired Males Of the 97 male C. vittiger locate d during field sampling 53 were solitary. An additional 25 males were paired with one or more females. Nine males were found in a total of four male only groups, while 10 males were found in four mixed sex groups (Fig. 2 ). We did not include these groups in further analyses bec ause of their rarity. Figure 2 Number of cactus patches and male C. vittiger found in multiple male and mixed groups. Of the remaining 78 males in the study, 25 were found paired with females and 53 were solitary. Adults 140 Males 85 Males with nymphs 28 Females 84 Females with nymphs 17 Males and females 29 Males and females with nymphs 13 Multiple Male >1 males found with no females 4 patches 9 males 2 males 3 3 males 1 Mixed Male and Female >1 males found with one or more females 4 patches 10 males 2 males 3 4 males 1

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JAKE THICKMAN & DR CHRISTINE M ILLER University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 4 N o significant difference was found in the body sizes of solitary and paired males ( 2 =0.073, df=1, p = 0 .787; Fig. 3 ). The number of cladodes per patch where paired males were located tended to be higher than the number of cladodes where only solitary males were found approaching significance ( 2 =3.609, df=1, p = 0 .057; Fig. 4 ). Figure 3 Solitary males and males paired with females did not differ in body size. Figur e 4 Males paired with females were found on larger cactus patches (p=0.057) than solitary males. Changes in Size Over Time Males were smaller later in the season ( 2 =16.240, df=6, p = 0 .013; Fig. 3), but no significant different was seen in female size over time ( 2 =9.486, df=6 p = 0 .148; Fig. 5 ). Figure 5. Male (blue) and female (pink) size trends over the sampling period. DISCUSSION Male Size Effects Contrary to our predictions, no significant difference was observed between the body sizes of Chelinidea vittiger males found paired with females and those found alone. This is a surprising result, as studies of other species of coreids have shown male size to be an important factor in both male male competitions (Procter et al., 2009) and female mate choice (Gillespie et al., n.d.) I t is possible that proximity to females does not accurately represent mating success for males in this case. H owever, it is also possible that male size is unreliable as an indicator of male quality, that male size plays only a small role in the mating system of the populati on, or that females value resource quality over male quality. Male size may not be reliable as a sexual signal for the population of C. vittiger at the Ordway Swisher Biological Station. For a female mating preference to develop, male sexual traits must be reliable indicators of male quality, allowing females to benefit from potentially costly mate choice (Andersson & Iwasa, 1996) If no such correlation between male traits and quality exists, mate choice may fail to evolve within a population ( Kokko, Jennions, & Brooks, 2006) In recent years models have been developed to predict possible environmental effects on sexual signals (Ingleby et al., 2010) One study made four predictions 4.2 4.25 4.3 4.35 4.4 4.45 4.5 Solitary Males Paired Males Mean Pronotum Width (mm) 0 10 20 30 40 50 60 70 Solitary Males Paired Males Mean Number of Cladodes per Patch 4.000 4.100 4.200 4.300 4.400 4.500 4.600 4.700 4.800 4.900 1 2 3 4 5 6 7 Mean Pronotum Width (mm) Week of Sampling

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INTEGRATING ENVIRONM ENTAL QUALITY AND SE XUAL SELECTION IN TH E CACTUS BUG University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 5 regarding what conditions may lead to sexual signals becoming unreliable signals of male quality: highly heterogeneous environments, low variation in genetic quality, harsh environments with high levels of juvenile mortality, and strong environmental influ ence relative to genetic quality influence on sexual trait expression (Higginson & Reader, 2009) While no measure of genetic quality was recorded for the study population of C. vittiger the presence of a harsh and heterogeneous environment is highly plausible. Cactus within the Ordway Swisher Biological Station is highly heterogeneous, especially on the habitat scale of C. vittiger as the cactus naturally grows in distinct patches with different areas of the station varying in density and size of patches found (Thickm an, unpublished data). Natural predators such as ants or spiders were also frequently found in areas containing cactus, suggesting potential for high predation on nymphs that lack mobility (Thickman, unpublished data). Size in many other insects can be adv ersely affected by resource quality and predator presence during development (Danner & Joern, 2004; Nageon de Lestang & Miller, 2009) While no significant relationship was found between patch size and adult male size, it was not possible to distinguish rearing environment from adult location in this study. Adult size is influenced by developmental environment, the cactus patch on which C. vittiger progress through 5 instars. Once fully developed, adults may trave l between different cactus patches, though their mobility is somewhat limited by short flight distances. Further research on the effects of patch size during nymphal stages is needed to more accurately determine the level of environmental influence on size in C. vittiger The effects of this harsh, heterogeneous environment may be the cause of the downward trend in male size that was observed over the course of the sampling period. Although we are unable to indentify causal environmental factor s from these data herbivory or the maturation of cactus cladodes may play a role. Herbivory by populations of deer or tortoises in the station depletes patches of cladodes reducing the quantity of resources available. New cactus cladodes are produced only early in the year, and C. vittiger are often found feeding on areas of new growth. Prickly pear cladodes in other Opuntia species have been shown to vary in chemical comp osition as they mature, exhibiting a decline in protein content over time (Rodriguez Felix & Cantwell, 1988) This may affect the quality of food resources available to C. vittiger populations. In any case, this observed decline in male size over the course of the mating season could potentially weaken selection for mate choice based on male size within the population as size becomes an unreliable indicator of male quality. It is also p ossible that size is no t a determinant of male attractiveness within the mating system present in the population of C. vittiger at the Ordway Swisher Biological Station. In either female defense or resource defense polygyny larger males may be better suited to defend females or resources critical to females from competitors, resulting in higher levels of mating success (Thornhill & Alcock, 1983) In the absence of aggressive behavior between males (none was witnessed during the field season) scramble competition polygyny may deve lop, in locate receptive females rather than his ability to aggressively exclude other males (Alcock, 1980; Nishida, 1989) In such a mating system male size could potentially play a much smaller role, as any advantage larger males might hold in direct competition or exclusion would be lost. Chelinidea vittiger were found at relatively low density throughout the station (Thickm an, unpublished data), suggesting that to spend energy efficiently, adult males should attempt to increase female encounters rather than defend territories. Further knowledge of adult movement patterns and social interaction is required to fully examine th e mating system of C. vittiger within the station. Cactus Patch Size Effects While no significant d ifference in body size between solitary and paired males was found analyses showed a potential difference in cactus patch size between the two groups, with paired males being found on larger overall patches. This result suggests that the size of the cactus patch a male is located on may play a role in sexual selection w ithin the population. The distribution of resources within a population can shape the development of mating systems, as more clumped distributions of important resources provide greater potential for individuals to monopolize them (Emlen & Oring, 1977) The uneven distribution pattern of cactus in the station holds potential for the monopolization of large c actus patches by C. vittiger males. Patches may also be economically defendable from an energetic standpoint, as no parental care is exhibited (Emlen & Oring, 1977) This potential for monopolization is somewhat supported by the lack of multiple male and mixed male and female groups found during the field season, presumably due to the possible monopolization and defense of cactus patches by individual males. I t is possible that cactus patch size may represent a valuable resource to female reproduction. Cactus cladodes are the primary food resource for C. vittiger and nymphs lack mobility. It is highly plausib le that nymphs remain on the same patch on which eggs were laid until adulthood; therefore the quality of the rearing environment selected by females could play a role in nymph al development ( Miller, 2008) Large patches may also provide greater defense from predators. When discovered in the field, both nymphs

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JAKE THICKMAN & DR CHRISTINE M ILLER University of Florida | Journal of Undergraduate Research | Volume 14, Issue 2 | Spring 2013 6 and adults often traveled downward into the cactus patch and leaf litter when disturbed, taking advantage of the natural defenses of the cactus. A larger patch may increase the effectiveness of such behavior, providing a greater area in which nymphs can seek cover. If certain C. vittiger males were able to monopolize larger cactus patches, a potentially valua ble resource to females, then female choice should be affected by both male and patch quality (Emlen & Orin g, 1977) Because male size may be an unreliable signal of male quality in this C. vittiger population it is possible that females place more emphasis on patch size when selecting a mate. To test this hypothesis, further information is required regardi ng the potential mechanisms for male resource monopolization and environmental effects on nymph development. ACKNOWLEDGEMENTS I would like to thank all members of the Miller Lab of Evolutionary Ecology at the University of Florida for their assistance with this study, especially Katherine Holmes for all of her hard work during the field season. This work was supported by the 2011 Thad Owens Memorial Fund Undergraduate Research Grant and NSF grant IOS 0926855 (to CWM). REFERENCES Alcock, J. (1980). Natural selection and the mating systems of solitary b ees. American s cientist 68 (2), 146 153. Andersson, M. (1994). Sexual s election Princeton: Princeton University Press. Andersson, M., & Iwasa, Y. (1996). Sexual selection. Trends in ecology & e volution 11 (2), 53 58. Cornwallis, C. K., & Uller, T. (2010). Towards an evolutionary ecology of sexual traits. Trends in ecology & evolution 25 (3), 145 152. Dan ner, B., & Joern, A. (2004). Development, growth and egg production of Ageneotettix deorum (Orthoptera: Acrididae) in response to spider predation risk and elevated resource quality. Ecological e ntomology 29 (1), 1 11. Emlen, S., & Oring, L. (1977). Ecology, sexual selection, and the evolution of mating systems. Science 197 (4300), 215 223. Gillespie, S., Tudor, S., Moore, A., & Miller, C. W. (n.d.). Both female and male mate discrimination vary by environmental context, but in diff erent ways. Hamlin, J. (1924). A review of the genus Chelinidea (Hemiptera Heteroptera) with biological data. Annals of the Entomological Society of America 17 (2), 193 208. Higginson, A. D., & Reader, T. (2009). Environmental heterogeneity, genotype by en vironment interactions and the reliability of sexual traits as indicators of mate quality. Proceedings of the Royal Society B: Biological sciences 276 (1659), 1153 1159. Ingleby, F. C., Hunt, J., & Hosken, D. J. (2010). The role of genotype by environment interactions in sexual selection. Journal of evolutionary biology 23 (10), 2031 2045. Kokko, H., Jennions, M. D., & B rooks, R. (2006). Unifying and testing models of sexual s election. Annual review of ecology, evolution, and s ystematics 37 (1), 43 66. Mil ler, C. W. (2008). Seasonal effects on offspring reproductive traits through maternal oviposition behavior. Behavioral e cology 19 (6), 1297 1304. Miller, C. W., & Em len, D. J. (2010). Across and within population differences in the size and scaling relationship of a sexually selected t rait in Leptoscelis tricolor (Hemiptera: Coreidae). Annals of the Entomological Society of America 103 (2), 209 215. Mills, S. C., Alatalo, R. V, Koskela, E., Mappes, J., Mappes, T., & Oksanen, T. (2007). Signal reliabi lity compromised by genotype by environment interaction and potential mechanisms for its preservation. Evolution 61 (7), 1748 17 57. Nageon de Lestang, F. (2010). Effects of environmental heterogeneity on male male competitive success, size, and s cali ng in a cactus bug Narnia femorata ( Hemiptera : Coreidae ). 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Settlement patterns of Mediterranean fruit flies in the tree canopy: An experimental a nalysis. Journal of insect b ehavior 20 (5), 453 472. Thornhill, R., & Alcock, J. (1983). The evolution of insect mating s ystem s (pp. 1 547). Cambridge: Harvard University Press.