Assortative mating of the halloween butterfly Dryadula phaetusa (Nymphalidae, Heliconiinae)


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Assortative mating of the halloween butterfly Dryadula phaetusa (Nymphalidae, Heliconiinae)

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Assortative mating of the halloween butterfly Dryadula phaetusa (Nymphalidae, Heliconiinae)
Translated Title:
Apareamiento selectivo de la mariposa de halloween Dryadula phaetusa (Nymphalidae Heliconiinae)
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Reynaud, Darin
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Text in English

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Butterflies--Reproduction ( lcsh )
Mariposas--Reproducción ( lcsh )
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
CIEE Fall 2000
CIEE Otoño 2000
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Abstract:
Possible size-assortative mating was investigated in a captive population of the butterfly, Dryadula phaetusa (Nymphalidae) by comparing the forewing sizes of individuals of mating pairs to the average size of forewings in the whole population. It was postulated that females would prefer larger males because they deposit larger spermatophores, and females would thereby increase their fitness. Males would also prefer larger females because larger females have a better chance of fighting off predators and thus higher fitness. Individuals that were feeding or courting were also measured to document size variation and investigate size-dependent behaviors. There was no evidence for positive assortative mating in D. phaetusa but individuals of the mating pairs showed a significantly larger average forewing length than the average forewing length of the courting and feeding population (captive population mean = 3.878 cm, mating population mean = 3.983cm, feeding population mean = 3.573, courting population mean = 3.501 cm). This larger average in size of the mating pairs suggests that there is non-random mating. This sexual selection may be a preference to larger individuals due to their higher fitness or it may be an avoidance behavior to inbreeding. A significantly smaller population than average in the garden suggests that inbreeding is occurring and therefore mate choice may be observed because the butterflies are using it as an avoidance mechanism to inbreeding, which often results in reduced fitness. ( , )
Abstract:
Examiné una población de mariposas Nymphalidae, Dryadula phaetusa, hay una preferencia para aparearse es por su tamaño. Comparé el tamaño de la segunda ala de las parejas que se aparean con las de la población. Mi expectativa es que las hembras deben escoger los machos más grandes para aumentar la aptitud genética de ella. La frecuencia de comportamientos como comiendo y cortejando estuvieron comparados también para examinar la preferencia del tamaño. Los compañeros que eran aparean muestran mas grande la segundo parte de la ala que la segundo parte de la ala longitud de los que cortejan y en las poblaciones comiendo. Pienso que los compañeros que son apareando son mas grandes porque ellos pueden proteger mejor y ellos tienen espermataphora mas grande. También, las mariposas no quieren aparearse con sus familiares y entonces, ellos escogen depende de tamaño. Porque ellos no tienen una gran selección ellos escogen quien quiere aparearse para tener mejor aptitud.
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Student affiliation: Department of Biology, Whitman College, Walla Walla, WA, USA
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Assortative Mating of the Halloween Butterfly Dryadula phaetusa, Nymphalidae, Heliconiinae Darin Reynaud Department of Biology, Whitman College, Walla Walla, WA, USA ABSTRACT Possible size assortative mating was investigated in a captive population of th e butterfly, Dryadula phaetusa Nymphalidae by comparing the forewing sizes of individuals of mating pairs to the average size of forewings in the whole population. It was postulated that females would prefer larger males because they deposit larger sperm atophores, and females would thereby increase their fitness. Males would also prefer larger females because larger females have a better chance of fighting off predators and thus higher fitness. Individuals that were feeding or courting were also measured to document size variation and investigate size dependent behaviors. There was no evidence for positive Assortative mating in D. phaetusa but individuals of the mating pairs showed a significantly larger average forewing length than the average forewing le ngth of the courting and feeding population captive population mean = 3.878 cm, mating population mean = 3.983cm, feeding population mean = 3.573, courting population mean = 3.501 cm. This larger average in size of the mating pairs suggests that there is non random mating. This sexual selection may be a preference to larger individuals due to their higher fitness or it may be an avoidance behavior to inbreeding. A significantly smaller population than average in the garden suggests that inbreeding is occu rring and therefore mate choice may be observed because the butterflies are using it as an avoidance mechanism to inbreeding, which often results in reduced fitness. RESUMEN Examin una poblacin de mariposas Nymphalidae, Dryadula phaetusa hay una prefere ncia para aparearse es por su tamao. Compare el tamao de la segunda ala de las parejas que se aparean con las de la poblacin. Mi expectativa es que las hembras deben escoger los machos ms grandes para aumentar la aptitud gentica de ella. La frecuencia de comportamientos como comiendo y cortejando estuvieron comparados tambin para examinar la preferencia del tamao. Los compaeros que eran aparean muestran mas grande la segundo parte de la ala que la segundo parte de la ala longitud de los que cortejan y en las poblaciones comiendo. Pienso que los compaeros que son apareando son mas grandes porque ellos pueden proteger mejor y ellos tienen espermataphora mas grande. Tambin, las mariposas no quieren aparearse con sus familiares y entonces, ellos escoge n depende de tamao. Porque ellos no tienen una gran seleccin ellos escogen quien quiere aparearse para tener mejor aptitud.

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INTRODUCTION In 1859, Darwin first outlined his ideas on sexual selection in The Origin of Species, making the distribution f rom natural selection by saying €this [sexual selection] depends, not on a struggle for existence, but on a struggle between males for the possession of females; the result is not death to the unsuccessful competitor, but few or no offspring Smith, 1989. A lcock defines sexual selection as €a component of individual selection that is created by the pressures males and females exert on conspecifics as they compete for mates and choose among potential partners Alcock 1984. According to Trivers, it is the in equality in parental investment per offspring between the sexes that generates competition and mate choice. Whichever sex makes the greater parental investment per offspring will be a limited and valuable resource for the other. Therefore, there will usual ly be competition among males for access to females, who will have the option of choosing among the many males that want to copulate with them. They should select individuals that will have the greatest possible effect on their fitness Alcock 1984. Sexu al selection can have profound effects on populations. Futuyma states that €the opportunity for selection exists when differences in some phenotypic characteristic result in consistent differences, on average, in rates of survival or reproduction 1986. Three major modes of selection that depend on the relationship between phenotype and fitness are directional, stabilizing and disruptive selection. Directional selection is when an extreme phenotype is favored and the population selects for a higher or low er value of a character than its current mean. An example of directional selection is for instance, if large only mate with large, therefore leaving the small and the middle sized individuals to be weeded out and eventually the population shifts to become all large individuals. Stabilizing selection is when intermediate phenotypes are most fit. For example, suppose the medium average sized forewing in a butterfly population is the most fit phenotype. Selection for medium sized forewings will then center o n this optimal value of character. Finally, disruptive selection is selection of two or more phenotypes and against the intermediates between the two. An example of disruptive selection is if the large individuals are mating with the large and the small ar e mating with the small, therefore excluding the middle sized individuals. Eventually, the two extreme phenotypes with high fitness will separate and possibly become two new species Futuyma 1986. All three types of selection can greatly alter a populatio n. Sexual selection is a component of individual selection, where each sex may choose a partner based on a number of preferred attributes. An example of discriminating between partners is seen in female damselflies Calopteryx maculata that behave as th ough they were selecting partners with characteristics likely to enhance their fitness Alcock 1984. They are more likely to accept the courtship of territorial mates than the copulatory attempts of subordinate intruders. As a result, they gain entry into superior oviposition sites controlled by their partners, and may also derive some genetic benefits. With butterflies, the preferred traits may be size and color. Alcock says it is a natural deduction that females should prefer dominant males as mates, wit h dominant referring to males with high male status, large body size, good foraging ability, superior survival

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skills and good physical condition 1984. Color may be a preferred trait because color provides protection from visually oriented predators and may be related to social signals used during courtship. Color and size may reflect fitness and therefore, sexual selection among butterflies is likely to be based on these characteristics. For butterflies, size is a sign of dominance and is not only repor ted as an important stimulus for courting it has also been reported to be an indicator of size of spermatophore, an important characteristic in mating. Among the checkered white butterfly, Pieris protodice Pieridae, Pierinae, females prefer larger males to smaller males in courtship, because larger males deposit larger spermatophores inside the female. A large spermatophore works to the advantage of the female, because she digests it and uses it for energy in making eggs. The male also benefits because it takes longer for the female to digest, forcing her to delay possible remating. The last male to mate with a female generally fathers her offspring; therefore, delaying her from remating increases the number of offspring that he fathers Scott 1986. This study focused on sexual selection and its consequences in the heliconiine butterfly, Dryadula phaetusa D. phaetusa Nymphalidae, also known as the €Halloween butterfly, ranges from Mexico to Paraguay and can be found in tropical lowland damp fields and marshes De Vries 1987. In Costa Rica, it occurs infrequently in open areas from sea level to 1200 meters. The genus has shorter and broader wings than other Heliconiines, and comparatively short antennae and an androconial patch on the males De Vries 1 987. The hostplant of the Halloween butterfly is Passiflora talamacensis Passifloraceae De Vries 1987. Adults feed on nectar of flowers and bird droppings and males sometimes sip mud Scott 1986. D. phaetusa occurs sporadically and roosts in small gro ups at night De Vries 1987. Most studies of sexual selection focus on densely populated species, since widely dispersed, locally rare species are difficult to study. However, the Monteverde Butterfly garden offers the rare chance to study the mate choic e of a normally widespread, but locally uncommon species. This leads to the question, does Dryadula phaetusa exhibit sexual selection and what is its implication for the population? MATERIALS AND METHODS The research was carried out in the lowland garden at The Butterfly Garden in Monteverde, Costa Rica from October 28, 2000 until November 15, 2000. The lowland garden simulates a low elevation environment of 0 500 meters. Before beginning the experiment, all adult and female Dryadula phaetusa in the garden were captured by hand or with butterfly nets. Individuals were sexed, and the thorax, tibia and forewing lengths were measured with a Spi brand caliper to 0.001cm. Each individual was given a unique mark with a black permanent pen on the underside of the abdomen. All individuals observed mating, courting or feeding were recaptured and identified. New individuals that were introduced into the garden during the study were similarly marked, measured and incorporated into the study.

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Most observations of matin g were made in the afternoon on sunny days. The majority of the observations of feeding and courting were taken during the sunniest parts of the day when the butterflies were most active. RESULTS A total of 41 butterflies were caught in the garden, consis ting of 24 males and 17 females. There were seven pairs of mating butterflies caught. Out of the whole population, 51 were caught while courting and of those, 46 were male and five were female. The number of males caught courting exceeds the number of indi viduals in the population because some of the butterflies were recaptures. Thirty one butterflies were caught feeding and of those 27 were male and four were female. A Spearman Rank Correlation test was used to test forewing length versus tibia length, th orax length versus forewing length, and thorax length versus tibia length. In the captive population of Dryadula phaetusa forewing length versus tibia as well as thorax length versus forewing length correlated significantly length Figure 1, r = 0.314, p = 0.0473, n = 41 Figure 2, r=0.326, p = 0.0392, n=41. The sizes, ranges, mean and standard deviation for forewing, thorax and tibia are reported in Table 1. Among the whole population thorax length versus tibia did not correlate significantly Figure 3, r= 0.144, p = 0.3638, n = 41. In both forewing comparisons, there was a significant correlation; therefore, forewing length was used for the following statistics. Because forewing length correlated with other measures of size, it was chosen as the varia ble to investigate possible sexual dimorphism between males and females of the whole population. An unpaired t test was used to compare the average forewing length of males and females. The average forewing for a male was 3.847 cm n=24 and the average fo rewing for a female was 3.922 cam n=17. The males and females did not differ significantly in forewing length t= 0.900, p=0.3736, d.f. = 39. A Turkey/Kramer one way ANOVA test was used to test for significant size differences between males and female s engaging in various behaviors. The captive populations of males had an average forewin g length of 3.847 cm, the captive population of females = 3.922 cm, mating males = 3.974 cm, mating females = 3.991 cm, feeding males = 3.542, feeding females = 3.787 cm, courting males = 3.474 and courting females = 3.762 cm fig. 4. It was determined that there was no significant size difference between males and females, engaging in various behaviors. A Sheffe post hoc test was run to investigate differences in si ze of individuals in pairwise combinations of behaviors. The means for each behavior were calculated captive population mean = 3.878 cm, mating population mean = 3.983 cm, feeding population mean = 3.573 cm, courting population m ean = 3.501 cm. It was determined that the mean of the whole population is larger than both the mean of the feeding population and the mean of the courting population Figure 5. The mean of the mating population is larger than both the mean of the feeding and the mean of the c ourting

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populations Figure 5. In general, the mean of the mating population was larger than the mean of the whole population but there was no significance measured Figure 1. To test for the possibility of assortative mating a Spearman Rank correlatio n and a Regression Plot were run and the forewing lengths of each mating pair were compared and found to have no significant correlation r=0.143, p value=0.7264, n=7Figure 6. A one sample t test was run to test for significance between the observed fo rewing mean of the captive population of D. phaetusa mean = 3.878 cm and the expected mean of D. phaedusa as reported by DeVries mean = 4.5cm 1987. Significance was observed between the expected and the observed mean suggesting that the population un der study was significantly smaller than the average, for forewing length p = 0.0002. DISCUSSION The findings reported here demonstrate that male forewing length and female forewing length in mating pairs are not significantly correlated and thus in a p opulation of Dryadula phaetusa there is no assortative mating. However, the mating pairs are larger in comparison to other behaviors suggesting that Alcock‚s argument that females may prefer dominant males as mates may be true 1984. Dominant refers to ma les with high male status, large body size, good foraging ability, superior survival skills or good physical condition 1984. It may be to the female‚s benefit to mate preferentially with a male of high status because he may have a genetic advantage and h is advantage would thus be passed down to her offspring. The non random mating observed in this study may lead to two different types of selection. As the average size of mating pairs is only a fraction larger than the average size of the population, dire ctional selection or stabilizing selection may exist. Because the extreme larger size in mating pairs is very close to the intermediate the average of the whole population, both selection pressures may be working together. Another possibility is that a s the larger butterflies are favored, disruptive selection may be occurring, where the larger females prefer the larger males, leaving the smaller ones to mate with one another, possibly resulting in two separate species Futuyma 1986. Non random mating on the basis of size has been intensely studied and is heavily supported. Body size often contributes to competitive ability because the larger males can win territories and fights. Increased competitive ability can lead to a greater chance at attracting a mate Krebs & Davies 1981. In addition, large males may be better able than small males to protect a female from being damaged by other males during mating Partridge 1983. One factor that limits this study‚s conclusions is that an entirely captive pop ulation was studied. Most of the butterflies living in the garden are reared in the garden; therefore inbreeding may decrease the genetic variability of the population. If this is the case, the general population may be smaller than perhaps a natural popul ation. Alcock writes that females can make mating choices purely on the basis of male genetic quality based on observations that females of some species actively avoid their brothers and fathers

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1984. This may be true if females are choosing from a large population, but if they only have a select few with the butterflies in the garden, inbreeding may be more common than usual. An alternative to the hypothesis already presented is the effect of inbreeding on the captive population. DeVries reports forewin g length to be 40 41 mm, a value much higher than the 38.78mm average found in the captive population of D. phaetusa When tested, there was a significant difference between the expected mean and the observed mean, therefore it has been tested and proved t hat the population under study was indeed smaller in forewing length average than D. phaetusa reported in DeVries 1987. Only 14 of the 41 butterflies captured were 40mm or greater and many were smaller. This high percentage of small individuals may be du e to inbreeding. The population studied was a captive population and abundant evidence from breeding experiments and captive populations of plants and animals demonstrates that breeding between close relatives results in inbreeding depression, a decrease i n the fitness of offspring Pusey and Wolfe 1996. Perhaps D. phaetusa responds to this lower fitness effect by using mate choice as an inbreeding avoidance behavior. By this example, size is fitness related, with small size signifying inbreeding depressio n. Through mate choice a captive population, such as the one in the garden, can prevent inbreeding from influencing the population beyond a certain level. The question then arises do natural populations experience this same mate choice if they are less at risk for inbreeding? Given that adults are widely dispersed and rare, there may be little need for inbreeding avoidance or sexual selection. If mates are rare, one mate can‚t be too selective. Therefore, it is only more remarkable that they demonstrate si ze selected mate choice. Another idea is that as a community roosting species, D. phaetusa mates may be able to assess many potential mates at once. If this opportunity is given on a nightly basis, mate choice may be easy. It may even explain why they roos t in groups. A study of roosting behavior would be a good extension to this study. ACKNOWLEDGEMENTS Thanks to the staff at El Jardin de las Mariposas for all your entertainment, knowledge and for finding 6/7 of the total copulations. Thanks to Jim Wolfe f or the use of the garden. Thanks to Andrew Rodstrom, Tim Kuhman and Stephanie McFarlane for your time, energy, patience, encouragement and lots of hugs. Thanks CIEE and Estacin Biolgica de Monteverde for a semester in paradise. Alan Masters, Karen Master s and Mauricio Garcia, your passion for life and Costa Rica has taught and inspired me, thanks. LITERATURE CITED Alcock, J. 1984. Animal Behavior, An Evolutionary Approach Third Edition. Sinauer Associates, Inc., Sunderland, Massachusetts.

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DeVries, P. J. 1987. The Butterflies of Costa Rica and Their Natural History. Princeton University Press, Princeton, New Jersey, USA. Futuyma, P.J. 1986. Evolutionary Biology. Sinauer Associates, Inc. Sunderland, Massachusetts. Halliday, T.R. 1983. In P. Bates on Ed.. Mate Choice, p.5. Cambridge University Press, Cambridge, England. Krebs, J.R. and Davies, N.B. 1981. An Introduction to Behavioral Ecology. Sinauer Associates, Inc. Sunderland, MA. Pusey A. and M. Wolf. 1996. Inbreeding avoidance in anima ls. Tree. 11:202Scott, J.A. 1986. The Butterflies of North America. Stanford University Press, Stanford, California. Smith, D.A.S. 1989. Mate selection in butterflies: Competition, coyness, choice and chauvinism. In R.I. Vane Wright and P.R. Ackery, Eds.. The Biology of Butterflies, pg. 225. Princeton University Press, Princeton, New Jersey.

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Table 1. Values of forewing, thorax and tibia measurements for Dryadula phaetusa were measured to find a standard unit of measurement while testing for a size preference in mating. Mean Standard Deviation Minimum Maximum Range Forewing 3.878 cm 0.265 cm 3.24 cm 4.44 cm 1.2 cm Thorax 0.778 cm 0.66 cm 0.68 cm 0.94 cm 0.26 cm Tibia 0.583 cm 0.052 cm 0.47 cm 0.7 cm 0.23 cm Figure 1. A scattergram of fore wing length versus tibia length for the captive population of Dryadula phaetusa n=41. There is a significant correlation between the two measurements r = 0.314, p = 0.0473.

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Figure 2. A scattergram of thorax length versus forewing length for the captive population of Dryadula phaetusa There is a significant correlation between the two measurements r = 0.326, p = 0.0392. Figure 3. A scattergram of thorax length versus tibia length of the captive population of Figure 3. A scattergram of tho rax length versus tibia length of the captive population of Dryadula phaetusa There is no significant correlation between thorax length and tibia length r = 0.144, p = 0.3638.

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Figure 4. Mean forewing length of Dryadula phaetusa males and females engaged in various behaviors cm. There is no significant size difference between males and females for a given behavior.

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Figure 5. Size comparisons for each behavioral category of Dryadula phaetusa showing that there is a significant difference between mean forewing length of certain behaviors. Whole population mean = 3.878 mm, mating population mean = 3.983 mm, feeding population mean = 3.573 mm, and courting population mean = 3.501 mm.

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Figure 6. There is no significant correlation bet ween the forewing length of mating males and females r = 0.0143, p = 0.7264, n = seven mating pairs.



PAGE 1

Assortative Mating of the Halloween Butterfly Dryadula phaetusa, Nymphalidae, Heliconiinae Darin Reynaud Department of Biology, Whitman College, Walla Walla, WA, USA ABSTRACT Possible size assortative mating was investigated in a captive population of th e butterfly, Dryadula phaetusa Nymphalidae by comparing the forewing sizes of individuals of mating pairs to the average size of forewings in the whole population. It was postulated that females would prefer larger males because they deposit larger sperm atophores, and females would thereby increase their fitness. Males would also prefer larger females because larger females have a better chance of fighting off predators and thus higher fitness. Individuals that were feeding or courting were also measured to document size variation and investigate size dependent behaviors. Ther e was no evidence for positive a ssortative mating in D. phaetusa but individuals of the mating pairs showed a significantly larger average forewing length than the average forewing le ngth of the courting and feeding population captive population mean = 3.878 cm, mating population mean = 3.983cm, feeding population mean = 3.573, courting population mean = 3.501 cm. This larger average in size of the mating pairs suggests that there is non random mating. This sexual selection may be a preference to larger individuals due to their higher fitness or it may be an avoidance behavior to inbreeding. A significantly smaller population than average in the garden suggests that inbreeding is occu rring and therefore mate choice may be observed because the butterflies are using it as an avoidance mechanism to inbreeding, which often results in reduced fitness. RESUMEN Examiné una población de mariposas Nymphalidae, Dryadula phaetusa , hay una prefere ncia para apa rearse es por su tamaño. Comparé el tamaño de la segunda ala de las parejas que se aparean con las de la población. Mi expectativa es que las hembras deben escoger los machos más grandes para aumentar la aptitud genética de ella. La frecuencia de comportamientos como comiendo y cortejando estuvieron comparados también para examinar la preferencia del tamaño. Los compañeros que eran aparean muestran mas grande la segundo parte de la ala que la segundo parte de la ala longitud de los que cortejan y en las poblaciones comiendo. Pienso que los compañeros que son apareando son mas grandes porque ellos pueden proteger mejor y ellos tienen espermataphora mas grande. También, las mariposas no quieren aparearse con sus familiares y entonces, ellos escoge n depende de tamaño. Porque ellos no tienen una gran selección ellos escogen quien quiere aparearse para tener mejor aptitud.

PAGE 2

INTRODUCTION In 1859, Darwin first outlined his ideas on sexual selection in The Origin of Species, making the distinction f rom natural selection by saying €this [sexual selection] depends, not on a struggle for existence, but on a struggle between males for the possession of females; the result is not death to the unsuccessful competitor, but few or no offspring Smith, 1989. Al cock defines sexual selection as €a component of individual selection that is created by the pressures males and females exert on conspecifics as they compete for mates and choose among potential partners Alcock 1984. According to Trivers, it is the ine quality in parental investment per offspring between the sexes that generates competition and mate choice. Whichever sex makes the greater parental investment per offspring will be a limited and valuable resource for the other. Therefore, there will usuall y be competition among males for access to females, who will have the option of choosing among the many males that want to copulate with them. They should select individuals that will have the greatest possible effect on their fitness Alcock 1984. Sexua l selection can have profound effects on populations. Futuyma states that €the opportunity for selection exists when differences in some phenotypic characteristic result in consistent differences, on average, in rates of survival or reproduction 1986. T hree major modes of selection that depend on the relationship between phenotype and fitness are directional, stabilizing and disruptive selection. Directional selection is when an extreme phenotype is favored and the population selects for a higher or lowe r value of a character than its current mean. An example of directional selection is for instance, if large only mate with large, therefore leaving the small and the middle sized individuals to be weeded out and eventually the population shifts to become a ll large individuals. Stabilizing selection is when intermediate phenotypes are most fit. For example, suppose the medium average sized forewing s in a butterfly population is the most fit phenotype. Selection for medium sized forewings will then center o n this optimal value of character. Finally, disruptive selection is selection of two or more phenotypes and against the intermediates between the two. An example of disruptive selection is if the large individuals are mating with the large and the small ar e mating with the small, therefore excluding the middle sized individuals. Eventually, the two extreme phenotypes with high fitness will separate and possibly become two new species Futuyma 1986. All three types of selection can greatly alter a populatio n. Sexual selection is a component of individual selection, where each sex may choose a partner based on a number of preferred attributes. An example of discriminating between partners is seen in female damselflies Calopteryx maculata , that behave as th ough they were selecting partners with characteristics likely to enhance their fitness Alcock 1984. They are more likely to accept the courtship of territorial mal es than the copulatory attempts of subordinate intruders. As a result, they gain entry into superior oviposition sites controlled by their partners, and may also derive some genetic benefits. With butterflies, the preferred traits may be size and color. Alcock says it is a natural deduction that females should prefer dominant males as mates, wit h dominant referring to males with high male status, large body size, good foraging ability, superior survival

PAGE 3

skills and good physical condition 1984. Color may be a preferred trait because color provides protection from visually oriented predators and may be related to social signals used during courtship. Color and size may reflect fitness and therefore, sexual selection among butterflies is likely to be based on these characteristics. For butterflies, size is a sign of dominance and is not only repor ted as an important stimulus for courting it has also been reported to be an indicator of size of spermatophore, an important characteristic in mating. Among the checkered white butterfly, Pieris protodice Pieridae, Pierinae, females prefer larger males to smaller males in courtship, because larger males deposit larger spermatophores inside the female. A large spermatophore works to the advantage of the female, because she digests it and uses it for energy in making eggs. The male also benefits because it takes longer for the female to digest, forcing her to delay possible remating. The last male to mate with a female generally fathers her offspring; therefore, delaying her from remating increases the number of offspring that he fathers Scott 1986. This study focused on sexual selection and its consequences in the heliconiine butterfly, Dryadula phaetusa . D. phaetusa Nymphalidae, also known as the €Halloween butterfly, ranges from Mexico to Paraguay and can be found in tropical lowland damp fields and marshes De Vries 1987. In Costa Rica, it occurs infrequently in open areas from sea level to 1200 meters. The genus has shorter and broader wings than other Heliconiines, and comparatively short antennae and an androconial patch on the males De Vries 1 987. The hostplant of the Halloween butterfly is Passiflora talamacensis Passifloraceae De Vries 1987. Adults feed on nectar of flowers and bird droppings and males sometimes sip mud Scott 1986. D. phaetusa occurs sporadically and roosts in small gro ups at night De Vries 1987. Most studies of sexual selection focus on densely populated species, since widely dispersed, locally rare species are difficult to study. However, the Monteverde Butterfly garden offers the rare chance to study the mate choic e of a normally widespread, but locally uncommon species. This leads to the question, does Dryadula phaetusa exhibit sexual selection and what is its implication for the population? MATERIALS AND METHODS The research was carried out in the lowland garden at The Butterfly Garden in Monteverde, Costa Rica from October 28, 2000 until November 15, 2000. The lowland garden simulates a low elevation environment of 0 500 meters. Before beginning the experiment, all adult male and female Dryadula phaetusa in the g arden were captured by hand or with butterfly nets. Individuals were sexed, and the thorax, tibia and forewing lengths were measured with a Spi brand caliper to 0.001cm. Each individual was given a unique mark with a black permanent pen on the underside of the abdomen. All individuals observed mating, courting or feeding were recaptured and identified. New individuals that were introduced into the garden during the study were similarly marked, measured and incorporated into the study.

PAGE 4

Most observations of mating were made in the afternoon on sunny days. The majority of the observations of feeding and courting were taken during the sunniest parts of the day when the butterflies were most active. RESULTS A total of 41 butterflies were caught in the garden, c onsisting of 24 males and 17 females. There were seven pairs of mating butterflies caught. Out of the whole population, 51 were caught while courting and of those, 46 were male and five were female. The number of males caught courting exceeds the number of individuals in the population because some of the butterflies were recaptures. Thirty one butterflies were caught feeding and of those 27 were male and four were female. A Spearman Rank Correlation test was used to test forewing length versus tibia lengt h, thorax length versus forewing length, and thorax length versus tibia length. In the captive population of Dryadula phaetusa , forewing length versus tibia as well as thorax length versus forewing length correlated significantly length Figure 1, r = 0.31 4, p = 0.0473, n = 41 Figure 2, r = 0.326, p = 0.0392, n=41. The sizes, ranges, mean and standard deviation for forewing, thorax and tibia are reported in Table 1. Among the whole population thorax length versus tibia did not correlate significantly Fi gure 3, r = 0.144, p = 0.3638, n = 41. In both forewing comparisons, there was a significant correlation; therefore, forewing length was used for the following statistics. Because forewing length correlated with other measures of size, it was chosen as t he variable to investigate possible sexual dimorphism between males and females of the whole population. An unpaired t test was used to compare the average forewing length of males and females. The average forewing for a male was 3.847 cm n=24 and the av erage fo rewing for a female was 3.922 c m n = 17. The males and females did not differ significantly in forewing length t= 0.900, p=0.3736, d.f. = 39. A Tu key/Kramer one way ANOVA test was used to test for significant size differences between males an d females engaging in various beha viors. The captive population of males had an average forewin g length of 3.847 cm, the captive population of females = 3.922 cm, mating males = 3.974 cm, mating females = 3.991 cm, feeding males = 3.542, feeding females = 3.787 cm, courting males = 3.474 and courting females = 3.762 cm fig. 4. It was determined that there was no significant size difference between males and females, engaging in various behaviors. A Sheffe post hoc test was run to investigate difference s in size of individuals in pairwise combinations of behaviors. The means for each behavior were calculated captive population mean = 3.878 cm, mating population mean = 3.983 cm, feeding population mean = 3.573 cm, courting population m ean = 3.501 cm. It was determined that the mean of the whole population is larger than both the mean of the feeding population and the mean of the courting population Figure 5. The mean of the mating population is larger than both the mean of the feeding and the mean o f the courting

PAGE 5

populations Figure 5. In general, the mean of the mating population was larger than the mean of the whole population but there was no significance measured Table 1. To test for the possibility of assortative mating a Spearman Rank corr elation and a Regression Plot were run and the forewing lengths of each mating pair were compared and found to have no significant correlation r = 0.143, p value=0.7264, n = 7Figure 6. A one sample t test was run to test for significance between the o bserved forewing mean of the captive population of D. phaetusa mean = 3.878 cm and the expected mean of D. phaedusa as reported by DeVries mean = 4.5cm 1987. Significance was observed between the expected and the observed mean suggesting that the pop ulation under study was significantly smaller than the average, for forewing length p = 0.0002. DISCUSSION The findings reported here demonstrate that male forewing length and female forewing length in mating pairs are not significantly correlated and t hus in a population of Dryadula phaetusa there is no assortative mating. However, the mating pairs are larger in comparison to other behaviors suggesting that Alcock‚s argument that females may prefer dominant males as mates may be true 1984. Dominant re fers to males with high male status, large body size, good foraging ability, superior survival skills or good physical condition 1984. It may be to the female‚s benefit to mate preferentially with a male of high status because he may have a genetic advan tage and t his advantage would thus be passed down to her offspring. The non random mating observed in this study may lead to two different types of selection. As the average size of mating pairs is only a fraction larger than the average size of the popul ation, directional selection or stabilizing selection may exist. Because the extreme larger size in mating pairs is very close to the intermediate the average of the whole population, both selection pressures may be working together. Another possibilit y is that as the larger butterflies are favored, disruptive selection may be occurring, where the larger females prefer the larger males, leaving the smaller ones to mate with one another, possibly resulting in two separate species Futuyma 1986. Non ran dom mating on the basis of size has been intensely studied and is heavily supported. Body size often contributes to competitive ability because the larger males can win territories and fights. Increased competitive ability can lead to a greater chance at a ttracting a mate Krebs & Davies 1981. In addition, large males may be better able than small males to protect a female from being damaged by other males during mating Partridge 1983. One factor that limits this study‚s conclusions is that an entirely captive population was studied. Most of the butterflies living in the garden are reared in the garden; therefore inbreeding may decrease the genetic variability of the population. If this is the case, the general population may be smaller than perhaps a na tural population. Alcock writes that females can make mating choices purely on the basis of male genetic quality based on observations that females of some species actively avoid their brothers and fathers 1984. This may be true if females are choosing f rom a large population, but if they only

PAGE 6

have a select few with the butterflies in the garden, inbreeding may be more common than usual. An alternative to the hypothesis already presented is the effect of inbreeding on the captive population. DeVries repo rts forewing length to be 40 41 mm, a value much higher than the 38.78mm average found in the captive population of D. phaetusa . When tested, there was a significant difference between the expected mean and the observed mean, therefore it has been tested a nd proved that the population under study was indeed smaller in forewing length average than D. phaetusa reported in DeVries 1987. Only 14 of the 41 butterflies captured were 40mm or greater and many were smaller. This high percentage of small individual s may be due to inbreeding. The population studied was a captive population and abundant evidence from breeding experiments and captive populations of plants and animals demonstrates that breeding between close relatives results in inbreeding depression, a decrease in the fitness of offspring Pusey and Wolfe 1996. Perhaps D. phaetusa responds to this lower fitness effect by using mate choice as an inbreeding avoidance behavior. By this example, size is fitness related, with small size signifying inbreedin g depression. Through mate choice a captive population, such as the one in the garden, can prevent inbreeding from influencing the population beyond a certain level. The question then arises do natural populations experience this same mate choice if they are less at risk for inbreeding? Given that adults are widely dispersed and rare, there may be little need for inbreeding avoidance or sexual selection. If mates are rare, one mate can‚t be too selective. Therefore, it is only more remarkable that they dem onstrate size selected mate choice. Ano ther idea is that as a communally roosting species, D. phaetusa mates may be able to assess many potential mates at once. If this opportunity is given on a nightly basis, mate choice may be easy. It may even explain why they roost in groups. A study of roosting behavior would be a good extension to this study. ACKNOWLEDGEMENTS Thanks to the staff at El Jardí n de las Mariposas for all your entertainment, knowledge and for finding 6/7 of the total copulations. Thanks t o Jim Wolfe for the use of the garden. Thanks to Andrew Rodstrom, Tim Kuhman and Stephanie McFarlane for your time, energy, patience, encouragement and lots of hugs. Thanks CIEE and Estación Biológica de Monteverde for a semester in paradise. Alan Masters, Karen Masters and Mauricio Garcia, your passion for life and Costa Rica has taught and inspired me, thanks. LITERATURE CITED Alcock, J. 1984. Animal Behavior, An Evolutionary Approach Third Edition. Sinauer Associates, Inc., Sunderland, Massachusetts . DeVries, P.J. 1987. The Butterflies of Costa Rica and Their Natural History. Princeton University Press, Princeton, New Jersey, USA.

PAGE 7

Futuyma, P.J. 1986. Evolutionary Biology. Sinauer Associates, Inc. Sunderland, Massachusetts. Halliday, T.R. 1983. In P. Bateson Ed.. Mate Choice, p.5. Cambridge University Press, Cambridge, England. Krebs, J.R. and Davies, N.B. 1981. An Introduction to Behavioral Ecology. Sinauer Associates, Inc. Sunderland, MA. Pusey A. and M. Wolf. 1996. Inbreeding avoid ance in animals. Tree. 11:202 Scott, J.A. 1986. The Butterflies of North America. Stanford University Press, Stanford, California. Smith, D.A.S. 1989. Mate selection in butterflies: Competition, coyness, choice and chauvinism. In R.I. Vane Wright an d P.R. Ackery, Eds.. The Biology of Butterflies, pg. 225. Princeton University Press, Princeton, New Jersey.

PAGE 8

_____________________________________________________________________________________ Table 1. Values of forewing, thorax and tibia measureme nts for Dryadula phaetusa were measured to find a standard unit of measurement while testing for a size preference in mating. _____________________________________________________________________________________ Mean Standard Deviation Minimum Maximum Ra nge Forewing 3.878 cm 0.265 cm 3.24 cm 4.44 cm 1.2 cm Thorax 0.778 cm 0.66 cm 0.68 cm 0.94 cm 0.26 cm Tibia 0.583 cm 0.052 cm 0.47 cm 0.7 cm 0.23 cm _____________________________________________________________________________________ Figure 1. A scattergram of forewing length versus tibia length for the captive population of Dryadula phaetusa n=41. There is a significant correlation between the two measurements r = 0.314, p = 0.0473. ___________________________________________________________ __________________________

PAGE 9

_____________________________________________________________________________________ Figure 2. A scattergram of thorax length versus forewing length for the captive population of Dryadula phaetusa . There is a significant correlation between the two measurements r = 0.326, p = 0.0392. _____________________________________________________________________________________ _____________________________________________________________________________________ Figure 3. A sca ttergram of thorax length versus tibia length of the captive population of Figure 3. A scattergram of thorax length versus tibia length of the captive population of Dryadula phaetusa . There is no significant correlation between t horax length and tibia le ngth r = 0.144, p = 0.3638. _____________________________________________________________________________________

PAGE 10

_____________________________________________________________________________________ Figure 4. Mean forewing length of Dryadula phae tusa males and females engaged in various behaviors cm. There is no significant size difference between males and females for a given behavior. _____________________________________________________________________________________

PAGE 11

_________________ ____________________________________________________________________ Figure 5. Size comparisons for each behavioral category of Dryadula phaetusa , showing that there is a significant difference between mean forewing length of certain behaviors. Whole po pulation mean = 3.878 mm, mating population mean = 3.983 mm, feeding population mean = 3.573 mm, and courting population mean = 3.501 mm. _____________________________________________________________________________________

PAGE 12

__________________________ ___________________________________________________________ Figure 6. There is no significant correlation between the forewing lengt h of mating males and females r = 0.0143, p = 0.7264, n = seven mating pairs. ___________________________________________ __________________________________________


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