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The role of size and defensive compounds in mate choice by Nyssodesmus python (Polydesmida: Platyrhacidae)

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Title:
The role of size and defensive compounds in mate choice by Nyssodesmus python (Polydesmida: Platyrhacidae)
Translated Title:
El papel del tamaño y los compuestos de defensa en la elección de pareja por Nyssodesmus python (Polydesmida: Platyrhacidae) ( )
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English
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Burns, Mercedes
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Millipedes--Central America   ( lcsh )
Sexual selection in animals   ( lcsh )
Biological fitness   ( lcsh )
Monteverde Biological Station (Costa Rica)   ( lcsh )
Costa Rica--Puntarenas--Monteverde Zone--Monteverde   ( lcsh )
Milpies--América Central
Selección sexual en animales
Salud biológica
Estación Biológica de Monteverde (Costa Rica)
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
Tropical Ecology Fall 2005
San Gerardo Biological Station (Costa Rica)
Species fitness
Ecologia Tropical Otono 2005
Estación Biológica de San Gerardo (Costa Rica)
Aptitud de especies
Genre:
Reports   ( lcsh )
Reports

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Abstract:
Large mates are preferred in a variety of species, presumably because larger size typically confers greater fitness. In chemically protected species, mates may prefer more toxic partners for the same reason. The common forest millipede Nyssodesmus python displays sexual dimorphism in body size and also produces a defensive compound containing hydrogen cyanide, making it an organism well suited to an experiment on mate preferences highlighting both size and presence of defense compounds. Here, 32 mate choice experiments showed a statistically significant preference between both sexes for larger mates; however, preference for mates with differing hydrogen cyanide ranks (based on a modification of the Grignard Sodium Picrate Test Seigler 1991) was not significant. Results suggested that mating with individuals possessing high quantities of defense compounds did not provide a significant fitness advantage. High mortality in this study may result from autotoxicity and exposure to unusually high amounts of ambient hydrogen cyanide during measurement of relative hydrogen cyanide levels. A new methodology is recommended in the future to better understand the role of chemically mediated mate choice without the hindrance of high study mortality.
Abstract:
Las parejas grandes son preferidas en una variedad de especies, supuestamente porque el tamaño grande normalmente otorga mejores adaptaciones. En especies con protección química, las parejas pueden también preferir parejas más tóxicas. El milpiés común del bosque, Nyssodesmus python, muestra dimorfismo sexual en tamaño del cuerpo y produce un químico de protección con cianuro de hidrógeno; estas características permiten usar a este milpiés en experimentos de preferencia de parejas de acuerdo al tamaño y la presencia de químicos protectores. En este estudio se condujeron 32 experimentos de elección de pareja que mostraron una preferencia estadística significativa por parejas grandes en ambos sexos; sin embargo, la preferencia por parejas con ámbitos de cianuro de hidrógeno diferentes (Basado en una modificación de la prueba del picrato de sodio de Grignard Seigler 1991) no fue significativa. Los resultados sugieren que el apareamiento con individuos con concentraciones altas de químicos protectores no proporcionaron una ventaja de adaptación significativa. La alta mortalidad en estos experimentos pudo ser el resultado de autotoxicicidad y exposición excesiva a cantidades altas de cianuro de hidrógeno en el ambiente durante las mediciones de los niveles relativos de este químico. Se recomienda una metodología nueva en el futuro con el fin de entender mejor el papel de la elección de la pareja basada en químicos sin tener el problema de la mortalidad desmedida.
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Text in English.
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Born Digital

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The Role of Size and Defensive Compounds in Mate Choice by Nyssodesmus python (Polydesmida: Platyrhacidae) Mercedes Burns Department of Biology, Macalester College ABSTRACT Large mates are preferred in a variety of species, presumably because larger s ize typically confers greater fitness. In chemically protected species, mates may prefer more toxic partners for the same reason. The common forest millipede Nyssodesmus python displays sexual dimorphism in body size and also produces a defensive compound containing hydrogen cyanide, making it an organism well suited to an experiment on mate preferences highlighting both size and presence of defense compounds. Here, 32 mate choice experiments showed a statistically significant preference between both sexes for larger mates; however, preference for mates with differing hydrogen cyanide ranks (based on a modification of the Grignard Sodium Picrate Test [Seigler 1991]) was not significant. Results suggested that mating with individuals possessing high quantitie s of defense compounds did not provide a significant fitness advantage. High mortality in this study may result from autotoxicity and exposure to unusually high amounts of ambient hydrogen cyanide during measurement of relative hydrogen cyanide levels. A n ew methodology is recommended in the future to better understand the role of chemically mediated mate choice without the hindrance of high study mortality. RESUMEN Las parejas grandes son preferidas en una variedad de especies, supuestamente porque el ta mao grande normalmente otorga mejores adaptaciones. En especies con proteccin qumica, las parejas pueden tambin preferir parejas ms txicas. El milpis comn del bosque, Nyssodesmus python muestra dimorfismo sexual en tamao del cuerpo y produce un q umico de proteccin con cianuro de hidrgeno; estas caractersticas permiten usar a este milpis en experimentos de preferencia de parejas de acuerdo al tamao y la presencia de qumicos protectores. En este studio se condujeron 32 experimentos de elecci n de pareja que mostraron una preferencia estadstica significativa por parejas grandes en ambos sexos; sin embargo, la preferencia por parejas con mbitos de cianuro de hidrgeno differentes (Basado en una modificacin de la prueba del picrato de sodio de Grignard [Seigler 1991]) no fue significativa. Los resultados sugieren que el apareamiento con individuos con concentraciones altas de qumicos protectores no proporcionaron una ventaja de adaptacin significativa. La alta mortalidad en estos experimentos pudo ser el resultado de autotoxicicidad y exposicin excesiva a cantidades altas de cianuro de hidrgeno en el ambiente durante las mediciones de los niveles relativos de este qumico. Se recomienda una metodologa nueva en el futuro con el fin de entend er mejor el papel de la eleccin de la pareja basada en qumicos sin tener el problema de la mortalidad desmedida. INTRODUCTION Mate choice is an important mechanism of sexual selection, a process Charles Darwin (1871) suggested was distinct from natural selection and that is usually based upon choosing mates with characters that confer greater fitness. One such criterion for mate

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choice is body size, which in many animals in an important determinant of mating success (Telford and Dangerfield 1990). Larger size in potential mates may signal exceptional health, longevity, or ability to effectively defend or compete against other suitors all traits conferring fitness to offspring (Alcock 2005; Telford and Dangerfield 1990). Theoretically, sexual selection sho uld occur in response to differences in parental investment, expenditures of time and energy made by both parents to raise offspring (Trivers 1972). Females, who typically invest more than males in the production of offspring with their nutrient rich and e nergetically costly eggs and greater parental care, tend to exhibit greater selectivity when deciding on a male to fertilize her gametes (Alcock 1984). Males are typically not limited in the number of energetically cheap sperm they can produce, and are ins tead selected to fertilize the eggs of as many different females as possible (Alcock 1984). However, males should also be selective when their investment is large (Trivers 1972). In some mating systems, males provide mates with energetically expensive copu latory gifts, or expend energy via copulatory displays, mate competition by mating with another male), or investment in parental care (Alcock 2005). One potentially impo rtant component of fitness in many organisms is chemical defense. Chemically mediated mate choice does not appear to be well documented in nature, however, some taxa have evolved chemical defenses that seem to act as mate attracting pheromones, namely Itho miine butterflies (Andrews 1983) and Arctiid moths (Iyengar and Eisner 2004) that use pyrrolyzidine alkaloids (PA) derived from flowers in this way. Iyengar and Eisner (2004) report that the female arctiid moth Utetheisa ornatrix exhibits mate choice based the PA levels measured in the courtship pheromones of males. This preference is hypothesized due to the fact that spermatophores (packets of sperm) with replenishing nutrients and PA defense compounds used in egg production are passed from male to female during mating. Thus, large males able to produce energetically costly spermatophores are selected for using the proxy of pheromones, which send an honest signal about the chemical defense he can offer to potential mates and offspring. Males were found to m ate on an opportunistic basis regardless of their heavy energy investment in reproduction because females only broadcasted attractant pheromones for a short period of the day and mate localization is a costly endeavor (Iyengar and Eisner 2004). Arthropods selection; however, dynamics of selection in tropical millipedes such as Nyssodesmus python are poorly understood. Nyssodesmus python is a diurnal, detritivorous millipede found primarily at roa dsides or forest floor pathways, and is easily identified by its large size (up to 100 millimeters in carapace length) and variable dull whitish yellow carapace with dark brown longitudinal stripes (Heisler 1983). Unlike most polydesmids, N. python has bee n studied for sexual selection related to body size. Sexual dimorphism in size is reportedly common among millipedes and is a consequence of the large ovary situated in the abdominal cavity, thus having larger volume than the male abdominal cavity (Telford and Dangerfield 1990). After mating, sperm is only allowed to fertilize eggs as they are laid, sometimes several days after copulation. To ensure paternity, N. python males engage in a proposed form of mate guarding in which individuals will ride on the back of the mated female for up to five days and forcibly flex her head over the genital openings when competitors are

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present so as to decrease her opportunities to mate again (Adolph and Geber, 1995; Heisler 1983). This behavior is presumably exhibited in response to sperm competition between males, which Heisler (1983) suggests is unusually high for N. python due to the high population density and relatively non seasonal breeding of the species, which increase the likelihood of males to encounter and vi e for receptive females. Associative mating a form of mate choice where individuals mate based on like phenotypes has not been found based on size in this species when studying differences between paired and unpaired N. python males (Hyatt 1993; Adolph and Geber 1995). Still, mate guarding behavior constrains N. python of male mate choice based on criteria such as large size to make the best of each individual mating. In addition, like most known milli pedes of the order Polydesmida, N. python produces a volatile spray from sacs located along the sides of the body containing hydrogen cyanide and benzaldehyde when threatened (Adolph and Geber, 1995; Borror et. al. 1989; Heisler 1983). Therefore, sexual se lection may be mediated chemically as well, prompting both sexes to prefer mates with higher hydrogen cyanide content. Given that many characters determine fitness, sexual selection may occur for several simultaneously. Nyssodesmus python provides a system to study sexual selection on two potentially fitness enhancing characters: body size and chemical defense. No research has yet confirmed whether mate choice based on presence or concentration of defensive compounds is displayed in N. python As is consist ent with previous works in other species, it is hypothesized that male and female N. python will preferentially mate with comparatively larger individuals that exhibit a higher concentration of hydrogen cyanide. Furthermore, body size and hydrogen cyanide concentration are expected to be statistically correlative. Here, evidence is examined for sexual selection based on body size and hydrogen cyanide content, as well as how these two characters are related. MATERIALS AND METHODS Subjects and Study Site 58 millipedes (36 females, 22 males) were collected along the forest paths and roads of the Estacin Biolgica de Monteverde and the Estacin Biolgica de San Gerardo (Gandhi 2005). Individuals were sexed and transported in plastic containers separated by se x. Groups of up to 15 individuals of the same sex were maintained in 15 gallon terrariums with a one inch substrate of dead leaves and dirt. Rotting wood was supplied in chunks as a food source. Hydrogen Cyanide and Size Testing The strength of hydrogen cyanide defense per millipede was analyzed using a modification of the Guignard Sodium Picrate Test (Seigler 1991). Indicator paper reactive to cyanide was made by dipping one centimeter by five centimeter cut pieces of Whatman No. 1 filter paper in sodium picrate solution (Baptiste 2002; Martin 2001). Papers were dried overnight on a plastic plate. Body size was measured via the length of each millipede carapace in centimeters from tip to tip with a clear ruler. Individuals were subsequently placed in a ca pped vial with a piece of sodium picrate paper fixed to the lid. The vial was agitated by gentle shaking until the millipede released hydrogen cyanide as detected by odor. It was then

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immediately capped, and agitated for another two minutes to encourage a full discharge of hydrogen cyanide, after which vials were left undisturbed with the millipede inside for 30 minutes. Following this latency, the millipede was removed from the vial and numbered with a paint pen. The sodium picrate paper was preserved unde r clear tape and the color ranked for intensity on a 0 5 scale. Mate Choice Testing Fifty five mate choices experiments were performed, where a male or female and two potential mates of differing criteria (ie: high and low cyanide defense, large or small body size) were placed in a rectangular plastic container. Groups were observed for up to one hour and the chosen mate was recorded after a pair assumed pre copulation position (male mounted on female with head clasp) (Heisler, 1983). Mating pairs were su bsequently separated and each individual was returned to their aquarium for at least an hour before any further trials. RESULTS FIGURE 1. Scatter distribution of carapace size (cm) and hydrogen cyanide rank (on a scale of 0 5) in male and female Nyssodesmus python (N = 58). Size for both males and females was shown to predict hydrogen cyanide rank (ANOVA: F = 137.45, p < 0.0001 for males; F = 237.92, p < 0.0001 for females).Bimodal patterns are seen for both male and female sets and are further explained in FIGURE 2. Size for b oth males and females was shown to predict hydrogen cyanide rank (ANOVA: F = 137.45, p < 0.0001 for males; F = 237.92, p < 0.0001 for females). Male and female N. python differed significantly in size (Mann Whitney U = 191.5, p=0.010) with a mean carapace length of 6.65 cm 0.547 (N = 22) for males and 7.23 cm 0.642 (N = 36) for females. However, male and female hydrogen cyanide ranks did not differ significantly, (Mann Whitney U = 390, p= 0.9219) (Figure 1). The mean HCN rank for males was 2.682 1.656 while the mean rank for females was barely different at 2.667 1.492.

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FIGURE 2. Distribution of carapace size (cm) and hydrogen cyanide rank (on a scale of 0 5) in male and female Nyssodesmus python of different apparent color morphs, Whitney U = 10, p<0.0001 for size between females; U = 2, p = 0.0007 for size between males; U = 93.5, p = 0.0853 for HCN between females; U = 19.5, p = 0.0286). The sample size for this study included two distinct morphs a yellow tan morph with two dark brown stripes on each side of the carapace as described in Heisler (1983) and dark brown to black individuals with cream stripes on each side of the carapace. species for the purpose of this study. Furthermore, a divergent distribution (Figure 2) is females and males are both larger and had marginally to significantly higher hydrogen betwe en females; U = 2, p = 0.0007 for size between males; U = 93.5, p = 0.0853 for HCN between females; U = 19.5, p = 0.0286). Sixteen mate choice tests with size as the factor revealed preferential choice of larger partner for both male and female choice (W ilcoxon sign rank: tied z = 2.521, tied p = 0.0103 for female choice; tied z = 1.890, tied p = 0.0584 for male choice) (Figure 3).

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FIGURE 3. Mean carapace size (in cms) for mated and nonmated Nyssodesmus python of both sexes. Results were derived from 16 mate choice tests in which choosers were male or female (Wilcoxon sign rank: tied z = 2.521, tied p = 0.0103 for female choice; tied z = 1.890, tied p = 0.0584 for male choice). Conversely, the 16 hydrogen cyanide mate choi ce tests showed no significant preference for higher HCN producing partners (Wilcoxon sign rank: tied z = 1.244 tied p = 2.012 for female choice; tied z = 1.014, tied p = 3.035 for male choice) (Figure 4). Nearly half of mating experiments did not resu lt in a mating event after one hour, even though potential mates provided differed in size and/or cyanide as per the methodology (Table 1). FIGURE 4. Mean carapace size (in cms) for mated and nonmated Nyssodesmus python of both sexes. Results were derived from 16 mate choice tests in which choosers were male or female (Wilcoxon sign rank: tied z = 1.244 tied p = 2.012 for female choice; tied z = 1.014, tied p = 3.035 for male choice).

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TABLE 1. Nonmating mate choice test ro unds in which no Nyssodesmus python pairing was observed over the duration of one hour. Criteria for ascertaining a pairing were the presence of a male mounted on the dorsum of the female and a head clasp in ead with his mandibles. C = Chooser Size Tests HCN Tests # C Sex C Size 1 st choice 2 nd choice # C Sex C HCN 1 st choice 2 nd choice 1 f 7.1 7.3 6.8 1 f 5 3 2 2 f 7.7 7.3 6.8 2 f 4 2 3 3 f 7.8 7.1 6.6 3 f 1 2 4 4 f 7.5 6.2 7.6 4 f 3 2 4 1 m 6 6.2 8.2 5 f 1 3 0 2 m 7.3 8 6.6 6 f 5 3 0 3 m 7.3 6.1 8 7 f 4 0 2 4 m 6.7 7.7 7.1 8 f 5 2 3 5 m 6.8 7.3 8 1 m 3 3 5 6 m 6 7.2 7.8 2 m 3 4 1 3 m 3 4 0 4 m 3 2 5 5 m 0 2 5 A surprisingly high mortality of millipedes (26%, N = 58) was observed u p to 24 hours following the hydrogen cyanide testing. Figures 5 and 6 show the mean differences between living and dead populations for size and hydrogen cyanide rank, respectively, as well as their comparisons to the mean size and rank for males and femal es. Dead individuals were marginally significantly larger (Mann Whitney U = 180.5, p = 0.0662) and scored much higher on the hydrogen cyanide test in comparison to all living individuals (Mann Whitney U = 90, p = 0.0001). FIGURE 5. Mean carapace lengths (in cm) for male (N = 22), female (N = 36), dead (N = 12), and living (N = 46) Nyssodesmus python. Death status was recorded less than 24 hours after hydrogen cyanide test was performed. Comparisons were made between male and fema le body lengths (Mann Whitney U = 191.5, p=0.010) and dead and living body lengths (Mann Whitney U = 180.5, p = 0.0662).

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FIGURE 6. Mean hydrogen cyanide rank on a scale of 0 5 for male (N = 22), female (N = 36), dead (N = 12) and living (N = 46) Nyssodesmus python. Death status was recorded less than 24 hours after hydrogen cyanide test was performed. Comparisons were made between male and female HCN scores (Mann Whitney U = 390, p= 0.9219) and dead and living HCN scores (Man n Whitney U = 90, p = 0.0001). DISCUSSION As expected from previous research (Adolph and Geber, 1995; Heisler 1983), female N. python were larger than males. However, although hydrogen cyanide rank was found to be analogous to carapace size (Figure 1), female HCN ranks were not shown to be statistically different from those of males. This finding suggests that, although females are generally larger than males, the concentration of defensive compounds is similar between the sexes. It is plausible that the size difference between males and females does not therefore include defensive compound sac size. The hypothesis of this study that both sexes would preferentially mate with larger individuals with higher cyanide ranks was partially supported by the data Males and females both chose mates with larger carapace size in 15 out of 16 total mate choice trials (8 of 8 for females, 7 of 8 for males), making size apparently a factor strongly influencing mating success for both sexes. However, neither sex was sho wn to preferentially select mates with higher HCN scores. There are many possible explanations for this lack of preference in defensive compound presence. N. python is blind (Adolph and Geber 1995), and it may be possible that individuals are simply not ab le to sense the defensive compounds of others through scent, touch, or another sense. Hydrogen cyanide content may have little to no bearing on reproductive fitness for individuals as a chemical that may be transmitted through spermatophores as in Utetheis a ornatrix (Iyengar and Eisner 2004). In this situation, mate choice effects would be expected to show greater significance if females had direct fitness benefits such as an increase in defense compounds for accepting the spermatophores of males with high hydrogen cyanide.

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It is important to note that measuring the amount of hydrogen cyanide released by a millipede following a disturbance may not be an accurate measure of the true chemical potential of an individual. Chapman (1998) gives the example of the phasmid genera Anismorpha and Poikilocerus in which females are able to discharge repellent chemical sprays up to five times over a short duration, after which it takes up to 15 days to replenish these resources. Furthermore, other insects only elicit thi s response when they physically disturb the phasmid vertebrate predators such as blue jays are sprayed even before they touch the insect (Chapman 1998). This suggests that millipedes tested for of compound proportional to their total chemical stores, or that some level of control over the amount of chemical released is available and contingent on the type of disturbance encountered. Thus, while less invasive than other methods, the sodium picrate test may not be the most honest in terms of expressing a proxy for the real volume of defense compound a millipede carries. The mortality in the study was an unexpected effect that seemed to be strongly linked to the hydrogen cyanide rank of individuals. Although no individuals were removed dead from the testing chamber following their latency time after agitation, those that were found dead up to 24 hours following the test were shown to have statistically significant higher HCN ranks than all of the livi ng individuals. Insects that store cyanogenic glycosides in the cuticle tend to be less sensitive to cyanide poisoning (Chapman 1998) but it is possible that, for individuals producing a large amount of defense compound, the 30 minutes spent in the chamber would be enough to cause autotoxicity. Recommendations for future work with Nyssodesmus python and chemically mediated mate choice strongly suggest performing a large quantity of choice tests with a larger sample before testing the hydrogen cyanide test, to avoid the mortality effect before allowing mating chances. This system would give a researcher the opportunity to test if hydrogen cyanide release really does have no bearing on mate choice, without the possible negative affects asserted by the hydrogen cyanide test. To summarize, sexual selection was shown to be operating for the characteristic of body size in N. python, whereas hydrogen cyanide concentration, although positively correlated with body size, was not a characteristic shown to influence mat e choice. Both males and females chose mates predictably based on these criteria. Body size is therefore considered to be a more reliable indicator of mating success than chemical defense in N. python mating systems. These findings serve to enrich our know ledge of the species as well as suggesting a base for further exploration in the topic of mate choice. ACKNOWLEDGEMENTS daily supply of millipedes and to my advisor, Alan Masters, for helping me formulate a research plan. Special thanks are due to Taegan McMahon and Gregory Burkard Jr., who were my constant comrades in the lower laboratory and brought me (more) millipedes and great conversation dail y. Finally, I am grateful to my host mother, Marjorie Trejos Rojas, who patiently listened through scores of gross explanations about millipede sex in my bogus Spanish.

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LITERATURE CITED Adolph, S.C, and Geber, M.A. 1995. Mate Guarding, Mating Success a nd Body Size in the Tropical Millipede Nyssodesmus Python (Peters) (Polydesmida: Platyrhacidae). The Southwestern Naturalist 40: 56 61. Andrews, C.C. 1983. Melinaea lilis imitata (Melineas, Army Ant Butterflies). In: Costa Rican Natural History D. H. Janz en, ed. The University of Chicago Press, Chicago, IL, pp. 736 738. Alcock, J. 2005. The Evolution of Reproductive Behavior. Animal Behavior Sinauer Associates, Inc. Sunderland, MA. 317. ---------. 1984. Male and Female Reproduction Strategies. Anim al Behavior: An Evolutionary Approach Sinauer Associates, Inc. Sunderland, MA. 341. Baptiste, K. 2002. Does Herbivory Induce Cyanide Production in Passiflora biflora ? University of California Education Abroad Program Spring 2002. Monteverde, Costa Rica. B orror, D.J., Triplehorn, C.A., and Johnson, N. F. 1989. Introduction to the Study of Insects Harcourt Brace College Publishers. Orlando, FL. 139. Chapman, R.F. 1998. The Insects: Structure and Function Cambridge University Press. Cambridge, England. 730 733. Darwin, C. 1871. The Descent of Man and Selection in Relation to Sex. Murray. London, England. Gandhi, S. 2005. Defense mechanisms of Nyssodesmus python (Polydesmidae). Tropical Ecology and Conservation (CIEE) Summer 2005. Monteverde, Costa Rica. Hya tt, P. 1993. The Mating Behavior of the Tropical Millipede Nyssodesmus python Tropical Ecology and Conservation (CIEE) Summer 1993. Monteverde, Costa Rica. Heisler, I.L. 1983. Nyssodesmus python (Milpies, Giant Forest Floor Millipede). In: Costa Rican Nat ural History D. H. Janzen, ed. The University of Chicago Press, Chicago, IL, pp. 747 748. Iyengar, V.K. and Eisner, T. 2004. Male indifference to female traits in an arctiid moth ( Utetheisa ornatrix ). Ecological Entomology 29: 281 284. Martin, M.S. 2001. Effect of Altitude on Cyanide Concentration, Leaf Toughness, and Herbivory in Passiflora biflora (Passifloraceae). Tropical Ecology and Conservation (CIEE) Fall 2001. Monteverde, Costa Rica. Seigler, D.S. 1991. Cyanide and Cyanogenic Glycosides. Herbivores : Their Interations with Secondary Plant Metabolites, Second Edition, Volume 1: The Chemical Participants (Rosenthal, G.A. and M.R. Berenbaum, eds.). Academic Press. San Diego, CA, USA. 35 70. Telford, S.R. and Dangerfield, J.M. 1990. Sex in Millipedes: La boratory Studies on Sexual Selection. Journal of Biological Education 24: 223 229. Trivers, R.L. 1972. Parental investment and sexual selection. In: Sexual Selection and the Descent of Man ed. Campbell, B. Aldine. Chicago, IL.


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El papel del tamao y los compuestos de defensa en la eleccin de pareja por Nyssodesmus python (Polydesmida: Platyrhacidae)
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Large mates are preferred in a variety of species, presumably because larger size typically confers greater fitness. In chemically protected species, mates may prefer more toxic partners for the same reason. The common forest millipede Nyssodesmus python displays sexual dimorphism in body size and also produces a defensive compound containing hydrogen cyanide, making it an organism well suited to an experiment on mate preferences highlighting both size and presence of defense compounds. Here, 32 mate choice experiments showed a statistically significant preference between both sexes for larger mates; however, preference for mates with differing hydrogen cyanide ranks (based on a modification of the Grignard Sodium Picrate Test [Seigler 1991]) was not significant. Results suggested that mating with individuals possessing high quantities of defense compounds did not provide a significant fitness advantage. High mortality in this study may result from autotoxicity and exposure to unusually high amounts of ambient hydrogen cyanide during measurement of relative hydrogen cyanide levels. A new methodology is recommended in the future to better understand the role of chemically mediated mate choice without the hindrance of high study mortality.
Las parejas grandes son preferidas en una variedad de especies, supuestamente porque el tamao grande normalmente otorga mejores adaptaciones. En especies con proteccin qumica, las parejas pueden tambin preferir parejas ms txicas. El milpis comn del bosque, Nyssodesmus python, muestra dimorfismo sexual en tamao del cuerpo y produce un qumico de proteccin con cianuro de hidrgeno; estas caractersticas permiten usar a este milpis en experimentos de preferencia de parejas de acuerdo al tamao y la presencia de qumicos protectores. En este estudio se condujeron 32 experimentos de eleccin de pareja que mostraron una preferencia estadstica significativa por parejas grandes en ambos sexos; sin embargo, la preferencia por parejas con mbitos de cianuro de hidrgeno diferentes (Basado en una modificacin de la prueba del picrato de sodio de Grignard [Seigler 1991]) no fue significativa. Los resultados sugieren que el apareamiento con individuos con concentraciones altas de qumicos protectores no proporcionaron una ventaja de adaptacin significativa. La alta mortalidad en estos experimentos pudo ser el resultado de autotoxicicidad y exposicin excesiva a cantidades altas de cianuro de hidrgeno en el ambiente durante las mediciones de los niveles relativos de este qumico. Se recomienda una metodologa nueva en el futuro con el fin de entender mejor el papel de la eleccin de la pareja basada en qumicos sin tener el problema de la mortalidad desmedida.
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