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Preferencia de las presas de Megaphobema mesomelas (theraphosidae) en relacin con la abundancia de las presas y la defensa qumica
Prey preference of Megaphobema mesomelas (theraphosidae) in relation to prey abundance and chemical defense
Food preference is an integral part of what defines a species niche. Predators within the arthropod community must be able to deal with a variety of anti-predatory defenses. Because spiders vary in their responses to prey defenses, it is difficult to predict how each spider species will cope with them.
This knowledge is important because spiders are common in nearly all ecosystems. In this experiment, I studied the feeding preference of Megaphobema mesomelas, a tropical spider of the family Theraphosidae, in relation to the abundance of potential prey species and prey chemical defenses. I found that cockroach nymphs were the most common potential prey item available, but that M. mesomelas typically did not eat this prey, preferring the less abundant crickets instead. I also discovered that the tarantulas prefer the chemically protected larvae of Danaus plexippus over the palatable larvae of Archeoprepona sp. Additional observations led me to the conclusion that prey preference of M. mesomelas is dependent on a combination of ideal factors, including not only abundance, but also texture and speed.
La preferencia de comida es una parte esencial que distingue el nicho de las especies. Los depredadores dentro de la comunidad tienen que manejar con una variedad de defensas para no comer. Debido a que las araas varan en sus respuestas a las defensas de las presas, es difcil predecir cmo cada especie se las arreglar. Este conocimiento es importante porque las araas son comunes en casi todos los ecosistemas. En este experimento, estudi la preferencia de las presas de Megaphobema mesomelas, una araa tropical de la familia Theraphosidae, en relacin a la abundancia de especies potenciales de presas y las defensas qumicas de las mismas. Encontr que las ninfas de cucarachas son la presa ms comn, pero que M. mesomelas figurativamente no come esta presa, y que prefiere a los grillos que tenan una abundancia menor en lugar de otros. Tambin descubr que las tarntulas prefieren las larvas de Danaus plexippus que tienen la proteccin qumica en vez de las larvas apetitosas de Archeoprepona sp. Observaciones adicionales me indican la conclusin que la preferencia de las presas de M. mesomelas es dependiente en un cmulo de factores ideales, incluyendo la textura y la velocidad adems de la abundancia.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--Cerro Plano
Alimentos de animales
Costa Rica--Puntarenas--Zona de Monteverde--Cerro Plano
Tropical Ecology Spring 2011
Ecologa Tropical Primavera 2011
t Monteverde Institute : Tropical Ecology
Prey preference of Megaphobema mesomelas (Theraphosidae) in relation to prey abundance and chemical defense Brendan Boyer Department of Biology, Whitman College ABSTRACT within the arthropod community must be able to deal with a variety of anti predatory defenses. Because spiders vary in their responses to prey defenses, it is difficult to predict how each spider species will cope with them. This knowledge is important b ecause spiders are common in nearly all ecosystems. In this experiment, I studied the feeding preference of Megaphobema mesomelas a tropical spider of the family Theraphosidae, in relation to the abundance of potential prey species and prey chemical defe nses. I found that cockroach nymphs were the most common potential prey item available, but that M. mesomelas typically did not eat this prey, preferring the less abundant crickets instead. I also discovered that the tarantulas prefer the chemically prot ected larvae of Danaus plexippus over the palatable larvae of Archeoprepona sp. Additional observations led me to the conclusion that prey preference of M. mesomelas is dependent on a combination of ideal factors, including not only abundance, but also te xture and speed. RESUMEN La preferencia de comida es una parte esencial que distingue el nicho de las especies. Los depredadores dentro de la comunidad tienen que manejar con una variedad de defensas para no comer. Debido a que las araas varia en sus respuestas a defensas de las presas, es difcil para predecir como cada especie se las arreglar. Este conocimiento es importante porque las araas son comunes en casi todos los ecosistemas. En este experimento, estudi la preferencia de presas de Megap hobema mesomelas una araa tropical de la familia Theraphosidae, en relacin a la abundancia de especies potenciales de presas y las defensas qumicas de las mismas. Encontr que las ninfas de cucarachas son la presa ms comn, pero que M. mesomelas figu rativamente no come esta presa, y que prefiere los grillos que tenan una abundancia menor en lugar de otros. Tambin descubr que las tarntulas prefiere las larvas de Danaus plexippus que tienen la proteccin qumica en vez de las larvas apetitosas de A rcheoprepona sp Observaciones adicionales me indican la conclusin que la preferencia de presas de M. mesomelas es dependiente en un cmulo de factores ideales, incluyendo la textura y la velocidad adems de la abundancia. INTRODUCTION Knowledge of what food a species consumes can play an important role in determining its evolutionary history (Darwin 1859). Consequently, such information is crucial when attempting to understand the morphology and behavior of any species. While some may generalize on whatever food is abundant, others may specialize to reduce competition pressure (Emerson et al. 2005). This can help secure a species in its own niche, but it can also make some species more vulnerable. If anything should significantly reduce the popu may be in greater danger of going extinct (Rezac et al. 2008). I t would be strategically sound to prefer the most abundant option i f a predator were to specia lize on one type of prey (Murdoch 196 9) However, preference by prey abundance may be countered by other factors, such as physical and chemical protection,
as is apparent in many arthropod species (Kakimoto et al 1997, Trigo 2000, Silva et al 2001, Rezac et al. 2008, Sloggett 2010, Souza e t al. 2011). One species of harvestman can deter predators by possessing an exceptionally tough exoskeleton (Souza et al. 2011), and the lepidopteran species Danaus plexippus contains cardiac glycosides (Trigo 2000), which trigger an emetic response from predators that would normally attempt to eat them (Kakimoto et al. 1997). Because spiders are a common predator among arthropods, it is possible that much of the arthropod chemical arsenal would be used to defend against spider predation. This has been d ocumented in the interaction between Nephila clavipes and alkaloid containing lepidopterans, in which the spider releases the insects from its web without attacking them (Silva et al. 2001). While this supports the idea that chemical protection, at least in the form of alkaloids, is an effective deterrent against spider predation, other experimental results suggest otherwise. Tests conducted on another species of spider and an alkaloid containing beetle demonstrated that the spiders would readily consume the chemically protected insects without any negative repercussions, effectively bypassing the defense mechanism (Sloggett 2010). Another case using chemically protected harvestmen as prey proved that the spiders used were unresponsive to the defense (Sou za et al. 2011). Given the inconsistency among spider responses when faced with chemical defenses, more information on their feeding preferences can help to draw a clearer picture of a common yet understudied branch of arthropods. In this experiment, I s tudied the correlation between the abundance of different prey morpho species and the feeding preference of a tarantula (Family: Theraphosid ae) living in premontane and lower montane tropical habitats. I also sought to determine whether the tarantula woul d respond to chemical defenses. Theraphosids of the species Megaphobema mesomelas were chosen as my test subjects. M. mesomelas is likely an important predator among ground dwelling arthropods due to their relative abundance in these habitats, yet there is almost no information published on this species. Like many theraphosids, these spiders are nocturnal, and will crawl to the front of their burrows at night to feed (Prez Miles et al. 2005). Tarantulas are very sensitive to vibration stimuli, and it i s by this means that they detect their prey (Stradling 1994, Prez Miles et al. 2005). I predicted that prey abundance would not affect tarantula feeding preference, as tarantulas in other experiments have eaten many types of prey offered to them, even th ose that they did not encounter normally in the wild (Stradling 1994). Since spiders have responded differently to chemical defenses in the past (Silva et al. 2001, Sloggett 2010, Souza et al. 2011), I had no predictions for how they would react when give n chemically protected prey. METHODS Collection & Study Site Nine tarantulas were collected from burrows situated in roadside ditches in Cerro Plano, Monteverde, Costa Rica, during April 2011. Collection periods started after sunset, when tarantulas w ould make themselves visible near the entrances of their burrows. A small stick was poked into the burrow behind each tarantula, followed by constant jabbing until the tarantula left. Because the tarantulas varied in size, their masses were recorded. S iz e s ranged from 2 to 18 grams, with an average mass of 9.22 grams. This was also done with the majority of prey items used to ensure that the ratio of prey to predator size did not vary considerably between feeding trials.
Each tarantula was placed in its own terrarium at the University of Georgia Laboratory in San Luis. Terrarium floors were covered with just enough dirt to cover the bottom. Each terrarium had one cardboard tunnel and a water dish that was refilled every three days. Terrariums were cov ered with chicken fence weighed down with rocks to prevent the tarantulas from escaping. Prey Morpho species Survey Prey surveys were taken in San Luis along roadside ditches, so as to gather an accurate sample of what would naturally be found near tara ntula burrows. They were taken after sunset, as this was the time that tarantulas were active. A total of eleven surveys were conducted over the same stretch of road, each lasting one hour. Distance covered each night was slightly under a kilometer. Su rveys included all ground dwelling arthropods that were approximately 2 cm or longer with a width greater than 1 cm. Smaller arthropods were excluded on the basis that they would not be large enough to Abundant Prey Feeding Trials Only cockroach nymphs, crickets, and beetles were among the surveyed prey used for each feeding trial. Adult roaches were not used because they were able to escape from the terrariums easily. Centipedes were excluded as the y were too fast to capture, and caterpillars were excluded as not enough were found to record significant results. Feeding trials were also conducted after sunset. Each feeding trial began by throwing a prey item near the head of a tarantula after sunset The prey was then left in the terrarium for at least one full day. Dead prey were removed from the terrariums. If a tarantula attacked a prey but did not eat it, then it was counted as a rejection. No more than two prey were put in any terrarium at a time. Novel Prey (Caterpillar) Feeding Trials Two different caterpillar species were used as prey for the novel prey feeding trials, because they were not likely to be encountered by tarantulas in the wild (pers. observation). The first species was Dan aus plexippus which is found throughout the temperate zone and the tropics. It accumulates toxic cardiac glycosides by feeding on the leaves of Asclepias curassavica (Trigo 2000, Haeger et al. 2010), and this makes it unpalatable to most predators (Kakim oto et al. 1997). Archeoprepona sp. was the other species used, and this feeds on Ocotea leaves (pers. observation) and has no chemical defenses. Novel prey feeding trials were conducted in the same manner as abundant prey feeding trials. RESULTS Prey Morpho species Survey There was significant diference between the number of potential prey organisms found (One Way ANOVA, F = 15.6, df = 5, p < 0.0 001; Fig. 1 ). Cockroach nymphs were significantly the most abundant type, composing 47.19% of the total pr ey found
Crickets and adult roaches were the the second most abundant morpho species, respectively comprising 27.83% and 12.44% of the total prey. The number of crickets surveyed HSD test, p > 0.05), but it was greater than the three least abundant prey morpho species he caterpillar was found during the entire survey, it was not significantly less abundant than the other three least abundant morpho Figu re 1. Average (+SD) percent abundance of prey (for tarantulas) organisms per day. Prey were suveyed along roadside ditches in San Luis after sunset. Numbers in parentheses indicate total number of individuals found for each prey organism in total. Letter s above each column indicate which species differed significantly from each other. Abundant Prey Feeding Trials Each tarantula was given prey 6 to 9 times over the course of 11 days. Total number of prey items eaten by each tarantula ranged from 1 to 8 Tarantulas significantly ate crickets more (when offered) than cockroach nymphs or beetles (X2 = 7.64, df = 2, p = 0.0219; Fig. 2). More than twice as many crickets were eaten than were rejected. The other insects were refused more often than they wer e eaten, with cockroach nymphs and beetles seeing a turndown rate of 60% and 75%, respectively. The cockroach nymphs were of a species that would secrete a volatile chemical when disturbed (pers. observation). While the tarantulas either tended to ignore or eat the cockroaches, there were two instances where the tarantulas successfully caught them, only to spit them out a few seconds later. One of the cockroaches secreted its chemical when this happened, and after spitting it out the tarantula walked away and began cleaning itself. Both the cockroaches succumbed to their attack wounds overnight, but neither were eaten. The tarantula that cleaned itself had eaten a cockroach nymph before, and when it was offered another one the following night, it ate it without hesitation.
The cockroach nymphs were sometimes able to escape from the terrariums overnight, so if no traces of the cockroach was found the day following a trial, the results were not counted. These insects were the fastest of the prey items used but a tarantula could still catch a sprinting cockroach if it ran past its head (pers. observation). Although terrarium before the tarantula decided to eat it. Beet les were typically an exception, not moving much after being placed in the terrariums. Tarantulas would often make contact with the prey, choose not to eat it, and then eat it later in the evening. Other times, tarantulas would eat the prey the moment it was placed in the terrarium. Prey legs, heads, and wings were usually discarded during consumption. Figure 2. Acceptance of prey organisms by tarantulas during feeding trials conducted in terrariums. This figure sums the results of trials conducted wi th nine separate tarantulas. Novel Prey (Caterpillar) Feeding Trials Tarantulas significantly preferred the larvae of Danaus plexippus over the larvae of Archeoprepona sp. (X2 = 6.31, df = 2, p = 0.0427; Fig. 3). Only two of the seven Archeoprepona larv ae were eaten. The feeding process was similar to that involving other prey, with the exception that nearly the entire prey was consumed. No visible remains were left after the monarchs were eaten, and only the heads of Archeoprepona sp. were discarded. Archeoprepona sp. was by far the slowest prey presented to the tarantulas during the entire experiment. Even the monarch caterpillars were noticeably faster in their movements. They were also the biggest prey offered, with an average mass nearly five t imes that of the biggest D. plexippus
Figure 3. Acceptance of caterpillars by tarantulas during feeding trials conducted in terrariums. This figure sums the results of trials conducted with nine separate tarantulas. DISCUSSION M. mesomelas did show a strong preference for some prey items over others, though it was not for the most abundant prey items. Even though cockroach nymphs significantly less abundant cric kets. The tarantulas also favored D. plexippus larvae, a prey item that was never likely to be present in their natural environment. This tells us that these tarantulas do discriminate between different prey items. It also means that there are one or mo re factors that are important in determining prey selection, and that prey abundance is not the main cause of this discrimination as it is with other species (Murdoch 1969). Chemical protection was the other factor that was tested in this experiment, and the results suggest that this too is not a driving factor, at least in the form of cardiac glycosides. The tarantulas did significantly prefer one type of caterpillar over the other, but it was the chemically protected monarch caterpillar, not the palatab le Archeoprepona sp., that they prefered. This cannot be attributed to navet as the tarantulas that ate the monarch caterpillars showed no decrease in fitness afterwards. While M. mesomelas may be immune to one type of chemical defense, they may still be vulnerable to others. Since the most abundant prey item also happened to be chemically protected in a different way than the D. plexippus larvae, it is possible that the cockroach nymph volatile chemical secretions were enough to deter the tarantulas most of the time. Chemical spray defenses have been found to effectively defend other insects from predation (Dossey et al. 2009). However, there was only one observed instance its release, and this was not enough to keep the tarantula from fatally wounding it. It also was not enough to spare other cockroach nymphs from suffering a similar fate when confronted with the same predator. Thus, while the chemical secretion of the c ockroach nymphs
may be able to deter them from some predators, it is not likely to have evolved as a mechanism for detering tarantulas. Chemical defenses aside, there were many morphological differences between the various prey items, and this also could The roach nymphs have very flat bodies covered by tough, plated carapaces, and the beetles have their backs covered by hardened elytra. Crickets, on the other hand, have very soft, squishy bodies that w ould pose little resistence to the chelicera of M. mesomelas This was documented in a previous study conducted with tarantulas where the spiders significantly prefered crickets over beetles because they were softer (Kosiba 2009). However, if the amount physical protection were the primary influence on tarantula prey selection, then all of the catterpillars should have been eaten, and yet most were refused. The attribute that caused the to be ignored by the majority of the tarantulas may have been their speed. Because tarantulas typically sense prey through vibrations ( Stradling 1994) the slow movement of Archeoprepona sp. may have rendered it nearly avoid most could be the slowest moving species, regardless of their palatability. It is evident that there are many variables that can influence prey selection, and to effectively test which one carries the most weight requires the use of prey species that differ i n one but share the rest. Further studies would benefit from using a palatable caterpillar that more closely matched the size and speed of D. plexippus larvae. Another means that could help clarify the trends would be to increase the sample size of all s pecies (predator and prey) involved. More trials should be conducted to increase the three samples were available for this study. Ultimately, it would apear that M. mesomelas prefers prey items that exhibit a balance of both the right speed and body type, and that prey abundance and chemical defenses play a minor role, if any, in impacting prey selection. ACKNOWLEDGEMENTS I would like to thank the University of Geor gia, San Luis de Monteverde for granting me permission to use their laboratory; Pablo Allen Monge for guiding me through the experiment and ordering the caterpillars I used; Anjali Kumar for connecting me with El Jardn de Mariposas in Monteverde and helpi ng me collect tarantulas; Scott Kosiba for answering my questions on how to prepare for the experiment; Miriam Cruz Salazar for always making sure I was well fed even in the midst of sudden changes to my schedule; Nathan Sellers for his constant assistanc e throughout various phases of the project; and finally Cody Will, whose knowledge on tarantulas made this project possible. LITERATURE CITED Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favoured race s in the struggle for life London: John Murray. Dossey, A. T., M. Gottardo, J. M. Whitaker, W. R. Roush, and A. S. Edison. 2009. Alkyldimethylpyrazines in the Defensive Spray of Phyllium westwoodii : A First for Order Phasmatodea Journal of Chemical Ecolo gy, Vol. 35: 861 870. Emerson, B. C. and N. Kolm. 2005. Species diversity can drive speciation. Nature 434: 1015 1017.
Haeger, J. F., D. Jordano, and M. L. Melndez. 2011. Status and conservation of Asclepiadaceae and Danaus in southern Spain Journal of I nsect Conservation, Vol. 15: 361 365. Kakimoto, S., H. Saito, and N. Matsuki. 1997. Involvement of a Peripheral Mechanism in the Emesis Induced by Cardiac Glycosides in Suncus murinus Biological & Pharmaceutical Bulletin 20: 486 489. Kosiba, S. 2009. Opti mal Foraging in Brachypelma smithi (Araneae: Theraphosid ae). Tropical Ecology and Conservation Council on International Educational Exchange, Spring 2009: 37 43. Murdoch, M. M. 1969. Switching in General Predators: Experiments on Predator Specificity and S tability of Prey Populations. Ecological Monographs, Vol. 39: 335 354. Perz Miles, F., F. G. Costa, C. Toscano Gadea, and A. Mignone. 2005. Ecology and behaviour of the 'road tarantulas' Eupalaestrus zveijenberght and Acanthoscurria suina (Araneae, Therap hosidae) from Uruguay Journal of Natural History 39: 483 498. Rezac, M., S. Pekar, and Y Lubin 2008. How oniscophagous spiders overcome woodlouse armour Journal of Zoology 275: 64 71. Silva, K. L. and J. R. Trigo. 2001. Structure activity relationships of pyrrolizidine alkaloids in insect chemical defense against the orb weaving spider Nephila clavipes Journal of Chemical Ecology 28: 657 668. Sloggett, J. J. 2010. Predation of ladybird beetles by the orb web spider Araneus diadematus BioControl 55: 63 1 638. Souza, E. and R. Willemart. 2011. Harvest ironman: heavy armature, and not its defensive secretions, protects a harvestman agai nst a spider. Animal Behaviour 81: 127 133. Stradling, D. J. 1994. Distribution and behavioral ecol spider in Trinidad. Biotropica 26: 84 97. Trigo, J. R. 2000. The Chemistry of Antipredator Def ense by Secondary Compounds in Neotropical Lepidoptera: Facts, Perspectives and Caveats. J. Braz. Chem. Soc., Vol. 11: 551 561.