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Fidelidad al sitio y tasa de la cancin como indicadores de la territorialidad en el Grillo Neotropical (Gryllidae)
Site fidelity and song rate as indicators of territoriality in neotropical cricket (Gryllidae)
Territoriality is a competitive behavior expressed commonly throughout the animal
kingdom in a wide variety of ways. To establish there is territoriality in simpler animals such as insects the practice of two specific types of behavior is a strong indicator. The first behavior is site fidelity, or the tendency of an animal to regularly return to previously occupied locations. The second behavior is the expression of aggression during encounters between two individuals in the presence of the aforementioned location. To test the presence of territoriality in a Neotropical cricket population, site fidelity was recorded as well as the response to an
intruders call being played in an individuals territory. The results of these experiments confirmed the presence of territoriality in the subject, indicated by a high percentage of site fidelity and the reduced song-rate in the response call to intruder playbacks.
La territorialidad es un comportamiento competitivo expresado a travs del reino animal en una gran variedad de formas. Para establecer la territorialidad en animales como insectos la prctica de dos tipos especficos de comportamiento son un fuerte indicador. El primer comportamiento es fidelidad de sitio, o la tendencia de un animal a regresar constantemente a sitios ocupados previamente. El segundo comportamiento es la agresin durante encuentros entre dos individuos en un mismo sitio. Para probar la presencia de territorialidad en una poblacin de grillos neotropicales, la fidelidad de sitio fue medida, as como la respuesta al
llamado de un "intruso" sonado en el territorio de un individuo. El resultado de este experimento confirma la presencia de territorialidad en el sujeto, indicado por un alto porcentaje de fidelidad de sitio y la reduccin de la tasa de canto en respuesta al llamado de un intruso.
Text in English.
Monteverde Biological Station (Costa Rica)
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Spring 2010
Ecologa Tropical Primavera 2010
t Monteverde Institute : Tropical Ecology
Site Fidelity and Song Rate as Indicators of Territoriality in Neotropical Cricket (Gryllidae) Hannah Bergemann University of Minnesota Twin Cities College of Food, Agriculture and Natural Resources RESUMEN La territorialidad es un comportamiento compe titivo expresado a travs del reino animal en una gran variedad de formas. Para establecer la territorialidad en animales como insectos la prctica de dos tipos especficos de comportamiento son un fuerte indicador. El primer comportamiento es fidelidad de sitio, o la tendencia de un animal a regresar constantemente a sitios ocupados previamente. El segundo comportamiento es la agresin durante encuentros entre dos individuos en un mismo sitio. Para probar la presencia de territorialidad en una poblaci n de grillos neotropicales, la fidelidad de sitio fue medida, as como la respuesta al llamado de un "intruso" sonado en el territorio de un individuo. El resultado de este experimento confirma la presencia de territorialidad en el sujeto, indicado por un alto porcentaje de fidelidad de sitio y la reduccin de la tasa de canto en respuesta al llamado de un intruso. ABSTRACT Territoriality is a competitive behavior expressed commonly throughout the animal kingdom in a wide variety of ways. To establish the re is territoriality in simpler animals such as insects the practice of two specific types of behavior is a strong indicator. The first behavior is site fidelity, or the tendency of an animal to regularly return to previously occupied locations. The second behavior is the expression of aggression during encounters between two individuals in the presence of the aforementioned location. To test the presence of territoriality in an Neotropical cricket population, site fidelity was recorded as well as the respo nse to an confirmed the presence of territoriality in the subject, indicated by a high percentage of site fidelity and the reduced song rate in the response call to intruder playbacks. INTRODUCTION An important aspect of ecosystem dynamics is the inter and intra specific competition for resources, which could include food, mates, or land. Territoriality is a competitive behavior that allows animals to take advanta ge of these sought after resources by defending them (Alexander 1961). Several behaviors have been associated with the expression of territoriality in varying
degrees of intensity and in various animals. However, in simple invertebrates such as insects, a combination of behaviors has been generally used to establish the presence of basic territoriality (Alexander 1961). The first of these includes the display of aggressive behavior in encounters between two individuals in an area. These aggressive behavior s may involve scent marking, vocal cues, or violent physical combat. The second indicator is the tendency of individuals to consistently remain within limited areas or regularly return to the same areas (Alexander 1961). While territoriality in insects com monly occurs with groups that will leave and return to areas, recognizing landmarks within their territory, it can also be expressed through individuals that stay within relatively small, single defensible areas without the ability to distinguish between o ther sites (Otte and Joern 1975). Studies have found that in field crickets (Orthoptera; Gryllidae) territorial behavior is complex (Alexander 1961). The males of most species display specialized aggressive behaviors associated with the establishment of dominance hierarchies and mate attraction such as physical combat or vocal displays (Alexander 1961, Otte and Joern 1975). One study which examined the relationship between acoustic behavior and territoriality in Platycleis albopunctata found that expel energy through physical fighting (Latimer 1981). Another study expounded upon the significance of acoustic display in Orthoptera, finding that through analysis o f song rate, the for example, whether the call was meant to display aggression or to attract a female (Shaw 1968). Additionally, a relationship was found between the song rate, or frequency of chirps, a nd territoriality in another study (Heiligenburg 1966). This study established that an increase in song rate demonstrated aggression in crickets. This could be because the increased song rate is an indicator of fitness, allowing a male cricket to signal hi testing for 1) the tendency of an individual to consistently return to the same areas and 2) the display of aggressive behavior between individuals using acoustics, specifically playback experiments. A previous CIEE study in the fall of 2009 focused on the acoustic communication of males in both competition and sexual selection, which implies the strong possibility for territoriality in this species (Ehlers 2009). I predict that individuals will show territorial behavior and subsequently, an increase in song rate from the territorial individual when an intruder song is played (Shaw 1968, Heilig enburg 1966). The second test for territoriality involves site fidelity, or the tendency of an individual to return to the same location (Switzer 1993). Materials and Methods Study site and species This study was conducted at the Monteverde Biological St ation in Monteverde, Costa Rica which is located in Holdridge Life zone 3. The subject species is a Neotropical cricket in
the Family Gryllidae that is common in the Monteverde area. Males were found in the patches of vegetation surrounding the gravel road that leads up to the Biological Station. This study focuses on male individuals for two main reasons. Firstly, male crickets are known to be territorial, and secondly, male crickets sing while female crickets do not. Through personal observation, this spe cies was found to be active right before dusk throughout the night. Males tend to climb onto short young saplings, low laying bushes and shrubs, and are often found perched atop leaves or along the stems of various types of vegetation (H. Bergemann, pers. obs.). Experiment 1 Site Fidelity 21 crickets were located and each was marked and color coded using colored paint pens. A corresponding color coded flag was placed at the site where the cricket was found. Each night, the presence or absence of each ma rked cricket within the vicinity of its flag was recorded. After data was collected, a chi squared test was performed on the results. Experiment 2 For the playback experiment, 12 territorial individuals were located. I simu lated an intruder by playing the call of a conspecific individual for 60 seconds directed at the territorial iPod and played back through another iPod attached to a Bose stereo. First the normal, unprovoked call of each individual was recorded for approximately 60 seconds. After the intruder call was played, the response was also recorded for approximately 60 seconds. Once all 24 calls from both normal calls and responses to intruders were collected, Raven Audio software was used for analysis. To determine song rate, two 30 second clips were taken determine song rate. After all of the song rates were collected, a paired t test was used on the data to check for a statistically significant difference between the means of the normal and playback song rates. Results Site Fidelity The average site fidelity was 79.22%. The maxim um number of times an individual was found is 7, and the minimum is 0. The chi squared test revealed individuals were found more in the same territory each night than expected by chance. (chi square = 32.09, p < 0.0001; Fig. 1 ).
Figure 1. Frequency of s ite fidelity by percent of time crickets were seen in the same location. There is a high frequency of individuals (12) who demonstrated site fidelity between 91 100% of the time, and very low frequency of individuals who demonstrated less than 60% site fid elity (5). Song Rate Analysis The average song rate of the normal calls was 37.70 chirps per 30 seconds compared to the average song rate of 31.91 chirps per 30 seconds in playbacks (t = 3.23, df = 23, p = 0.0037; Fig. 2). This indicates that the playb ack song rate is in fact, lower than the normal song rate. 8.78 12.56
Figure 2. The average song rates of normal and playback calls. The normal song rate (SD 8.78) is significantly higher than the playback song rate (SD 12.56), meaning that individuals gene rally had a higher number of chirps per 30 seconds in the normal call compared to the playback call. Discussion This study found that this cricket species demonstrates the two criteria that indicate territoriality; the tendency of individuals to return t o previously occupied locations, or stay in these locations, and the expression of aggressive behavior between individuals. The test of site fidelity produced strong results that were consistent with my original hypothesis. As indicated in Figure 1, an ave rage of 79.22% site fidelity, and 12 individuals or 57% of the population with 100% site fidelity strongly support this hypothesis. Only 5 individuals showed less than 60% site fidelity, which can be accounted for with individual mortality. During the 10 d ay testing period, it was expected that several of the 21 individuals would disappear as a result of predation related mortality, which was accounted for in the collected data. A separate study which also examined territoriality in Orthoptera found that th e highest site fidelity among their subjects was an The test for a response to the playback was expected to result in an increased song rate in the playback call as an aggressive response to an intruder. However, the opposite was found in that the song rate actually decreased in response to conspecific playback. Several studies have found similar results in their experiments with playbacks to territorial Orthopteran. In encounters between two males, studies have concluded that dominant individuals have a higher, more consistent song rate than the subordinate individuals, who had a more intermittent and slow song rate (Alexander 1961, Latimer 1981). This becomes logical when one considers the a dvantage behind having a higher song rate in mate attraction situations. The individual who chirps more has a better chance of attracting a female, thus providing the basis of this finding. Meanwhile, when a cricket chirps more slowly in encounters with a more aggressive, dominant cricket, it produces an honest signal that allows the first cricket to avoid fighting by demonstrating that he is not a threat (Latimer 1981, Searcy and Nowicki). The reasoning to produce this honest signal that indicates lower fi tness is because fighting is costly in terms of energy and also risky (Searcy and Nowicki). The avoidance of fighting would be the best option for a cricket that believes he is faced with a stronger, faster chirping competitor. Additionally, this outcome w ould have been the same, should they have actually fought, because the individual who was more aggressive and willing to fight still would have won (Searcy and Nowicki). p layback would be lower, the results still pose some interesting dilemmas. Because the playback
response calls have such significantly lower song rates, it would imply that the individuals tested f the intruder cricket. This conflicts with the findings of the previously mentioned studies, where the cricket whose territory was invaded usually assumed dominance. The likelihood that the majority of the crickets tested in my study are mostly subordinat e is very low, which indicates an underlying factor that influenced their unusual behavior. One probable explanation is that the playback used to incite a response in the crickets was not properly modulated. A higher volume used to playback the call may ha ve song rate is consistent and fairly rapid, so in the simulated encounter, there was no indication of h may have influenced the behavior of the tested individuals. The misleading and dishonest qualities of the playback call incited an honest signal from the crickets. Despite the unexpected results of the playback experiment, the general outcomes of this s tudy still strongly indicate the presence of territorial behavior in the subject. Although the responses to the playback of an intruder elicited lower song rates, thus subordinate behavior instead of the expected aggressive responses, this result demonstra tes that dominance hierarchy exists in this species, which has been found to be a function of territoriality. Acknowledgements I would like to thank all of the CIEE staff for their support and insight throughout this project. Thank you to Anjali Kumar fo r being my advisor, and to Yimen Araya for his extra help. Also, thank you to David and Colleen for entertaining me on all those long sober nights at the station.
Literature Cited Alexander, Richard D. Aggressiveness, Territoriality, and Sexual Behavior in Field Crickets (Orthoptera: Gryllidae). 1961. Behavior: 17: 130 223. Biology, University of Wisconsin Madison. CIEE Fall 2009. Heiligenburg, Walter The Stimulation of Territorial Singing in House Crickets (Acheta Domesticus). 1966. Zeitschrift fur vergleichende Physiologie: 53: 114 129 Latimer, William. The Acoustic Behaviour of Platycleis albopunctata (Goeze) (Orthoptera, Tettigoniidae). 1981. Beha vior: 76: 182 206. Otte, D., Joern, A. Insect Territoriality and its Evolution: Population Studies of Desert Grasshoppers on Creosote Bushes. 1975. Journal of Animal Ecology: 44: 29 54. Searcy, William A., Nowicki, Stephen. Bird Song and the Problem of Ho nest Communication. American Scientist: 96: 114 121. Shaw, Kenneth C. An Analysis of the Phonoresponse of Males of the True Katydid, Pterophylla camellifolia (Fabricius) (Orthoptera: Tettigoniidae). 1968. Behavior: 31: 203 260. Switzer, Paul V. Site fideli ty in predictable and unpredictable habitats. 1993. Evolutionary Ecology: 7: 533 555.