SPATIAL DYNAMICS of NIGHT ROOSTING in Heliconius Erato Petiverana (LEPIDOPTERA: NYMPHALIDAE)
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SPATIAL DYNAMICS OF NIGHT ROOSTING IN Heliconius erato petiverana (LEPIDOPTERA: NYMPHALIDAE) By CHRISTIAN SALCEDO A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2006 Copyright 2006 by Christian Salcedo “Perfect as the wing of a butterfly may be, it will never enable the butterfly to fly if unsupported by the air. Facts are the air of science. Without them a man of science can never rise.” Adaptation from phrase by Ivan Pavlov (1849-1936) ACKNOWLEDGMENTS This work could not have been possible without the great help and advice of Dr. Thomas C. Emmel, Dr. Miriam Medina Hay-Roe, Dr. Jacqueline Y. Miller, Dr. Andrei Sourakov, and Dr. Peter Teal. I want to thank also Ernesto Rodriguez, from El Bosque Nuevo Preserve, Costa Rica, for providing some of the species used in this study. Student assistants Vanessa Walthall and Kari Ellison contributed with help in breeding and colony maintenance. iv TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iv LIST OF FIGURES .......................................................................................................... vii ABSTRACT..................................................................................................................... viii CHAPTER 1 INTRODUCTION AND RESEARCH GOALS ..........................................................1 2 HELICONIUS BUTTERFLIES ...................................................................................3 3 ROOSTING PATTERNS IN HELICONIUS...............................................................5 Introduction...................................................................................................................5 Roosting in Heliconius .................................................................................................5 Objectives .....................................................................................................................8 Materials and Methods .................................................................................................8 Study Organisms ...................................................................................................8 Butterfly Rearing...................................................................................................9 Butterfly Marking and Wing Length Measurement ..............................................9 Roost Structure ......................................................................................................9 Butterfly Roost Recruitment................................................................................10 Statistical Analysis for Roost Structure...............................................................11 Determining Level of Clustering..................................................................11 Analyzing Spatial Distribution related to Individual Traits .........................11 Results.........................................................................................................................12 Butterfly Roost Recruitment and General Observations.....................................12 Level of Clustering..............................................................................................12 Spatial Distribution related to Sex, Age, and Size ..............................................13 Discussion...................................................................................................................13 Within Roost Interactions and Trends.................................................................13 Level of Gregariousness......................................................................................16 Conclusions.................................................................................................................17 LITERATURE CITED ......................................................................................................21 v BIOGRAPHICAL SKETCH .............................................................................................26 vi LIST OF FIGURES Figure page 3-1. Species used in this study ...........................................................................................18 3-2. Measurement of butterfly wing length. ......................................................................18 3-3. Example of grid used to locate each individual position in a roost............................19 3-4. Heliconius erato sub-aggregations in captivity..........................................................20 vii Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science SPATIAL DYNAMICS OF NIGHT ROOSTING IN Heliconius erato petiverana (LEPIDOPTERA: NYMPHALIDAE) By Christian Salcedo August 2006 Chair: Thomas C. Emmel Major Department: Entomology and Nematology Communal roosting occurs when multiple insects of one or more species assemble in close proximity to one another for a certain period of time. Roosts may be synchronized with circadian rhythms (day-night cycles), or with seasons, or they can be permanent. Some species within the genus Heliconius display night roosting behavior. This particular behavior has been addressed several times over more than a century, but there is still no clear explanation for it. In order to better understand this behavior, I analyzed clustering and roost structure related to individual’s sex, age, and size using Heliconius erato petiverana individuals. The results show that the roost is frequently formed by sub aggregations of individuals but there is no spatial pattern related to sex, age, or size. This suggests that the roost spatial distribution is not affected by selective pressure on these particular traits. Other factors that could be involved in the formation and structure of the roost are discussed. viii CHAPTER 1 INTRODUCTION AND RESEARCH GOALS Communal roosting occurs when multiple insects of one or more species assemble in close proximity to one another for a certain period of time (Yackel 1999). Roosts may be synchronized with circadian rhythms (day-night cycles), or seasons, or they can be permanent (Waller and Gilbert 1982). This behavior has been documented in several insect groups such as butterflies, moths, dragonflies, bees, and wasps for over a century, and in each case authors have proposed different hypotheses to explain its relevance (Evans and Linsley 1960; Benson and Emmel 1973; Joseph 1982; Greig and DeVries 1986; Rehfeldt 1993). Nevertheless, the functional and adaptive significance of gregarious roosting in insects is not fully understood. It is a complex behavior indeed and several factors might have been responsible for its evolution. An interesting group where communal roosting occurs and that can help to further understand this behavior is Heliconius butterflies. Heliconius butterflies belong to the family Nymphalidae within the order Lepidoptera (Penz 1999). They comprise a widespread genus over the tropical and subtropical regions of the New World (Brown 1981; Emsley 1965; Turner 1981) and have been subject to a wide range of studies due to their abundance and relative ease in breeding under laboratory conditions. Even though considerable information has been published on their genetics, ecology, and behavior (Benson 1971, 1972; Cook et al. 1976; Crane 1955, 1957; Jiggins et al. 2001; Jones 1930; Mallet 1980, 1986; Mallet and Gilbert 1 2 1995; Mavárez et al. 2006; Murawski and Gilbert 1986; Poulton 1931; Salcedo 2003; Turner 1971, 1975, 1981; Waller and Gilbert 1982; Young 1978), little is known about their night roosting habits (Beebe 1949; Cook et al. 1976; Crane 1955, 1957; Edwards 1881; Jones 1930; Mallet 1980, 1986; Mallet and Gilbert 1995; Murawski and Gilbert 1986; Poulton 1931; Turner 1971, 1975, 1981; Waller and Gilbert 1982; Young 1978). In the typical situation, several individuals (males and females) begin to land usually on twigs, tendrils, and dry leaves under the shade of a tree just before sunset. After sunset, usually a group has been formed and most of them remain together until sunrise (Crane 1955, 1957; Jones 1930; pers. observations). In addition to the mentioned studies on Heliconius roosting, several authors have speculated about the possible function of this type of aggregation, and within these speculations, the evolution of this behavior as a social trait has been frequently addressed (Benson 1971; Gilbert 1975, 1977; Mallet 1986; Turner 1981). However, to date, only one author has gathered evidence that suggests that avoidance of disturbance and predation are likely reasons to explain this behavior (Mallet 1986; Mallet and Gilbert 1995). Why do Heliconius species roost gregariously? The answer remains in obscurity. Consequently, further observations and experiments are required. The present work analyzes roost structure patterns to further understand this remarkably complex behavior. CHAPTER 2 HELICONIUS BUTTERFLIES History, Distribution and Biology: Relevant Facts and Traits Heliconius butterflies do not represent a rare isolated genus for the scientific community. They have been studied and described since the times of Darwin, and have became a widespread model in evolutionary studies (Turner 1981). In Wallace’s contributions to the Theory of Natural Selection, he wrote: There is in South America